Merge pull request #232 from aykhans/feat/script-http

feat: add http function to script engines
Merge remote-tracking branch 'origin/main' into feat/script-http
2026-10-12 20:58:52 +00:00 · 2026-10-04 22:12:38 +04:00 · 2026-10-04 22:10:02 +04:00 · 2026-10-04 22:04:01 +04:00 · 2026-10-04 21:59:36 +04:00 · 2026-10-04 21:58:53 +04:00
35 changed files with 2490 additions and 492 deletions
@@ -24,4 +24,4 @@ jobs:
      - name: golangci-lint
        uses: golangci/golangci-lint-action@v9
        with:
-          version: v2.13.2
+          version: v2.14.0
@@ -67,7 +67,7 @@ nix run github:aykhans/sarin/release -- -U http://example.com -r 100 -c 10
 Install into your profile:

 ```sh
-nix profile install github:aykhans/sarin/release
+nix profile add github:aykhans/sarin/release
 ```

 Or add it to your own flake via the overlay:
@@ -5,7 +5,7 @@ vars:
  BIN_DIR: ./bin
  GO_VERSION:
    sh: sed -n 's/^go //p' go.mod
-  GOLANGCI_LINT_VERSION: v2.13.2
+  GOLANGCI_LINT_VERSION: v2.14.0
  GOLANGCI: "{{.BIN_DIR}}/golangci-lint-{{.GOLANGCI_LINT_VERSION}}"

 tasks:
@@ -51,7 +51,8 @@ func main() {
 		ctx,
 		combinedConfig.Methods, combinedConfig.URL, *combinedConfig.Timeout,
 		*combinedConfig.Concurrency, combinedConfig.Requests, combinedConfig.Duration,
-		*combinedConfig.Progress == config.ConfigProgressTypeBar, *combinedConfig.Insecure, combinedConfig.Params, combinedConfig.Headers,
+		*combinedConfig.Progress == config.ConfigProgressTypeBar, *combinedConfig.Insecure, *combinedConfig.MaxResponseBody,
+		combinedConfig.Params, combinedConfig.Headers,
 		combinedConfig.Cookies, combinedConfig.Bodies, combinedConfig.Proxies, combinedConfig.Values,
 		*combinedConfig.Output != config.ConfigOutputTypeNone,
 		*combinedConfig.DryRun, *combinedConfig.LogLevel, *combinedConfig.LogFile,
@@ -75,7 +76,7 @@ func main() {
 		}),
 	)

-	srn.Start(ctx, stopCtrl)
+	srn.Start(stopCtrl)

 	switch *combinedConfig.Output {
 	case config.ConfigOutputTypeNone:
@@ -14,32 +14,33 @@ Use `-s` or `--show-config` to see the final merged configuration before sending

 > **Note:** For CLI flags with `string / []string` type, the flag can be used once with a single value or multiple times to provide multiple values.

-| Name                        | YAML                                | CLI                                          | ENV                              | Default | Description                  |
-| --------------------------- | ----------------------------------- | -------------------------------------------- | -------------------------------- | ------- | ---------------------------- |
-| [Help](#help)               | -                                   | `-help` / `-h`                               | -                                | -       | Show help message            |
-| [Version](#version)         | -                                   | `-version` / `-v`                            | -                                | -       | Show version and build info  |
-| [Show Config](#show-config) | `showConfig`<br>(boolean)           | `-show-config` / `-s`<br>(boolean)           | `SARIN_SHOW_CONFIG`<br>(boolean) | `false` | Show merged configuration    |
-| [Config File](#config-file) | `configFile`<br>(string / []string) | `-config-file` / `-f`<br>(string / []string) | `SARIN_CONFIG_FILE`<br>(string)  | -       | Path to config file(s)       |
-| [URL](#url)                 | `url`<br>(string)                   | `-url` / `-U`<br>(string)                    | `SARIN_URL`<br>(string)          | -       | Target URL (HTTP/HTTPS)      |
-| [Method](#method)           | `method`<br>(string / []string)     | `-method` / `-M`<br>(string / []string)      | `SARIN_METHOD`<br>(string)       | `GET`   | HTTP method(s)               |
-| [Timeout](#timeout)         | `timeout`<br>(duration)             | `-timeout` / `-T`<br>(duration)              | `SARIN_TIMEOUT`<br>(duration)    | `10s`   | Request timeout              |
-| [Concurrency](#concurrency) | `concurrency`<br>(number)           | `-concurrency` / `-c`<br>(number)            | `SARIN_CONCURRENCY`<br>(number)  | `1`     | Number of concurrent workers |
-| [Requests](#requests)       | `requests`<br>(number)              | `-requests` / `-r`<br>(number)               | `SARIN_REQUESTS`<br>(number)     | -       | Total requests to send       |
-| [Duration](#duration)       | `duration`<br>(duration)            | `-duration` / `-d`<br>(duration)             | `SARIN_DURATION`<br>(duration)   | -       | Test duration                |
-| [Log Level](#log-level)     | `logLevel`<br>(string)              | `-log-level` / `-l`<br>(string)              | `SARIN_LOG_LEVEL`<br>(string)    | `error` | Runtime log levels to emit   |
-| [Log File](#log-file)       | `logFile`<br>(string)               | `-log-file` / `-w`<br>(string)               | `SARIN_LOG_FILE`<br>(string)     | -       | Write runtime logs to a file |
-| [Progress](#progress)       | `progress`<br>(string)              | `-progress` / `-p`<br>(string)               | `SARIN_PROGRESS`<br>(string)     | `bar`   | Progress display (bar/none)  |
-| [Output](#output)           | `output`<br>(string)                | `-output` / `-o`<br>(string)                 | `SARIN_OUTPUT`<br>(string)       | `table` | Output format for stats      |
-| [Dry Run](#dry-run)         | `dryRun`<br>(boolean)               | `-dry-run` / `-z`<br>(boolean)               | `SARIN_DRY_RUN`<br>(boolean)     | `false` | Generate without sending     |
-| [Insecure](#insecure)       | `insecure`<br>(boolean)             | `-insecure` / `-I`<br>(boolean)              | `SARIN_INSECURE`<br>(boolean)    | `false` | Skip TLS verification        |
-| [Body](#body)               | `body`<br>(string / []string)       | `-body` / `-B`<br>(string / []string)        | `SARIN_BODY`<br>(string)         | -       | Request body                 |
-| [Params](#params)           | `params`<br>(object)                | `-param` / `-P`<br>(string / []string)       | `SARIN_PARAM`<br>(string)        | -       | URL query parameters         |
-| [Headers](#headers)         | `headers`<br>(object)               | `-header` / `-H`<br>(string / []string)      | `SARIN_HEADER`<br>(string)       | -       | HTTP headers                 |
-| [Cookies](#cookies)         | `cookies`<br>(object)               | `-cookie` / `-C`<br>(string / []string)      | `SARIN_COOKIE`<br>(string)       | -       | HTTP cookies                 |
-| [Proxy](#proxy)             | `proxy`<br>(string / []string)      | `-proxy` / `-X`<br>(string / []string)       | `SARIN_PROXY`<br>(string)        | -       | Proxy URL(s)                 |
-| [Values](#values)           | `values`<br>(string / []string)     | `-values` / `-V`<br>(string / []string)      | `SARIN_VALUES`<br>(string)       | -       | Template values (key=value)  |
-| [Lua](#lua)                 | `lua`<br>(string / []string)        | `-lua`<br>(string / []string)                | `SARIN_LUA`<br>(string)          | -       | Lua script(s)                |
-| [Js](#js)                   | `js`<br>(string / []string)         | `-js`<br>(string / []string)                 | `SARIN_JS`<br>(string)           | -       | JavaScript script(s)         |
+| Name                                    | YAML                                | CLI                                          | ENV                                 | Default | Description                   |
+| --------------------------------------- | ----------------------------------- | -------------------------------------------- | ----------------------------------- | ------- | ----------------------------- |
+| [Help](#help)                           | -                                   | `-help` / `-h`                               | -                                   | -       | Show help message             |
+| [Version](#version)                     | -                                   | `-version` / `-v`                            | -                                   | -       | Show version and build info   |
+| [Show Config](#show-config)             | `showConfig`<br>(boolean)           | `-show-config` / `-s`<br>(boolean)           | `SARIN_SHOW_CONFIG`<br>(boolean)    | `false` | Show merged configuration     |
+| [Config File](#config-file)             | `configFile`<br>(string / []string) | `-config-file` / `-f`<br>(string / []string) | `SARIN_CONFIG_FILE`<br>(string)     | -       | Path to config file(s)        |
+| [URL](#url)                             | `url`<br>(string)                   | `-url` / `-U`<br>(string)                    | `SARIN_URL`<br>(string)             | -       | Target URL (HTTP/HTTPS)       |
+| [Method](#method)                       | `method`<br>(string / []string)     | `-method` / `-M`<br>(string / []string)      | `SARIN_METHOD`<br>(string)          | `GET`   | HTTP method(s)                |
+| [Timeout](#timeout)                     | `timeout`<br>(duration)             | `-timeout` / `-T`<br>(duration)              | `SARIN_TIMEOUT`<br>(duration)       | `10s`   | Request timeout               |
+| [Max Response Body](#max-response-body) | `maxResponseBody`<br>(size)         | `-max-response-body` / `-S`<br>(size)        | `SARIN_MAX_RESPONSE_BODY`<br>(size) | `10MiB` | Largest response body to read |
+| [Concurrency](#concurrency)             | `concurrency`<br>(number)           | `-concurrency` / `-c`<br>(number)            | `SARIN_CONCURRENCY`<br>(number)     | `1`     | Number of concurrent workers  |
+| [Requests](#requests)                   | `requests`<br>(number)              | `-requests` / `-r`<br>(number)               | `SARIN_REQUESTS`<br>(number)        | -       | Total requests to send        |
+| [Duration](#duration)                   | `duration`<br>(duration)            | `-duration` / `-d`<br>(duration)             | `SARIN_DURATION`<br>(duration)      | -       | Test duration                 |
+| [Log Level](#log-level)                 | `logLevel`<br>(string)              | `-log-level` / `-l`<br>(string)              | `SARIN_LOG_LEVEL`<br>(string)       | `error` | Runtime log levels to emit    |
+| [Log File](#log-file)                   | `logFile`<br>(string)               | `-log-file` / `-w`<br>(string)               | `SARIN_LOG_FILE`<br>(string)        | -       | Write runtime logs to a file  |
+| [Progress](#progress)                   | `progress`<br>(string)              | `-progress` / `-p`<br>(string)               | `SARIN_PROGRESS`<br>(string)        | `bar`   | Progress display (bar/none)   |
+| [Output](#output)                       | `output`<br>(string)                | `-output` / `-o`<br>(string)                 | `SARIN_OUTPUT`<br>(string)          | `table` | Output format for stats       |
+| [Dry Run](#dry-run)                     | `dryRun`<br>(boolean)               | `-dry-run` / `-z`<br>(boolean)               | `SARIN_DRY_RUN`<br>(boolean)        | `false` | Generate without sending      |
+| [Insecure](#insecure)                   | `insecure`<br>(boolean)             | `-insecure` / `-I`<br>(boolean)              | `SARIN_INSECURE`<br>(boolean)       | `false` | Skip TLS verification         |
+| [Body](#body)                           | `body`<br>(string / []string)       | `-body` / `-B`<br>(string / []string)        | `SARIN_BODY`<br>(string)            | -       | Request body                  |
+| [Params](#params)                       | `params`<br>(object)                | `-param` / `-P`<br>(string / []string)       | `SARIN_PARAM`<br>(string)           | -       | URL query parameters          |
+| [Headers](#headers)                     | `headers`<br>(object)               | `-header` / `-H`<br>(string / []string)      | `SARIN_HEADER`<br>(string)          | -       | HTTP headers                  |
+| [Cookies](#cookies)                     | `cookies`<br>(object)               | `-cookie` / `-C`<br>(string / []string)      | `SARIN_COOKIE`<br>(string)          | -       | HTTP cookies                  |
+| [Proxy](#proxy)                         | `proxy`<br>(string / []string)      | `-proxy` / `-X`<br>(string / []string)       | `SARIN_PROXY`<br>(string)           | -       | Proxy URL(s)                  |
+| [Values](#values)                       | `values`<br>(string / []string)     | `-values` / `-V`<br>(string / []string)      | `SARIN_VALUES`<br>(string)          | -       | Template values (key=value)   |
+| [Lua](#lua)                             | `lua`<br>(string / []string)        | `-lua`<br>(string / []string)                | `SARIN_LUA`<br>(string)             | -       | Lua script(s)                 |
+| [Js](#js)                               | `js`<br>(string / []string)         | `-js`<br>(string / []string)                 | `SARIN_JS`<br>(string)              | -       | JavaScript script(s)          |

 ---

@@ -168,6 +169,20 @@ Valid time units: `ns`, `us` (or `µs`), `ms`, `s`, `m`, `h`

 **Examples:** `5s`, `300ms`, `1m20s`

+## Max Response Body
+
+Largest response body sarin reads. Defaults to `10MiB`, and `0` removes the limit.
+
+The value is a whole number of bytes, with or without a unit: `B`, `KB`, `MB`, `GB`, `TB` are powers of 1000, `KiB`, `MiB`, `GiB`, `TiB` are powers of 1024. Case is ignored and a space before the unit is allowed. The largest accepted value is `1GiB`, because one response can take a few times the limit in memory.
+
+**Examples:** `1048576`, `10MiB`, `2MB`, `512KiB`
+
+A server can declare a `Content-Length` far larger than it sends, and the body is allocated from that declared length, so without a limit a single response can exhaust memory. A response above the limit fails with `response body exceeds the body limit` and is counted like any other error.
+
+The limit applies per response in flight, and a declared Content-Length is allocated up to it, so the worst case is a few times the limit times the concurrency. It covers the requests scripts make as well.
+
+A compressed response in a script is decoded first, so the limit covers the decoded size as well as the bytes that arrived. Decoding holds both at once, so a brotli or zstd body peaks at over ten times the limit. Ratios are unbounded, and 106 bytes of brotli can hold 64MiB, so with `0` a tiny download can still exhaust memory.
+
 ## Concurrency

 Number of concurrent workers. Must be between 1 and 100,000,000. Defaults to `1`.
@@ -188,7 +203,7 @@ Valid time units: `ns`, `us` (or `µs`), `ms`, `s`, `m`, `h`

 Runtime log levels to emit, comma-separated. Valid levels: `info`, `error`. Defaults to `error`.

- `error`: errors that occur while generating or sending a request
+- `error`: errors that occur while generating a request. A request that fails to send is counted in the results instead
 - `info`: every completed response

 Leave empty to disable logging entirely.
@@ -219,6 +234,10 @@ Using `none` disables output and reduces memory usage since response statistics

 Generate requests without sending them. Useful for testing templates.

+A script's own `http` calls are still sent, since a script needs them to build the request you are inspecting. Only the requests sarin generates are held back.
+
+The duration in the results is how long generating each request took, templates and scripts included, so it is a different measurement from the response time a real run reports. A request with neither renders once before the run and reports `0s`.
+
 ## Insecure

 Skip TLS certificate verification.
@@ -456,7 +475,9 @@ function transform(req)
 end
 ```

-> **Note:** Header, parameter, and cookie values can be a single string or a table (array) for multiple values per key (e.g. `{"val1", "val2"}`).
+> **Note:** Header, parameter, and cookie values can be a single string or a table (array) for multiple values per key (e.g. `{"val1", "val2"}`). Numbers and booleans are converted to text, other values are skipped.
+
+> **Note:** Lua scripts can send their own HTTP requests with `http` and work with JSON through the global `json` table. See [HTTP Requests in Scripts](#http-requests-in-scripts).

 **YAML example:**

@@ -521,7 +542,9 @@ function transform(req) {
 }
 ```

-> **Note:** Header, parameter, and cookie values can be a single string or an array for multiple values per key (e.g. `["val1", "val2"]`).
+> **Note:** Header, parameter, and cookie values can be a single string or an array for multiple values per key (e.g. `["val1", "val2"]`). Numbers and booleans are converted to text, other values are skipped.
+
+> **Note:** JavaScript scripts can send their own HTTP requests with `http`. See [HTTP Requests in Scripts](#http-requests-in-scripts).

 **YAML example:**

@@ -554,3 +577,105 @@ js:
 ```sh
 SARIN_JS='function transform(req) { req.headers["X-Custom"] = "my-value"; return req; }'
 ```
+
+## HTTP Requests in Scripts
+
+Lua and JavaScript scripts can send their own HTTP requests and use the response in the main request, for example to fetch a fresh token or CSRF value from another endpoint.
+
+**Function:**
+
+```
+http(url, opts)
+```
+
+`url` must be an absolute `http` or `https` URL. `opts` is optional.
+
+**Options:**
+
+| Option         | Type                        | Default | Description                                                                                            |
+| -------------- | --------------------------- | ------- | ------------------------------------------------------------------------------------------------------ |
+| `method`       | string                      | `GET`   | HTTP method, sent as given                                                                             |
+| `headers`      | table/object                | -       | Request headers, same shape as `req.headers`                                                           |
+| `params`       | table/object                | -       | Query parameters, added to any already in the URL                                                      |
+| `cookies`      | table/object                | -       | Cookies, sent as a single `Cookie` header                                                              |
+| `body`         | string                      | `""`    | Request body                                                                                           |
+| `timeout`      | duration string (`"500ms"`) | `10s`   | Request timeout (the `timeout` config does not apply), applied to each hop when redirects are followed |
+| `insecure`     | boolean                     | `false` | Skip TLS verification (the `insecure` config does not apply)                                           |
+| `maxRedirects` | number                      | `0`     | How many redirects to follow, `0` follows none, maximum 100                                            |
+
+Unknown options and values of the wrong type raise an error.
+
+**Response:**
+
+| Field / Method | Description                                                        |
+| -------------- | ------------------------------------------------------------------ |
+| `status`       | Status code (number)                                               |
+| `headers`      | Response headers, each key holding an array of values              |
+| `body`         | Response body (string)                                             |
+| `header(name)` | First value of a header, case-insensitive; `nil`/`null` if missing |
+| `cookie(name)` | Value of a cookie from `Set-Cookie`; `nil`/`null` if missing       |
+| `json()`       | JavaScript only: parses the body as JSON                           |
+| `bytes()`      | JavaScript only: the body as a `Uint8Array`                        |
+
+In Lua, call the helpers as `res:header("name")` or `res.header("name")`.
+
+> **Note:** JavaScript strings hold UTF-16, so `body` replaces bytes that are not valid UTF-8 and `body.length` counts code units rather than bytes. Use `bytes()` for binary responses and for the real byte count. Lua strings keep the bytes as they arrived.
+
+**Behavior:**
+
+- **Runs on every request:** the requests are sent each time `transform` runs, so every main request triggers them. Script globals persist across requests within a worker, so a script can cache a value itself (e.g. `if not token then token = login() end`).
+- **Same proxy:** when proxies are configured, a script's requests go through the same proxy as the main request they belong to.
+- **Status codes:** non-2xx responses are returned normally; check `status` in the script.
+- **Errors:** network failures and timeouts raise a script error. The main request is not sent and the error is counted in the results. Scripts can catch it with `pcall` (Lua) or `try`/`catch` (JavaScript).
+- **Response bodies:** the [Max Response Body](#max-response-body) config bounds these requests too, and a body above it raises an error the script can catch.
+- **Compressed responses:** sarin asks for no encoding, so this only applies when a script sends `Accept-Encoding` itself or the server compresses unasked. A `gzip`, `deflate`, `br` or `zstd` body is then decoded before the script sees it, and `Content-Encoding` and `Content-Length` are both dropped from `headers` because neither describes what the script receives. Anything else is left raw with its header intact: an empty body, `identity`, a `Content-Encoding` sarin does not recognise, and two or more codings at once. A body that fails to decode raises an error the script can catch, though `br` carries no checksum, so damage to a brotli body can go unnoticed.
+- **Only inside `transform`:** `http` works anywhere while `transform` runs, including in helper functions it calls. Calling it at the top level of a script raises an error, because that code also runs during config validation and when each worker starts.
+- **Dry run:** script requests are still sent in dry-run mode.
+- **Not measured:** script requests are not included in the results.
+
+**Lua JSON:**
+
+Lua has no built-in JSON support, so Lua scripts get a global `json` table:
+
+- `json.decode(string)`: arrays become 1-indexed tables and `null` becomes `nil` (so `null` array items leave holes).
+- `json.encode(value)`: a table with keys exactly `1..n` becomes an array, any other table becomes an object, and an empty table becomes `{}`.
+
+JavaScript scripts use the built-in `JSON` object.
+
+**Lua example:**
+
+```lua
+function transform(req)
+    local res = http("https://api.example.com/login", {
+        method = "POST",
+        headers = { ["Content-Type"] = "application/json" },
+        body = json.encode({ user = "test", password = "secret" }),
+        timeout = "2s",
+    })
+    if res.status ~= 200 then
+        error("login failed: " .. res.status)
+    end
+
+    req.headers["Authorization"] = "Bearer " .. json.decode(res.body).token
+    return req
+end
+```
+
+**JavaScript example:**
+
+```javascript
+function transform(req) {
+    const res = http("https://api.example.com/login", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ user: "test", password: "secret" }),
+        timeout: "2s",
+    });
+    if (res.status !== 200) {
+        throw new Error("login failed: " + res.status);
+    }
+
+    req.headers["Authorization"] = "Bearer " + res.json().token;
+    return req;
+}
+```
@@ -857,7 +857,7 @@ progress: none

 ## Runtime Logging

-`--log-level` selects which runtime logs Sarin emits (comma-separated `info` and `error`, default `error`). `error` covers request and generation errors, `info` covers every completed response (status, duration, headers, body). Logs appear in the progress log box on an interactive terminal, go to stderr when piped, or go to a file with `--log-file`.
+`--log-level` selects which runtime logs Sarin emits (comma-separated `info` and `error`, default `error`). `error` covers errors raised while generating a request, `info` covers every completed response (status, duration, headers, body). A request that fails to send is counted in the results rather than logged. Logs appear in the progress log box on an interactive terminal, go to stderr when piped, or go to a file with `--log-file`.

 **Log responses and errors:**

@@ -883,9 +883,8 @@ sarin -U http://example.com -r 1000 -l info -o json > stats.json 2> run.log
 ```yaml
 url: http://example.com
 requests: 1000
-concurrency: 10
-logLevel: info,error
-logFile: ./run.log
+logLevel: info
+output: json
 ```

 </details>
@@ -1087,3 +1086,139 @@ js: "@./scripts/body.js"
 ```

 </details>
+
+**Fetch a value from another endpoint before each request:**
+
+Scripts can send their own requests with `http`. Here each request first fetches a CSRF token and sends it in a header and cookie. See [HTTP Requests in Scripts](configuration.md#http-requests-in-scripts) for all options.
+
+```sh
+sarin -U http://example.com/api/orders -r 1000 -c 10 \
+  -M POST \
+  -lua 'function transform(req)
+    local res = http("http://example.com/csrf")
+    req.headers["X-CSRF-Token"] = json.decode(res.body).token
+    req.cookies["session"] = res:cookie("session")
+    return req
+  end'
+```
+
+<details>
+<summary>YAML equivalent</summary>
+
+```yaml
+url: http://example.com/api/orders
+requests: 1000
+concurrency: 10
+method: POST
+lua: |
+    function transform(req)
+        local res = http("http://example.com/csrf")
+        req.headers["X-CSRF-Token"] = json.decode(res.body).token
+        req.cookies["session"] = res:cookie("session")
+        return req
+    end
+```
+
+</details>
+
+**Log in once per worker and reuse the token (JavaScript):**
+
+```yaml
+url: http://example.com/api/profile
+requests: 1000
+concurrency: 10
+js: |
+    let token;
+
+    function login() {
+        const res = http("http://example.com/login", {
+            method: "POST",
+            headers: { "Content-Type": "application/json" },
+            body: JSON.stringify({ user: "test", password: "secret" }),
+        });
+        if (res.status !== 200) throw new Error("login failed: " + res.status);
+        return res.json().token;
+    }
+
+    function transform(req) {
+        if (!token) token = login();
+        req.headers["Authorization"] = "Bearer " + token;
+        return req;
+    }
+```
+
+**Chain two calls, then use the result in the URL path (Lua):**
+
+```yaml
+url: http://example.com/api/placeholder
+requests: 500
+concurrency: 10
+lua: |
+    function transform(req)
+        local token = json.decode(http("http://example.com/login", {
+            method = "POST",
+            body = json.encode({ user = "test", password = "secret" }),
+            headers = { ["Content-Type"] = "application/json" },
+        }).body).token
+
+        local orders = json.decode(http("http://example.com/api/orders", {
+            headers = { Authorization = "Bearer " .. token },
+        }).body)
+
+        req.path = "/api/orders/" .. orders[1].id
+        req.headers["Authorization"] = "Bearer " .. token
+        return req
+    end
+```
+
+**Refresh a token only when it expires (JavaScript):**
+
+```yaml
+url: http://example.com/api/profile
+requests: 5000
+concurrency: 20
+js: |
+    let token;
+    let expiresAt = 0;
+
+    function transform(req) {
+        if (Date.now() >= expiresAt) {
+            const res = http("http://example.com/login", { method: "POST", body: "{}" });
+            token = res.json().token;
+            expiresAt = Date.now() + 60000; // refresh once a minute per worker
+        }
+        req.headers["Authorization"] = "Bearer " + token;
+        return req;
+    }
+```
+
+**Keep sending the main request even when the extra call fails:**
+
+A failed `http` call raises an error, which skips the main request and records the failure in the results. Catch it to decide yourself:
+
+```sh
+sarin -U http://example.com/api/items -r 1000 -c 10 \
+  -lua 'function transform(req)
+    local ok, res = pcall(http, "http://example.com/api/flags")
+    if ok and res.status == 200 then
+      req.headers["X-Flags"] = res.body
+    else
+      req.headers["X-Flags"] = "default"
+    end
+    return req
+  end'
+```
+
+The JavaScript equivalent:
+
+```sh
+sarin -U http://example.com/api/items -r 1000 -c 10 \
+  -js 'function transform(req) {
+    try {
+      req.headers["X-Flags"] = http("http://example.com/api/flags").body;
+    } catch (e) {
+      req.headers["X-Flags"] = "default";
+    }
+    return req;
+  }'
+```
@@ -2,9 +2,9 @@

 Sarin supports Go templates in URL paths, methods, bodies, headers, params, cookies, and values.

-> **Note:** Templating in URL host and scheme is not supported. Only the path portion of the URL can contain templates.
+> **Note:** Templating in URL host and scheme is not supported. Only the path and query string of the URL can contain templates.

-> **Note:** Template rendering happens before the request is sent. The request timeout (`-T` / `timeout`) only governs the HTTP request itself and starts _after_ templates have finished rendering, so slow template functions (e.g. captcha solvers, remote `file_Read`) cannot cause a request timeout no matter how long they take.
+> **Note:** Template rendering happens before the request is sent. The request timeout (`-T` / `timeout`) only governs the HTTP request itself and starts _after_ templates have finished rendering, so slow template functions (e.g. captcha solvers, remote `file_Read`) cannot cause a request timeout no matter how long they take. Downloads have their own limits: 30 seconds and 64MiB per file.

 ## Table of Contents

@@ -29,6 +29,7 @@ Sarin supports Go templates in URL paths, methods, bodies, headers, params, cook
    - [Generate](#generate)
    - [Auth](#auth)
    - [Address](#address)
+    - [Airline](#airline)
    - [Game](#game)
    - [Beer](#beer)
    - [Car](#car)
@@ -151,7 +152,7 @@ body: '{{ json_Encode (dict_Str "key1" "value1" "key2" "value2") }}'
 | ------------------------ | ------------------------------------------- | ------------------------------------------------------------------------------- |
 | `time_NowUnix`           | Current Unix timestamp (seconds)            | `{{ time_NowUnix }}` → `1735689600`                                             |
 | `time_NowUnixMilli`      | Current Unix timestamp (milliseconds)       | `{{ time_NowUnixMilli }}` → `1735689600123`                                     |
-| `time_NowRFC3339`        | Current time in RFC3339 format              | `{{ time_NowRFC3339 }}` → `"2026-02-26T21:00:00Z"`                              |
+| `time_NowRFC3339`        | Current time in RFC3339 format              | `{{ time_NowRFC3339 }}` → `"2026-02-26T21:00:00+04:00"` (local time)            |
 | `time_Format(layout, t)` | Format a `time.Time` value with a Go layout | `{{ time_Format "2006-01-02" (strings_ToDate "2024-05-10") }}` → `"2024-05-10"` |

 ### Crypto Functions
@@ -165,6 +166,8 @@ body: '{{ json_Encode (dict_Str "key1" "value1" "key2" "value2") }}'

 ### Body Functions

+> **Note:** These are available in `body` only, not in the URL, headers, params, cookies or values.
+
 | Function                         | Description                                                                                                                                                                                                 | Example                                                             |
 | -------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------- |
 | `body_FormData(pairs ...string)` | Create multipart form data from key-value pairs. Automatically sets the `Content-Type` header. Values starting with `@` are treated as file references (local path or URL). Use `@@` to escape literal `@`. | `{{ body_FormData "field1" "value1" "file" "@/path/to/file.pdf" }}` |
@@ -387,7 +390,7 @@ These functions are powered by [gofakeit](https://github.com/brianvoe/gofakeit)
 | Function                                                                                      | Description                                                 | Example                                               |
 | --------------------------------------------------------------------------------------------- | ----------------------------------------------------------- | ----------------------------------------------------- |
 | `fakeit_Username`                                                                             | Username                                                    | `"Daniel1364"`                                        |
-| `fakeit_Password(upper bool, lower bool, numeric bool, special bool, space bool, length int)` | Generate password with specified character types and length | `{{ fakeit_Password true true true false false 16 }}` |
+| `fakeit_Password(lower bool, upper bool, numeric bool, special bool, space bool, length int)` | Generate password with specified character types and length | `{{ fakeit_Password true true true false false 16 }}` |

 ### Address

@@ -521,7 +524,7 @@ These functions are powered by [gofakeit](https://github.com/brianvoe/gofakeit)
 | `fakeit_LoremIpsumSentence(wordCount int)`                                               | Lorem ipsum sentence with specified word count  | `{{ fakeit_LoremIpsumSentence 5 }}`           |
 | `fakeit_LoremIpsumParagraph(paragraphs int, sentences int, words int, separator string)` | Lorem ipsum paragraphs with specified structure | `{{ fakeit_LoremIpsumParagraph 1 3 5 "\n" }}` |
 | `fakeit_Question`                                                                        | Random question                                 | `"What is the marginal gain from fear?"`      |
-| `fakeit_Quote`                                                                           | Random quote                                    | `"Energy is contagious—so is fear"`           |
+| `fakeit_Quote`                                                                           | Random quote                                    | `"Energy is contagious so is fear"`           |
 | `fakeit_Phrase`                                                                          | Random phrase                                   | `"how many siblings do you have"`             |
 | `fakeit_PhraseNoun`                                                                      | Noun phrase                                     | `"a tribe"`                                   |
 | `fakeit_PhraseVerb`                                                                      | Verb phrase                                     | `"fully integrate a system"`                  |
@@ -9,7 +9,7 @@ require (
 	charm.land/lipgloss/v2 v2.0.6
 	github.com/brianvoe/gofakeit/v7 v7.17.1
 	github.com/charmbracelet/x/term v0.2.2
-	github.com/dop251/goja v0.0.0-20260911104922-fabc3b8078ad
+	github.com/dop251/goja v0.0.0-20261004174310-e21e22eac913
 	github.com/joho/godotenv v1.5.1
 	github.com/valyala/fasthttp v1.74.0
 	github.com/yuin/gopher-lua v1.1.2
@@ -23,28 +23,28 @@ require (
 	github.com/aymerick/douceur v0.2.0 // indirect
 	github.com/charmbracelet/colorprofile v0.4.3 // indirect
 	github.com/charmbracelet/harmonica v0.2.0 // indirect
-	github.com/charmbracelet/ultraviolet v0.0.0-20260910203606-6c9e17dc7a16 // indirect
+	github.com/charmbracelet/ultraviolet v0.0.0-20261001125412-878653296cfd // indirect
 	github.com/charmbracelet/x/ansi v0.11.8 // indirect
-	github.com/charmbracelet/x/exp/slice v0.0.0-20260906004030-3986e9119cf9 // indirect
+	github.com/charmbracelet/x/exp/slice v0.1.0 // indirect
 	github.com/charmbracelet/x/termios v0.1.1 // indirect
 	github.com/charmbracelet/x/windows v0.2.2 // indirect
 	github.com/clipperhouse/displaywidth v0.11.0 // indirect
 	github.com/clipperhouse/uax29/v2 v2.7.0 // indirect
 	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
-	github.com/dlclark/regexp2/v2 v2.8.0 // indirect
+	github.com/dlclark/regexp2/v2 v2.8.2 // indirect
 	github.com/go-sourcemap/sourcemap v2.1.4+incompatible // indirect
-	github.com/google/pprof v0.0.0-20260906184651-6331bc6350fe // indirect
+	github.com/google/pprof v0.0.0-20261003200830-ebaad5f31b4d // indirect
 	github.com/gorilla/css v1.0.1 // indirect
-	github.com/klauspost/compress v1.20.0 // indirect
+	github.com/klauspost/compress v1.20.1 // indirect
 	github.com/lucasb-eyer/go-colorful v1.4.1 // indirect
 	github.com/mattn/go-runewidth v0.0.30 // indirect
 	github.com/microcosm-cc/bluemonday v1.0.27 // indirect
-	github.com/molecule-man/go-brrr v1.1.0 // indirect
+	github.com/molecule-man/go-brrr v1.2.0 // indirect
 	github.com/muesli/cancelreader v0.2.2 // indirect
 	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
-	github.com/xo/terminfo v1.0.0 // indirect
+	github.com/xo/terminfo v1.2.0 // indirect
 	github.com/yuin/goldmark v1.8.6 // indirect
 	github.com/yuin/goldmark-emoji v1.0.6 // indirect
 	golang.org/x/sync v0.23.0 // indirect
@@ -24,14 +24,14 @@ github.com/charmbracelet/colorprofile v0.4.3 h1:QPa1IWkYI+AOB+fE+mg/5/4HRMZcaXex
 github.com/charmbracelet/colorprofile v0.4.3/go.mod h1:/zT4BhpD5aGFpqQQqw7a+VtHCzu+zrQtt1zhMt9mR4Q=
 github.com/charmbracelet/harmonica v0.2.0 h1:8NxJWRWg/bzKqqEaaeFNipOu77YR5t8aSwG4pgaUBiQ=
 github.com/charmbracelet/harmonica v0.2.0/go.mod h1:KSri/1RMQOZLbw7AHqgcBycp8pgJnQMYYT8QZRqZ1Ao=
-github.com/charmbracelet/ultraviolet v0.0.0-20260910203606-6c9e17dc7a16 h1:/hSCZO1puetbGd2JikHKN0nAcb2mFevzxJElFoduhaw=
-github.com/charmbracelet/ultraviolet v0.0.0-20260910203606-6c9e17dc7a16/go.mod h1:JFvmQPZh3uAu7PpSZU91bcR0BrIy24HhW+0EsW1uGxE=
+github.com/charmbracelet/ultraviolet v0.0.0-20261001125412-878653296cfd h1:W0HOPGkwRaAmV20plE/f+Rk6Xn+IdR8G0qHsGXcOnF8=
+github.com/charmbracelet/ultraviolet v0.0.0-20261001125412-878653296cfd/go.mod h1:D9SVXVpAsD1pP6xR6BiERbe4+dUU6EW9US1TFrNutGA=
 github.com/charmbracelet/x/ansi v0.11.8 h1:JMFwp0CgDC2+jcOB162HH5k7I3FVbgFSMMYg7dSPBQQ=
 github.com/charmbracelet/x/ansi v0.11.8/go.mod h1:ZNN+3mXny/516oTQPLMPIBeSINvNJJQ8uQXDgbeJxY0=
 github.com/charmbracelet/x/exp/golden v0.0.0-20250806222409-83e3a29d542f h1:pk6gmGpCE7F3FcjaOEKYriCvpmIN4+6OS/RD0vm4uIA=
 github.com/charmbracelet/x/exp/golden v0.0.0-20250806222409-83e3a29d542f/go.mod h1:IfZAMTHB6XkZSeXUqriemErjAWCCzT0LwjKFYCZyw0I=
-github.com/charmbracelet/x/exp/slice v0.0.0-20260906004030-3986e9119cf9 h1:zW96vcZ3zDetccerbN7sS/sCydOE/l/9S+C94pozsaU=
-github.com/charmbracelet/x/exp/slice v0.0.0-20260906004030-3986e9119cf9/go.mod h1:vqEfX6xzqW1pKKZUUiFOKg0OQ7bCh54Q2vR/tserrRA=
+github.com/charmbracelet/x/exp/slice v0.1.0 h1:m46dwuDf3M5SFYMRhxYkLC6z9WnENGVKbcbJlp13/og=
+github.com/charmbracelet/x/exp/slice v0.1.0/go.mod h1:vqEfX6xzqW1pKKZUUiFOKg0OQ7bCh54Q2vR/tserrRA=
 github.com/charmbracelet/x/term v0.2.2 h1:xVRT/S2ZcKdhhOuSP4t5cLi5o+JxklsoEObBSgfgZRk=
 github.com/charmbracelet/x/term v0.2.2/go.mod h1:kF8CY5RddLWrsgVwpw4kAa6TESp6EB5y3uxGLeCqzAI=
 github.com/charmbracelet/x/termios v0.1.1 h1:o3Q2bT8eqzGnGPOYheoYS8eEleT5ZVNYNy8JawjaNZY=
@@ -44,32 +44,32 @@ github.com/clipperhouse/uax29/v2 v2.7.0 h1:+gs4oBZ2gPfVrKPthwbMzWZDaAFPGYK72F0NJ
 github.com/clipperhouse/uax29/v2 v2.7.0/go.mod h1:EFJ2TJMRUaplDxHKj1qAEhCtQPW2tJSwu5BF98AuoVM=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/dlclark/regexp2/v2 v2.8.0 h1:CekDhPLGfm+GAJmPVFIG+5dqMIQPkyHJll7BbdneDfw=
-github.com/dlclark/regexp2/v2 v2.8.0/go.mod h1:avUrQvPaLz2DrFNHJF0taWAFFX2C1GMSSoeiqFjcBmU=
-github.com/dop251/goja v0.0.0-20260911104922-fabc3b8078ad h1:5gT9hQ9h8CplLZfAGgFMcoLqQS6I9T2sZwQOCXy8/3s=
-github.com/dop251/goja v0.0.0-20260911104922-fabc3b8078ad/go.mod h1:u8yZRUavu+N4EnFFy6J5fVtjE7lEcZ2YyV2GcBXY9c8=
+github.com/dlclark/regexp2/v2 v2.8.2 h1:uTTe+cxADL4be+dv4DZwsBIEPCQ6RHPlnVuKrtLqSx0=
+github.com/dlclark/regexp2/v2 v2.8.2/go.mod h1:avUrQvPaLz2DrFNHJF0taWAFFX2C1GMSSoeiqFjcBmU=
+github.com/dop251/goja v0.0.0-20261004174310-e21e22eac913 h1:0haZHQSJqA4qFZvnKIRgekXrPo2sQ2oRvFemiNHhcp0=
+github.com/dop251/goja v0.0.0-20261004174310-e21e22eac913/go.mod h1:GrCNKUrVplXZj0juFue0Cj2Xt/eL2Nuu2+y3UTanEo8=
 github.com/go-sourcemap/sourcemap v2.1.4+incompatible h1:a+iTbH5auLKxaNwQFg0B+TCYl6lbukKPc7b5x0n1s6Q=
 github.com/go-sourcemap/sourcemap v2.1.4+incompatible/go.mod h1:F8jJfvm2KbVjc5NqelyYJmf/v5J0dwNLS2mL4sNA1Jg=
 github.com/goccy/go-yaml v1.19.2 h1:PmFC1S6h8ljIz6gMRBopkjP1TVT7xuwrButHID66PoM=
 github.com/goccy/go-yaml v1.19.2/go.mod h1:XBurs7gK8ATbW4ZPGKgcbrY1Br56PdM69F7LkFRi1kA=
-github.com/google/pprof v0.0.0-20260906184651-6331bc6350fe h1:QAinXoAFJdGQYztXn3VpFey7KCwpedbZ/EkzbplQ0cY=
-github.com/google/pprof v0.0.0-20260906184651-6331bc6350fe/go.mod h1:jl5iWTm0/hd5PjEYEOuwAJ57L/CibdZfrqZ5XA5GrCk=
+github.com/google/pprof v0.0.0-20261003200830-ebaad5f31b4d h1:kZK3FQM86gr3cgJHMP7rPn8A3Z75mt+ZaVBoqMlpPF0=
+github.com/google/pprof v0.0.0-20261003200830-ebaad5f31b4d/go.mod h1:jl5iWTm0/hd5PjEYEOuwAJ57L/CibdZfrqZ5XA5GrCk=
 github.com/gorilla/css v1.0.1 h1:ntNaBIghp6JmvWnxbZKANoLyuXTPZ4cAMlo6RyhlbO8=
 github.com/gorilla/css v1.0.1/go.mod h1:BvnYkspnSzMmwRK+b8/xgNPLiIuNZr6vbZBTPQ2A3b0=
 github.com/hexops/gotextdiff v1.0.3 h1:gitA9+qJrrTCsiCl7+kh75nPqQt1cx4ZkudSTLoUqJM=
 github.com/hexops/gotextdiff v1.0.3/go.mod h1:pSWU5MAI3yDq+fZBTazCSJysOMbxWL1BSow5/V2vxeg=
 github.com/joho/godotenv v1.5.1 h1:7eLL/+HRGLY0ldzfGMeQkb7vMd0as4CfYvUVzLqw0N0=
 github.com/joho/godotenv v1.5.1/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
-github.com/klauspost/compress v1.20.0 h1:a3C1ke2ohxFymNlb2HWAHjDeKCI90scRskErZkR0ezA=
-github.com/klauspost/compress v1.20.0/go.mod h1:LUdAzn7YLVvxLpc7y3V1m40wESHTgc1422pwwBSKYuI=
+github.com/klauspost/compress v1.20.1 h1:T7kKElXUMXrUJ2E9QhQhxFtcK5rPyLdsGZvdbLMPdiQ=
+github.com/klauspost/compress v1.20.1/go.mod h1:LUdAzn7YLVvxLpc7y3V1m40wESHTgc1422pwwBSKYuI=
 github.com/lucasb-eyer/go-colorful v1.4.1 h1:1EO+WB73+EH8EVbzlrG3KLAfEypQWVHIBqlTf+2hNss=
 github.com/lucasb-eyer/go-colorful v1.4.1/go.mod h1:R4dSotOR9KMtayYi1e77YzuveK+i7ruzyGqttikkLy0=
 github.com/mattn/go-runewidth v0.0.30 h1:+KUuiDA4fF0R1p5FeueHefjDm+GIM+kWfFnDjybOPgk=
 github.com/mattn/go-runewidth v0.0.30/go.mod h1:3qAiGCV4Koz/yuveO58qUefmUTRm8r0IGEXZ9jeHp/8=
 github.com/microcosm-cc/bluemonday v1.0.27 h1:MpEUotklkwCSLeH+Qdx1VJgNqLlpY2KXwXFM08ygZfk=
 github.com/microcosm-cc/bluemonday v1.0.27/go.mod h1:jFi9vgW+H7c3V0lb6nR74Ib/DIB5OBs92Dimizgw2cA=
-github.com/molecule-man/go-brrr v1.1.0 h1:rLBGSXA393y+5ttCBLkW34CHESop5tahrOFxVv/XDEU=
-github.com/molecule-man/go-brrr v1.1.0/go.mod h1:7ybW6/7gA3oKY45jOfVNjSJDtrr6ea4tzbsTkjmQDC4=
+github.com/molecule-man/go-brrr v1.2.0 h1:dOJU45BC3Gc2WXoxAW0rgCAgVuu2gruAOGncfieA0Jw=
+github.com/molecule-man/go-brrr v1.2.0/go.mod h1:7ybW6/7gA3oKY45jOfVNjSJDtrr6ea4tzbsTkjmQDC4=
 github.com/muesli/cancelreader v0.2.2 h1:3I4Kt4BQjOR54NavqnDogx/MIoWBFa0StPA8ELUXHmA=
 github.com/muesli/cancelreader v0.2.2/go.mod h1:3XuTXfFS2VjM+HTLZY9Ak0l6eUKfijIfMUZ4EgX0QYo=
 github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 h1:Jamvg5psRIccs7FGNTlIRMkT8wgtp5eCXdBlqhYGL6U=
@@ -82,8 +82,8 @@ github.com/valyala/bytebufferpool v1.0.0 h1:GqA5TC/0021Y/b9FG4Oi9Mr3q7XYx6Kllzaw
 github.com/valyala/bytebufferpool v1.0.0/go.mod h1:6bBcMArwyJ5K/AmCkWv1jt77kVWyCJ6HpOuEn7z0Csc=
 github.com/valyala/fasthttp v1.74.0 h1:wMS9fnO2QTALozYx5pId2Vi7ZwU/epUkY8i/KPWCHoU=
 github.com/valyala/fasthttp v1.74.0/go.mod h1:3ARmLamUcw7ElxVtC8PXaGzQ6VEuvnetlkrwIklQBSE=
-github.com/xo/terminfo v1.0.0 h1:2ZpYzqWzyyytjk3TP6aJVDhkMAkc99/1xKQdA3TDTBY=
-github.com/xo/terminfo v1.0.0/go.mod h1:RbqR21r5mrJuqunuUZ/Dhy/avygyECGrLceyNeo4LiM=
+github.com/xo/terminfo v1.2.0 h1:d0ZTOCpuGE0lwSAOs0zcJjwz3jWQyqcRt9XbGJpCOl4=
+github.com/xo/terminfo v1.2.0/go.mod h1:lGzkSo8Fe7IRh/w+Gqz7n5mDog4FVXCj3gy/DYTBqio=
 github.com/yuin/goldmark v1.8.6 h1:d0VcaP1sx9GkFVkoW+KtggpGi2KZ965i14b0+bDQST4=
 github.com/yuin/goldmark v1.8.6/go.mod h1:ip/1k0VRfGynBgxOz0yCqHrbZXhcjxyuS66Brc7iBKg=
 github.com/yuin/goldmark-emoji v1.0.6 h1:QWfF2FYaXwL74tfGOW5izeiZepUDroDJfWubQI9HTHs=
@@ -94,8 +94,6 @@ go.aykhans.me/utils v1.0.7 h1:ClHXHlWmkjfFlD7+w5BQY29lKCEztxY/yCf543x4hZw=
 go.aykhans.me/utils v1.0.7/go.mod h1:0Jz8GlZLN35cCHLOLx39sazWwEe33bF6SYlSeqzEXoI=
 go.yaml.in/yaml/v4 v4.0.0-rc.6 h1:1h7H1ohdUh93/FyE4YaDa1Zh64K6VVbjF4K6WUxMtH4=
 go.yaml.in/yaml/v4 v4.0.0-rc.6/go.mod h1:aZqd9kCMsGL7AuUv/m/PvWLdg5sjJsZ4oHDEnfPPfY0=
-golang.org/x/exp v0.0.0-20231006140011-7918f672742d h1:jtJma62tbqLibJ5sFQz8bKtEM8rJBtfilJ2qTU199MI=
-golang.org/x/exp v0.0.0-20231006140011-7918f672742d/go.mod h1:ldy0pHrwJyGW56pPQzzkH36rKxoZW1tw7ZJpeKx+hdo=
 golang.org/x/net v0.59.0 h1:5zfYln+w5XCxwrnMMJPufRgNoXEaGxl0wo5GqPXyues=
 golang.org/x/net v0.59.0/go.mod h1:2DA/G1UfVbCpQPeWTmMPGY7Cs2PkBkwu743bVX5PIVg=
 golang.org/x/sync v0.23.0 h1:KameEIfc1IkluZyXWLn39Wd4tURc6GbCiISGiZm2bQk=
@@ -0,0 +1,40 @@
+package config
+
+import (
+	"flag"
+	"fmt"
+
+	"go.aykhans.me/sarin/internal/types"
+	"go.yaml.in/yaml/v4"
+)
+
+var (
+	_ flag.Value       = (*byteSize)(nil)
+	_ yaml.Unmarshaler = (*byteSize)(nil)
+)
+
+// byteSize is a size in bytes that reads and prints unit suffixes such as 10MiB.
+type byteSize uint64
+
+func (size byteSize) String() string {
+	return types.FormatByteSize(uint64(size))
+}
+
+// Set fills the size from a command line value.
+func (size *byteSize) Set(value string) error {
+	parsed, err := types.ParseByteSize(value)
+	if err != nil {
+		return err
+	}
+	*size = byteSize(parsed)
+	return nil
+}
+
+// UnmarshalYAML reads the raw scalar so a size parses the same way here as on the command line.
+func (size *byteSize) UnmarshalYAML(node *yaml.Node) error {
+	if node.Kind != yaml.ScalarNode {
+		return fmt.Errorf("expected a size, but got %s", yamlNodeName(node))
+	}
+
+	return size.Set(node.Value)
+}
@@ -20,32 +20,33 @@ Simple usage:

 Flags:
  General Config:
-    -h, -help                     Help for sarin
-    -v, -version                  Version for sarin
-    -s, -show-config   bool       Show the final config after parsing all sources (default %v)
-    -f, -config-file   string     Path to the config file (local file / http URL)
-    -c, -concurrency   uint       Number of concurrent requests (default %d)
-    -r, -requests      uint       Number of total requests
-    -d, -duration      time       Maximum duration for the test (e.g. 30s, 1m, 5h)
-    -l, -log-level     string     Runtime log levels to emit, comma-separated (possible values: info, error) (default %s)
-    -w, -log-file      string     Write runtime logs to this file instead of the terminal/stderr
-    -p, -progress      string     Progress display (possible values: bar, none) (default '%v')
-    -o, -output        string     Output format (possible values: table, json, yaml, none) (default '%v')
-    -z, -dry-run       bool       Run without sending requests (default %v)
+    -h, -help                         Help for sarin
+    -v, -version                      Version for sarin
+    -s, -show-config       bool       Show the final config after parsing all sources (default %v)
+    -f, -config-file       []string   Path to the config file (local file / http URL)
+    -c, -concurrency       uint       Number of concurrent requests (default %d)
+    -r, -requests          uint       Number of total requests
+    -d, -duration          time       Maximum duration for the test (e.g. 30s, 1m, 5h)
+    -l, -log-level         string     Runtime log levels to emit, comma-separated (possible values: info, error) (default %s)
+    -w, -log-file          string     Write runtime logs to this file instead of the terminal/stderr
+    -p, -progress          string     Progress display (possible values: bar, none) (default '%v')
+    -o, -output            string     Output format (possible values: table, json, yaml, none) (default '%v')
+    -z, -dry-run           bool       Run without sending the generated requests (default %v)

  Request Config:
-    -U, -url           string     Target URL for the request
-    -M, -method        []string   HTTP method for the request (default %s)
-    -B, -body          []string   Body for the request (e.g. "body text")
-    -P, -param         []string   URL parameter for the request (e.g. "key1=value1")
-    -H, -header        []string   Header for the request (e.g. "key1: value1")
-    -C, -cookie        []string   Cookie for the request (e.g. "key1=value1")
-    -X, -proxy         []string   Proxy for the request (e.g. "http://proxy.example.com:8080")
-    -V, -values        []string   List of values for templating (e.g. "key1=value1")
-    -T, -timeout       time       Timeout for the request (e.g. 400ms, 3s, 1m10s) (default %v)
-    -I, -insecure      bool       Skip SSL/TLS certificate verification (default %v)
-        -lua           []string   Lua script for request transformation (inline or @file/@url)
-        -js            []string   JavaScript script for request transformation (inline or @file/@url)`
+    -U, -url               string     Target URL for the request
+    -M, -method            []string   HTTP method for the request (default %s)
+    -B, -body              []string   Body for the request (e.g. "body text")
+    -P, -param             []string   URL parameter for the request (e.g. "key1=value1")
+    -H, -header            []string   Header for the request (e.g. "key1: value1")
+    -C, -cookie            []string   Cookie for the request (e.g. "key1=value1")
+    -X, -proxy             []string   Proxy for the request (e.g. "http://proxy.example.com:8080")
+    -V, -values            []string   List of values for templating (e.g. "key1=value1")
+    -T, -timeout           time       Timeout for the request (e.g. 400ms, 3s, 1m10s) (default %v)
+    -S, -max-response-body size       Largest response body to read (e.g. 10MiB, 2MB), 0 for no limit (default %s)
+    -I, -insecure          bool       Skip SSL/TLS certificate verification (default %v)
+        -lua               []string   Lua script for request transformation (inline or @file/@url)
+        -js                []string   JavaScript script for request transformation (inline or @file/@url)`

 var _ IParser = ConfigCLIParser{}

@@ -97,18 +98,19 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 		dryRun       bool

 		// Request config
-		urlInput   string
-		methods    = stringSliceArg{}
-		bodies     = stringSliceArg{}
-		params     = stringSliceArg{}
-		headers    = stringSliceArg{}
-		cookies    = stringSliceArg{}
-		proxies    = stringSliceArg{}
-		values     = stringSliceArg{}
-		timeout    time.Duration
-		insecure   bool
-		luaScripts = stringSliceArg{}
-		jsScripts  = stringSliceArg{}
+		urlInput        string
+		methods         = stringSliceArg{}
+		bodies          = stringSliceArg{}
+		params          = stringSliceArg{}
+		headers         = stringSliceArg{}
+		cookies         = stringSliceArg{}
+		proxies         = stringSliceArg{}
+		values          = stringSliceArg{}
+		timeout         time.Duration
+		maxResponseBody byteSize
+		insecure        bool
+		luaScripts      = stringSliceArg{}
+		jsScripts       = stringSliceArg{}
 	)

 	{
@@ -143,8 +145,8 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 		flagSet.StringVar(&output, "output", "", "Output format (possible values: table, json, yaml, none)")
 		flagSet.StringVar(&output, "o", "", "Output format (possible values: table, json, yaml, none)")

-		flagSet.BoolVar(&dryRun, "dry-run", false, "Run without sending requests")
-		flagSet.BoolVar(&dryRun, "z", false, "Run without sending requests")
+		flagSet.BoolVar(&dryRun, "dry-run", false, "Run without sending the generated requests")
+		flagSet.BoolVar(&dryRun, "z", false, "Run without sending the generated requests")

 		// Request config
 		flagSet.StringVar(&urlInput, "url", "", "Target URL for the request")
@@ -174,6 +176,9 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 		flagSet.DurationVar(&timeout, "timeout", 0, "Timeout for the request (e.g. 400ms, 15s, 1m10s)")
 		flagSet.DurationVar(&timeout, "T", 0, "Timeout for the request (e.g. 400ms, 15s, 1m10s)")

+		flagSet.Var(&maxResponseBody, "max-response-body", "Largest response body to read (e.g. 1048576, 10MiB, 2MB), 0 for no limit")
+		flagSet.Var(&maxResponseBody, "S", "Largest response body to read (e.g. 1048576, 10MiB, 2MB), 0 for no limit")
+
 		flagSet.BoolVar(&insecure, "insecure", false, "Skip SSL/TLS certificate verification")
 		flagSet.BoolVar(&insecure, "I", false, "Skip SSL/TLS certificate verification")

@@ -258,6 +263,8 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 			config.Values = append(config.Values, values...)
 		case "timeout", "T":
 			config.Timeout = new(timeout)
+		case "max-response-body", "S":
+			config.MaxResponseBody = new(uint64(maxResponseBody))
 		case "insecure", "I":
 			config.Insecure = new(insecure)
 		case "lua":
@@ -286,6 +293,7 @@ func (parser ConfigCLIParser) PrintHelp() {

 		Defaults.Method,
 		Defaults.RequestTimeout,
+		types.FormatByteSize(Defaults.MaxResponseBody),
 		Defaults.Insecure,
 	)
 }
@@ -27,27 +27,29 @@ import (
 )

 var Defaults = struct {
-	UserAgent      string
-	Method         string
-	RequestTimeout time.Duration
-	Concurrency    uint
-	ShowConfig     bool
-	Progress       ConfigProgressType
-	Insecure       bool
-	Output         ConfigOutputType
-	DryRun         bool
-	LogLevel       string
+	UserAgent       string
+	Method          string
+	RequestTimeout  time.Duration
+	MaxResponseBody uint64
+	Concurrency     uint
+	ShowConfig      bool
+	Progress        ConfigProgressType
+	Insecure        bool
+	Output          ConfigOutputType
+	DryRun          bool
+	LogLevel        string
 }{
-	UserAgent:      "Sarin/" + version.Version,
-	Method:         "GET",
-	RequestTimeout: time.Second * 10,
-	Concurrency:    1,
-	ShowConfig:     false,
-	Progress:       ConfigProgressTypeBar,
-	Insecure:       false,
-	Output:         ConfigOutputTypeTable,
-	DryRun:         false,
-	LogLevel:       "error",
+	UserAgent:       "Sarin/" + version.Version,
+	Method:          "GET",
+	RequestTimeout:  time.Second * 10,
+	MaxResponseBody: 10 << 20, // 10 MiB
+	Concurrency:     1,
+	ShowConfig:      false,
+	Progress:        ConfigProgressTypeBar,
+	Insecure:        false,
+	Output:          ConfigOutputTypeTable,
+	DryRun:          false,
+	LogLevel:        "error",
 }

 var (
@@ -82,28 +84,29 @@ var (
 )

 type Config struct {
-	ShowConfig  *bool               `yaml:"showConfig,omitempty"`
-	Files       []types.ConfigFile  `yaml:"files,omitempty"`
-	Methods     []string            `yaml:"methods,omitempty"`
-	URL         *url.URL            `yaml:"url,omitempty"`
-	Timeout     *time.Duration      `yaml:"timeout,omitempty"`
-	Concurrency *uint               `yaml:"concurrency,omitempty"`
-	Requests    *uint64             `yaml:"requests,omitempty"`
-	Duration    *time.Duration      `yaml:"duration,omitempty"`
-	Progress    *ConfigProgressType `yaml:"progress,omitempty"`
-	Output      *ConfigOutputType   `yaml:"output,omitempty"`
-	Insecure    *bool               `yaml:"insecure,omitempty"`
-	DryRun      *bool               `yaml:"dryRun,omitempty"`
-	Params      types.Params        `yaml:"params,omitempty"`
-	Headers     types.Headers       `yaml:"headers,omitempty"`
-	Cookies     types.Cookies       `yaml:"cookies,omitempty"`
-	Bodies      []string            `yaml:"bodies,omitempty"`
-	Proxies     types.Proxies       `yaml:"proxies,omitempty"`
-	Values      []string            `yaml:"values,omitempty"`
-	Lua         []string            `yaml:"lua,omitempty"`
-	Js          []string            `yaml:"js,omitempty"`
-	LogLevel    *string             `yaml:"logLevel,omitempty"`
-	LogFile     *string             `yaml:"logFile,omitempty"`
+	ShowConfig      *bool               `yaml:"showConfig,omitempty"`
+	Files           []types.ConfigFile  `yaml:"files,omitempty"`
+	Methods         []string            `yaml:"methods,omitempty"`
+	URL             *url.URL            `yaml:"url,omitempty"`
+	Timeout         *time.Duration      `yaml:"timeout,omitempty"`
+	MaxResponseBody *uint64             `yaml:"maxResponseBody,omitempty"`
+	Concurrency     *uint               `yaml:"concurrency,omitempty"`
+	Requests        *uint64             `yaml:"requests,omitempty"`
+	Duration        *time.Duration      `yaml:"duration,omitempty"`
+	Progress        *ConfigProgressType `yaml:"progress,omitempty"`
+	Output          *ConfigOutputType   `yaml:"output,omitempty"`
+	Insecure        *bool               `yaml:"insecure,omitempty"`
+	DryRun          *bool               `yaml:"dryRun,omitempty"`
+	Params          types.Params        `yaml:"params,omitempty"`
+	Headers         types.Headers       `yaml:"headers,omitempty"`
+	Cookies         types.Cookies       `yaml:"cookies,omitempty"`
+	Bodies          []string            `yaml:"bodies,omitempty"`
+	Proxies         types.Proxies       `yaml:"proxies,omitempty"`
+	Values          []string            `yaml:"values,omitempty"`
+	Lua             []string            `yaml:"lua,omitempty"`
+	Js              []string            `yaml:"js,omitempty"`
+	LogLevel        *string             `yaml:"logLevel,omitempty"`
+	LogFile         *string             `yaml:"logFile,omitempty"`
 }

 func (config Config) MarshalYAML() (any, error) {
@@ -177,6 +180,9 @@ func (config Config) MarshalYAML() (any, error) {
 	if config.Timeout != nil {
 		addField(content, "timeout", toNode(*config.Timeout), "")
 	}
+	if config.MaxResponseBody != nil {
+		addField(content, "maxResponseBody", toNode(types.FormatByteSize(*config.MaxResponseBody)), "")
+	}
 	if config.Concurrency != nil {
 		addField(content, "concurrency", toNode(*config.Concurrency), "")
 	}
@@ -301,6 +307,9 @@ func (config *Config) Merge(newConfig *Config) {
 	if newConfig.Timeout != nil {
 		config.Timeout = newConfig.Timeout
 	}
+	if newConfig.MaxResponseBody != nil {
+		config.MaxResponseBody = newConfig.MaxResponseBody
+	}
 	if newConfig.Concurrency != nil {
 		config.Concurrency = newConfig.Concurrency
 	}
@@ -379,6 +388,9 @@ func (config *Config) SetDefaults() {
 	if config.Timeout == nil {
 		config.Timeout = &Defaults.RequestTimeout
 	}
+	if config.MaxResponseBody == nil {
+		config.MaxResponseBody = &Defaults.MaxResponseBody
+	}
 	if config.Concurrency == nil {
 		config.Concurrency = new(Defaults.Concurrency)
 	}
@@ -454,6 +466,14 @@ func (config Config) Validate() error {
 		validationErrors = append(validationErrors, types.NewFieldValidationError("Timeout", "0", errors.New("timeout must be greater than 0")))
 	}

+	if config.MaxResponseBody != nil && *config.MaxResponseBody > types.ByteSizeLimit {
+		validationErrors = append(validationErrors, types.NewFieldValidationError(
+			"MaxResponseBody",
+			types.FormatByteSize(*config.MaxResponseBody),
+			errors.New("max response body must not exceed "+types.FormatByteSize(types.ByteSizeLimit)),
+		))
+	}
+
 	if config.ShowConfig == nil {
 		validationErrors = append(validationErrors, types.NewFieldValidationError("ShowConfig", "", errors.New("showConfig field is required")))
 	}
@@ -632,12 +652,19 @@ func ReadAllConfigs() *Config {

 	for _, configFile := range append(envConfig.Files, cliConf.Files...) {
 		fileConfig, err := parseConfigFile(configFile, 10)
+
+		// A nested configFile fails under its own name, not the one given on the command line.
+		path := configFile.Path()
+		if configFileErr, ok := errors.AsType[types.ConfigFileError](err); ok {
+			path = configFileErr.Path
+		}
+
 		_ = utilsErr.MustHandle(err,
 			utilsErr.OnType(func(err types.ConfigFileReadError) error {
 				cliParser.PrintHelp()
 				fmt.Fprint(os.Stderr, lipgloss.Sprintln(
 					StyleYellow.Render(
-						fmt.Sprintf("\nFailed to read config file (%s): ", configFile.Path())+err.Error(),
+						fmt.Sprintf("\nFailed to read config file (%s): ", path)+err.Error(),
 					),
 				))
 				os.Exit(1)
@@ -646,14 +673,14 @@ func ReadAllConfigs() *Config {
 			utilsErr.OnType(func(err types.UnmarshalError) error {
 				fmt.Fprint(os.Stderr, lipgloss.Sprintln(
 					StyleYellow.Render(
-						fmt.Sprintf("\nFailed to parse config file (%s): ", configFile.Path())+err.Error(),
+						fmt.Sprintf("\nFailed to parse config file (%s): ", path)+err.Error(),
 					),
 				))
 				os.Exit(1)
 				return nil
 			}),
 			utilsErr.OnType(func(err types.FieldParseErrors) error {
-				printParseErrors(fmt.Sprintf("CONFIG FILE '%s'", configFile.Path()), err.Errors...)
+				printParseErrors(fmt.Sprintf("CONFIG FILE '%s'", path), err.Errors...)
 				os.Exit(1)
 				return nil
 			}),
@@ -669,7 +696,7 @@ func ReadAllConfigs() *Config {

 // parseConfigFile recursively parses a config file and its nested files up to maxDepth levels.
 // Returns the merged configuration or an error if parsing fails.
-// It can return the following errors:
+// It can return the following errors, each wrapped in a types.ConfigFileError naming the file:
 // - types.ConfigFileReadError
 // - types.UnmarshalError
 // - types.FieldParseErrors
@@ -677,7 +704,7 @@ func parseConfigFile(configFile types.ConfigFile, maxDepth int) (*Config, error)
 	configFileParser := NewConfigFileParser(configFile)
 	fileConfig, err := configFileParser.Parse()
 	if err != nil {
-		return nil, err
+		return nil, types.NewConfigFileError(configFile.Path(), err)
 	}

 	if maxDepth <= 0 {
@@ -715,7 +742,7 @@ func validateScriptSource(script string) error {
 		return types.ErrScriptEmpty
 	}

-	// Not a file/URL reference - it's an inline script
+	// Not a file/URL reference, so it's an inline script
 	if !strings.HasPrefix(script, "@") {
 		return nil
 	}
@@ -725,7 +752,7 @@ func validateScriptSource(script string) error {
 		return nil
 	}

-	// It's a file or URL reference - validate the source
+	// It's a file or URL reference, so validate the source
 	source := script[1:] // Remove the @ prefix

 	if source == "" {
@@ -744,7 +771,7 @@ func validateScriptSource(script string) error {
 		return nil
 	}

-	// It's a file path - basic validation (not empty, checked above)
+	// It's a file path, already checked to be non-empty above
 	return nil
 }

@@ -822,7 +849,7 @@ func (m printConfigModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 }

 func (m printConfigModel) View() tea.View {
-	// AltScreen and MouseMode were program options in bubbletea v1; in v2 they
+	// AltScreen and MouseMode were program options in bubbletea v1, in v2 they
 	// are view properties, so every return path has to declare them.
 	newView := func(s string) tea.View {
 		v := tea.NewView(s)
@@ -195,6 +195,22 @@ func (parser ConfigENVParser) Parse() (*Config, error) {
 		}
 	}

+	if maxResponseBody := parser.getEnv("MAX_RESPONSE_BODY"); maxResponseBody != "" {
+		maxResponseBodyParsed, err := types.ParseByteSize(maxResponseBody)
+		if err != nil {
+			fieldParseErrors = append(
+				fieldParseErrors,
+				types.NewFieldParseError(
+					parser.getFullEnvName("MAX_RESPONSE_BODY"),
+					maxResponseBody,
+					err,
+				),
+			)
+		} else {
+			config.MaxResponseBody = &maxResponseBodyParsed
+		}
+	}
+
 	if insecure := parser.getEnv("INSECURE"); insecure != "" {
 		insecureParsed, err := utilsParse.ParseString[bool](insecure)
 		if err != nil {
@@ -1,13 +1,17 @@
 package config

 import (
+	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"net/url"
 	"os"
 	"path/filepath"
+	"reflect"
+	"strconv"
 	"strings"
 	"time"

@@ -79,10 +83,18 @@ func fetchHTTP(ctx context.Context, url string) ([]byte, error) {
 		return nil, types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
 	}

-	data, err := io.ReadAll(resp.Body)
+	// A declared size above the limit is refused before anything is read.
+	if resp.ContentLength > types.RemoteFetchLimit {
+		return nil, types.NewHTTPFetchError(url, types.ErrRemoteFileTooLarge)
+	}
+
+	data, err := io.ReadAll(io.LimitReader(resp.Body, types.RemoteFetchLimit+1))
 	if err != nil {
 		return nil, types.NewHTTPFetchError(url, err)
 	}
+	if len(data) > types.RemoteFetchLimit {
+		return nil, types.NewHTTPFetchError(url, types.ErrRemoteFileTooLarge)
+	}

 	return data, nil
 }
@@ -126,7 +138,7 @@ func (ss *stringOrSliceField) UnmarshalYAML(node *yaml.Node) error {
 		*ss = slice
 		return nil
 	default:
-		return fmt.Errorf("expected a string or a sequence of strings, but got %v", node.Kind)
+		return fmt.Errorf("expected a string or a sequence of strings, but got %s", yamlNodeName(node))
 	}
 }

@@ -147,7 +159,7 @@ func (kv *keyValuesField) UnmarshalYAML(node *yaml.Node) error {
 		// Handle sequence of maps: [{key1: value1}, {key2: value2}]
 		for _, item := range node.Content {
 			if item.Kind != yaml.MappingNode {
-				return fmt.Errorf("expected a mapping in sequence, but got %v", item.Kind)
+				return fmt.Errorf("expected a mapping in sequence, but got %s", yamlNodeName(item))
 			}
 			if err := kv.unmarshalMapping(item); err != nil {
 				return err
@@ -155,7 +167,7 @@ func (kv *keyValuesField) UnmarshalYAML(node *yaml.Node) error {
 		}
 		return nil
 	default:
-		return fmt.Errorf("expected a mapping or sequence of mappings, but got %v", node.Kind)
+		return fmt.Errorf("expected a mapping or sequence of mappings, but got %s", yamlNodeName(node))
 	}
 }

@@ -166,7 +178,7 @@ func (kv *keyValuesField) unmarshalMapping(node *yaml.Node) error {
 		valueNode := node.Content[i+1]

 		if keyNode.Kind != yaml.ScalarNode {
-			return fmt.Errorf("expected a string key, but got %v", keyNode.Kind)
+			return fmt.Errorf("expected a string key, but got %s", yamlNodeName(keyNode))
 		}

 		key := keyNode.Value
@@ -178,12 +190,12 @@ func (kv *keyValuesField) unmarshalMapping(node *yaml.Node) error {
 		case yaml.SequenceNode:
 			for _, v := range valueNode.Content {
 				if v.Kind != yaml.ScalarNode {
-					return fmt.Errorf("expected string values in array for key %q, but got %v", key, v.Kind)
+					return fmt.Errorf("expected string values in array for key %q, but got %s", key, yamlNodeName(v))
 				}
 				values = append(values, v.Value)
 			}
 		default:
-			return fmt.Errorf("expected a string or array of strings for key %q, but got %v", key, valueNode.Kind)
+			return fmt.Errorf("expected a string or array of strings for key %q, but got %s", key, yamlNodeName(valueNode))
 		}

 		*kv = append(*kv, types.KeyValue[string, []string]{Key: key, Value: values})
@@ -192,28 +204,29 @@ func (kv *keyValuesField) unmarshalMapping(node *yaml.Node) error {
 }

 type configYAML struct {
-	ShowConfig   *bool              `yaml:"showConfig"`
-	ConfigFiles  stringOrSliceField `yaml:"configFile"`
-	Concurrency  *uint              `yaml:"concurrency"`
-	RequestCount *uint64            `yaml:"requests"`
-	Duration     *time.Duration     `yaml:"duration"`
-	LogLevel     *string            `yaml:"logLevel"`
-	LogFile      *string            `yaml:"logFile"`
-	Progress     *string            `yaml:"progress"`
-	Output       *string            `yaml:"output"`
-	DryRun       *bool              `yaml:"dryRun"`
-	URL          *string            `yaml:"url"`
-	Method       stringOrSliceField `yaml:"method"`
-	Bodies       stringOrSliceField `yaml:"body"`
-	Params       keyValuesField     `yaml:"params"`
-	Headers      keyValuesField     `yaml:"headers"`
-	Cookies      keyValuesField     `yaml:"cookies"`
-	Proxies      stringOrSliceField `yaml:"proxy"`
-	Values       stringOrSliceField `yaml:"values"`
-	Timeout      *time.Duration     `yaml:"timeout"`
-	Insecure     *bool              `yaml:"insecure"`
-	Lua          stringOrSliceField `yaml:"lua"`
-	Js           stringOrSliceField `yaml:"js"`
+	ShowConfig      *bool              `yaml:"showConfig"`
+	ConfigFiles     stringOrSliceField `yaml:"configFile"`
+	Concurrency     *uint              `yaml:"concurrency"`
+	RequestCount    *uint64            `yaml:"requests"`
+	Duration        *time.Duration     `yaml:"duration"`
+	LogLevel        *string            `yaml:"logLevel"`
+	LogFile         *string            `yaml:"logFile"`
+	Progress        *string            `yaml:"progress"`
+	Output          *string            `yaml:"output"`
+	DryRun          *bool              `yaml:"dryRun"`
+	URL             *string            `yaml:"url"`
+	Method          stringOrSliceField `yaml:"method"`
+	Bodies          stringOrSliceField `yaml:"body"`
+	Params          keyValuesField     `yaml:"params"`
+	Headers         keyValuesField     `yaml:"headers"`
+	Cookies         keyValuesField     `yaml:"cookies"`
+	Proxies         stringOrSliceField `yaml:"proxy"`
+	Values          stringOrSliceField `yaml:"values"`
+	Timeout         *time.Duration     `yaml:"timeout"`
+	MaxResponseBody *byteSize          `yaml:"maxResponseBody"`
+	Insecure        *bool              `yaml:"insecure"`
+	Lua             stringOrSliceField `yaml:"lua"`
+	Js              stringOrSliceField `yaml:"js"`
 }

 // ParseYAML parses YAML config file arguments into a Config object.
@@ -226,9 +239,26 @@ func (parser ConfigFileParser) ParseYAML(data []byte) (*Config, error) {
 		parsedData = &configYAML{}
 	)

-	err := yaml.Unmarshal(data, &parsedData)
-	if err != nil {
-		return nil, types.NewUnmarshalError(err)
+	decoder := yaml.NewDecoder(bytes.NewReader(data))
+	decoder.KnownFields(true)
+	// The decoder returns a bare io.EOF for an empty file, errors.Is would also match a wrapped one.
+	if err := decoder.Decode(parsedData); err != nil && err != io.EOF { //nolint:errorlint
+		return nil, types.NewUnmarshalError(yamlError(err))
+	}
+
+	// Decode reads one document, so anything after the first "---" would be dropped silently.
+	for {
+		var next yaml.Node
+		err := decoder.Decode(&next)
+		if err == io.EOF { //nolint:errorlint
+			break
+		}
+		if err != nil {
+			return nil, types.NewUnmarshalError(yamlError(err))
+		}
+		if !isEmptyDocument(&next) {
+			return nil, types.NewUnmarshalError(types.ErrYAMLMultipleDocuments)
+		}
 	}

 	var fieldParseErrors []types.FieldParseError
@@ -290,6 +320,9 @@ func (parser ConfigFileParser) ParseYAML(data []byte) (*Config, error) {

 	config.Values = append(config.Values, parsedData.Values...)
 	config.Timeout = parsedData.Timeout
+	if parsedData.MaxResponseBody != nil {
+		config.MaxResponseBody = new(uint64(*parsedData.MaxResponseBody))
+	}
 	config.Insecure = parsedData.Insecure
 	config.Lua = append(config.Lua, parsedData.Lua...)
 	config.Js = append(config.Js, parsedData.Js...)
@@ -300,3 +333,186 @@ func (parser ConfigFileParser) ParseYAML(data []byte) (*Config, error) {

 	return config, nil
 }
+
+// configYAMLTypeName is the Go type name the yaml package puts in its messages.
+var configYAMLTypeName = reflect.TypeFor[configYAML]().String()
+
+// isEmptyDocument reports whether a document holds nothing, which a trailing "---" produces.
+// A null tag alone is not enough, since a user can hang it on a mapping that does hold content.
+func isEmptyDocument(node *yaml.Node) bool {
+	if len(node.Content) == 0 {
+		return true
+	}
+	root := node.Content[0]
+	return root.Kind == yaml.ScalarNode && root.Tag == "!!null"
+}
+
+// yamlError rewrites yaml decode errors so they read like config file errors.
+// Errors from building the config arrive collected, everything earlier arrives on its own.
+func yamlError(err error) error {
+	var loadErrors *yaml.LoadErrors
+	if errors.As(err, &loadErrors) && len(loadErrors.Errors) > 0 {
+		var builder strings.Builder
+		for i, loadError := range loadErrors.Errors {
+			if i > 0 {
+				builder.WriteString("\n")
+			}
+			builder.WriteString(yamlErrorText(loadError))
+		}
+		return errors.New(builder.String())
+	}
+
+	if loadError, ok := errors.AsType[*yaml.LoadError](err); ok {
+		return errors.New(yamlErrorText(loadError))
+	}
+
+	return err
+}
+
+// yamlErrorText renders one error as "line 3: while scanning a plain scalar: found a tab character".
+// The mark is where reading gave up and the context mark starts the construct that broke.
+func yamlErrorText(loadError *yaml.LoadError) string {
+	var builder strings.Builder
+	if loadError.Mark.Line > 0 {
+		fmt.Fprintf(&builder, "line %d: ", loadError.Mark.Line)
+	}
+	if loadError.ContextMsg != "" {
+		builder.WriteString(loadError.ContextMsg + ": ")
+	}
+	builder.WriteString(yamlErrorMessage(loadError.Message))
+
+	start := loadError.ContextMark
+	if loadError.ContextMsg != "" && start.Line > 0 && start.Line != loadError.Mark.Line {
+		fmt.Fprintf(&builder, " (started at line %d)", start.Line)
+	}
+
+	return builder.String()
+}
+
+// yamlTypeNames maps the Go types and YAML tags the yaml package prints to words a user knows.
+var yamlTypeNames = map[string]string{
+	"!!str":            "text",
+	"!!int":            "a number",
+	"!!float":          "a number",
+	"!!bool":           "true or false",
+	"!!seq":            "a list",
+	"!!map":            "a mapping",
+	"!!null":           "an empty value",
+	"!!timestamp":      "a date",
+	"!!binary":         "binary data",
+	"string":           "text",
+	"bool":             "true or false",
+	"uint":             "a whole number of 0 or more",
+	"uint64":           "a whole number of 0 or more",
+	"time.Duration":    "a duration such as 10s",
+	configYAMLTypeName: "a mapping of config keys",
+}
+
+// yamlNodeName names what a node holds, so an error can say "a list" instead of a kind number.
+// A tag that contradicts the shape is kept, since the shape alone would read as a contradiction.
+func yamlNodeName(node *yaml.Node) string {
+	if name, ok := yamlTypeNames[node.Tag]; ok && yamlTagKind(node.Tag) == node.Kind {
+		return name
+	}
+	if node.Tag == "" {
+		return yamlKindName(node.Kind)
+	}
+	return yamlKindName(node.Kind) + " tagged " + node.Tag
+}
+
+// yamlTagKind returns the node shape a tag belongs on, so a mismatch can be spotted.
+func yamlTagKind(tag string) yaml.Kind {
+	switch tag {
+	case "!!seq":
+		return yaml.SequenceNode
+	case "!!map":
+		return yaml.MappingNode
+	default:
+		return yaml.ScalarNode
+	}
+}
+
+// yamlKindName names a node by its shape, for when its tag says nothing a user would know.
+func yamlKindName(kind yaml.Kind) string {
+	switch kind {
+	case yaml.SequenceNode:
+		return "a list"
+	case yaml.MappingNode:
+		return "a mapping"
+	case yaml.AliasNode:
+		return "an alias"
+	default:
+		return "a value"
+	}
+}
+
+// yamlErrorMessage turns "field x not found in type config.configYAML" into "unknown key "x"".
+func yamlErrorMessage(message string) string {
+	if field, ok := strings.CutSuffix(message, " not found in type "+configYAMLTypeName); ok {
+		if name, ok := strings.CutPrefix(field, "field "); ok {
+			return fmt.Sprintf("unknown key %q", name)
+		}
+	}
+
+	// The user controls the value, so the rightmost separator is the only one that is ours.
+	// After "as a" comes the tag the user wrote, after "into" comes the type the key accepts.
+	if names, ok := strings.CutPrefix(message, "cannot construct "); ok {
+		at, width, tagged := -1, 0, false
+		for index, separator := range []string{" into ", " as a "} {
+			if found := strings.LastIndex(names, separator); found > at {
+				at, width, tagged = found, len(separator), index == 1
+			}
+		}
+		if at >= 0 {
+			got, want := yamlGotName(names[:at]), yamlWantedName(names[at+width:])
+			if tagged {
+				return fmt.Sprintf("%s is not %s", got, want)
+			}
+			return fmt.Sprintf("expected %s, but got %s", want, got)
+		}
+	}
+
+	return strings.ReplaceAll(message, configYAMLTypeName, "the config file")
+}
+
+// yamlWantedName names what a key accepts, which the yaml package prints as a Go type.
+func yamlWantedName(text string) string {
+	if name, ok := yamlTypeNames[text]; ok {
+		return name
+	}
+	return text
+}
+
+// yamlGotName names what the file held, which the yaml package prints as a tag and sometimes a value.
+// A tag the user would not know stays in the text, since without it the message reads as nonsense.
+func yamlGotName(text string) string {
+	// A collection tag carries no backticks, so a scalar wearing one arrives glued to it.
+	for _, tag := range []string{"!!seq", "!!map"} {
+		if value, ok := strings.CutPrefix(text, tag); ok && value != "" {
+			return yamlValueText(value) + " tagged " + tag
+		}
+	}
+
+	tag, value, _ := strings.Cut(text, " ")
+	value = strings.TrimSuffix(strings.TrimPrefix(value, "`"), "`")
+
+	name, known := yamlTypeNames[tag]
+	switch {
+	case known && value != "":
+		return yamlValueText(value)
+	case known:
+		return name
+	case value != "":
+		return yamlValueText(value) + " tagged " + tag
+	default:
+		return "a value tagged " + tag
+	}
+}
+
+// yamlValueText quotes a value, saying so when the yaml package shortened it to seven characters.
+func yamlValueText(value string) string {
+	if start, ok := strings.CutSuffix(value, "..."); ok && len(start) == 7 {
+		return "a value starting with " + strconv.Quote(start)
+	}
+	return strconv.Quote(value)
+}
@@ -193,7 +193,7 @@ func validateTemplateURLPath(urlPath string, funcMap template.FuncMap) []types.F

 func ValidateTemplates(config *Config) []types.FieldValidationError {
 	// Create template function map using the same functions as sarin package
-	// Use nil for fileCache during validation - templates are only parsed, not executed
+	// fileCache is nil because templates are only parsed here, not executed
 	randSource := sarin.NewDefaultRandSource()
 	funcMap := sarin.NewDefaultTemplateFuncMap(randSource, nil)

@@ -5,26 +5,102 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/base64"
-	"math"
 	"net"
 	"net/http"
 	"net/url"
 	"time"

 	"github.com/valyala/fasthttp"
-	"github.com/valyala/fasthttp/fasthttpproxy"
 	"go.aykhans.me/sarin/internal/types"
 	utilsSlice "go.aykhans.me/utils/slice"
 	"golang.org/x/net/proxy"
 )

-type HostClientGenerator func() *fasthttp.HostClient
+// clientIndexGenerator returns the index of the host client (and proxy) to use next.
+type clientIndexGenerator func() int

-func safeUintToInt(u uint) int {
-	if u > math.MaxInt {
-		return math.MaxInt
+// preferIPv4Resolver hides a host's IPv6 addresses whenever it also has IPv4 ones.
+type preferIPv4Resolver struct{}
+
+func (preferIPv4Resolver) LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error) {
+	return lookupPreferred(ctx, host, true)
+}
+
+// preferIPv6Resolver is the mirror of preferIPv4Resolver, used for the fallback dial.
+type preferIPv6Resolver struct{}
+
+func (preferIPv6Resolver) LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error) {
+	return lookupPreferred(ctx, host, false)
+}
+
+// lookupPreferred resolves host and keeps only the wanted family, unless that family is absent.
+func lookupPreferred(ctx context.Context, host string, wantIPv4 bool) ([]net.IPAddr, error) {
+	ips, err := net.DefaultResolver.LookupIPAddr(ctx, host)
+	if err != nil {
+		return nil, err
+	}
+
+	preferred := make([]net.IPAddr, 0, len(ips))
+	for _, ip := range ips {
+		if (ip.IP.To4() != nil) == wantIPv4 {
+			preferred = append(preferred, ip)
+		}
+	}
+	if len(preferred) > 0 {
+		return preferred, nil
+	}
+	return ips, nil
+}
+
+// preferIPv4DialTimeoutShare leaves half the timeout for the IPv6 retry.
+const preferIPv4DialTimeoutShare = 2
+
+// preferIPv4Dialer dials over IPv4 and retries over IPv6 alone when that fails.
+type preferIPv4Dialer struct {
+	ipv4 *fasthttp.TCPDialer
+	ipv6 *fasthttp.TCPDialer
+}
+
+// dialerConcurrency matches the limit on fasthttp own package-level dialer.
+const dialerConcurrency = 1000
+
+// dialer is shared so all clients use one DNS cache.
+var dialer = &preferIPv4Dialer{
+	ipv4: &fasthttp.TCPDialer{Resolver: preferIPv4Resolver{}, Concurrency: dialerConcurrency},
+	ipv6: &fasthttp.TCPDialer{Resolver: preferIPv6Resolver{}, Concurrency: dialerConcurrency},
+}
+
+// DialTimeout dials addr, which must carry a port.
+func (d *preferIPv4Dialer) DialTimeout(addr string, timeout time.Duration) (net.Conn, error) {
+	// fasthttp passes 0 from its pooled dial path.
+	if timeout <= 0 {
+		timeout = fasthttp.DefaultDialTimeout
+	}
+	deadline := time.Now().Add(timeout)
+
+	conn, err := d.ipv4.DialDualStackTimeout(addr, timeout/preferIPv4DialTimeoutShare)
+	if err == nil {
+		return conn, nil
+	}
+
+	remaining := time.Until(deadline)
+	if remaining <= 0 {
+		return nil, err
+	}
+
+	conn, fallbackErr := d.ipv6.DialDualStackTimeout(addr, remaining)
+	if fallbackErr != nil {
+		// Report the IPv4 error, not the fallback one.
+		return nil, err
+	}
+	return conn, nil
+}
+
+// dialFunc fills in the default port, which fasthttp skips for a custom dial func.
+func (d *preferIPv4Dialer) dialFunc(isTLS bool) fasthttp.DialFuncWithTimeout {
+	return func(addr string, timeout time.Duration) (net.Conn, error) {
+		return d.DialTimeout(fasthttp.AddMissingPort(addr, isTLS), timeout)
 	}
-	return int(u)
 }

 // NewHostClients creates a list of fasthttp.HostClient instances for the given proxies.
@@ -38,6 +114,7 @@ func NewHostClients(
 	maxConns uint,
 	requestURL *url.URL,
 	skipVerify bool,
+	maxResponseBody uint64,
 ) ([]*fasthttp.HostClient, error) {
 	isTLS := requestURL.Scheme == "https"

@@ -66,6 +143,9 @@ func NewHostClients(
 				MaxConnDuration:               timeout,
 				WriteTimeout:                  timeout,
 				ReadTimeout:                   timeout,
+				MaxResponseBodySize:           safeUint64ToInt(maxResponseBody),
+				MaxIdemponentCallAttempts:     1,
+				ReadBufferSize:                64 << 10, // 64 KiB
 				DisableHeaderNamesNormalizing: true,
 				DisablePathNormalizing:        true,
 				NoDefaultUserAgentHeader:      true,
@@ -77,8 +157,9 @@ func NewHostClients(
 	}

 	client := &fasthttp.HostClient{
-		MaxConns: safeUintToInt(maxConns),
-		IsTLS:    isTLS,
+		MaxConns:    safeUintToInt(maxConns),
+		IsTLS:       isTLS,
+		DialTimeout: dialer.dialFunc(isTLS),
 		TLSConfig: &tls.Config{
 			InsecureSkipVerify: skipVerify, //nolint:gosec
 		},
@@ -87,6 +168,9 @@ func NewHostClients(
 		MaxConnDuration:               timeout,
 		WriteTimeout:                  timeout,
 		ReadTimeout:                   timeout,
+		MaxResponseBodySize:           safeUint64ToInt(maxResponseBody),
+		MaxIdemponentCallAttempts:     1,
+		ReadBufferSize:                64 << 10, // 64 KiB
 		DisableHeaderNamesNormalizing: true,
 		DisablePathNormalizing:        true,
 		NoDefaultUserAgentHeader:      true,
@@ -94,40 +178,45 @@ func NewHostClients(
 	return []*fasthttp.HostClient{client}, nil
 }

-// NewProxyDialFunc creates a dial function for the given proxy URL.
+// NewProxyDialFunc creates a dial function for the given proxy URL with a fixed timeout.
 // It can return the following errors:
 //   - types.ProxyUnsupportedSchemeError
+//   - types.ErrProxyNoContextDialer
 func NewProxyDialFunc(ctx context.Context, proxyURL *url.URL, timeout time.Duration) (fasthttp.DialFunc, error) {
-	var (
-		dialer fasthttp.DialFunc
-		err    error
-	)
+	dial, err := NewProxyDialFuncWithTimeout(ctx, proxyURL)
+	if err != nil {
+		return nil, err
+	}

+	return func(addr string) (net.Conn, error) {
+		return dial(addr, timeout)
+	}, nil
+}
+
+// NewProxyDialFuncWithTimeout creates a dial function for the given proxy URL that takes the timeout per call.
+// It can return the following errors:
+//   - types.ProxyUnsupportedSchemeError
+//   - types.ErrProxyNoContextDialer
+func NewProxyDialFuncWithTimeout(ctx context.Context, proxyURL *url.URL) (fasthttp.DialFuncWithTimeout, error) {
 	switch proxyURL.Scheme {
 	case "socks5":
-		dialer, err = fasthttpSocksDialerDualStackTimeout(ctx, proxyURL, timeout, true)
-		if err != nil {
-			return nil, err
-		}
+		return fasthttpSocksDialer(ctx, proxyURL, true)
 	case "socks5h":
-		dialer, err = fasthttpSocksDialerDualStackTimeout(ctx, proxyURL, timeout, false)
-		if err != nil {
-			return nil, err
-		}
+		return fasthttpSocksDialer(ctx, proxyURL, false)
 	case "http":
-		dialer = fasthttpproxy.FasthttpHTTPDialerDualStackTimeout(proxyURL.String(), timeout)
+		return fasthttpConnectDialer(proxyURL, false), nil
 	case "https":
-		dialer = fasthttpHTTPSDialerDualStackTimeout(proxyURL, timeout)
+		return fasthttpConnectDialer(proxyURL, true), nil
 	default:
 		return nil, types.NewProxyUnsupportedSchemeError(proxyURL.Scheme)
 	}
-
-	return dialer, nil
 }

-// The returned dial function can return the following errors:
+// fasthttpSocksDialer creates a SOCKS5 dial function that takes the timeout per call.
+// It can return the following errors:
+//   - types.ErrProxyNoContextDialer
 //   - types.ProxyDialError
-func fasthttpSocksDialerDualStackTimeout(ctx context.Context, proxyURL *url.URL, timeout time.Duration, resolveLocally bool) (fasthttp.DialFunc, error) {
+func fasthttpSocksDialer(ctx context.Context, proxyURL *url.URL, resolveLocally bool) (fasthttp.DialFuncWithTimeout, error) {
 	netDialer := &net.Dialer{}

 	// Parse auth from proxy URL if present
@@ -149,66 +238,92 @@ func fasthttpSocksDialerDualStackTimeout(ctx context.Context, proxyURL *url.URL,

 	proxyStr := proxyURL.String()

-	// Assert to ContextDialer for timeout support
+	// Timeouts need DialContext, which proxy.SOCKS5 always provides with a net.Dialer.
 	contextDialer, ok := socksDialer.(proxy.ContextDialer)
 	if !ok {
-		// Fallback without timeout (should not happen with net.Dialer)
-		return func(addr string) (net.Conn, error) {
-			conn, err := socksDialer.Dial("tcp", addr)
-			if err != nil {
-				return nil, types.NewProxyDialError(proxyStr, err)
-			}
-			return conn, nil
-		}, nil
+		return nil, types.ErrProxyNoContextDialer
 	}

 	// Return dial function that uses context with timeout
-	return func(addr string) (net.Conn, error) {
+	return func(addr string, timeout time.Duration) (net.Conn, error) {
 		deadline := time.Now().Add(timeout)

+		addrs := []string{addr}
 		if resolveLocally {
-			host, port, err := net.SplitHostPort(addr)
+			var err error
+			addrs, err = resolveIPv4First(ctx, addr, timeout)
 			if err != nil {
 				return nil, types.NewProxyDialError(proxyStr, err)
 			}
+		}

-			dnsCtx, dnsCancel := context.WithTimeout(ctx, timeout)
-			ips, err := net.DefaultResolver.LookupIP(dnsCtx, "ip", host)
-			dnsCancel()
-			if err != nil {
-				return nil, types.NewProxyDialError(proxyStr, err)
-			}
-			if len(ips) == 0 {
-				return nil, types.NewProxyDialError(proxyStr, types.NewProxyResolveError(host))
+		// Try every resolved address, so one unreachable family does not fail the dial.
+		var lastErr error
+		for _, candidate := range addrs {
+			remaining := time.Until(deadline)
+			if remaining <= 0 {
+				return nil, types.NewProxyDialError(proxyStr, context.DeadlineExceeded)
 			}

-			// Use the first resolved IP
-			addr = net.JoinHostPort(ips[0].String(), port)
+			dialCtx, dialCancel := context.WithTimeout(ctx, remaining)
+			conn, err := contextDialer.DialContext(dialCtx, "tcp", candidate)
+			dialCancel()
+			if err == nil {
+				return conn, nil
+			}
+			lastErr = err
 		}
-
-		// Use remaining time for dial
-		remaining := time.Until(deadline)
-		if remaining <= 0 {
-			return nil, types.NewProxyDialError(proxyStr, context.DeadlineExceeded)
-		}
-
-		dialCtx, dialCancel := context.WithTimeout(ctx, remaining)
-		defer dialCancel()
-
-		conn, err := contextDialer.DialContext(dialCtx, "tcp", addr)
-		if err != nil {
-			return nil, types.NewProxyDialError(proxyStr, err)
-		}
-		return conn, nil
+		return nil, types.NewProxyDialError(proxyStr, lastErr)
 	}, nil
 }

+// resolveIPv4First resolves the host of addr into candidate addresses, IPv4 ones first.
+// It can return the following errors:
+//   - types.ProxyResolveError
+func resolveIPv4First(ctx context.Context, addr string, timeout time.Duration) ([]string, error) {
+	host, port, err := net.SplitHostPort(addr)
+	if err != nil {
+		return nil, err
+	}
+
+	dnsCtx, dnsCancel := context.WithTimeout(ctx, timeout)
+	ips, err := net.DefaultResolver.LookupIP(dnsCtx, "ip", host)
+	dnsCancel()
+	if err != nil {
+		return nil, err
+	}
+	if len(ips) == 0 {
+		return nil, types.NewProxyResolveError(host)
+	}
+
+	addrs := make([]string, 0, len(ips))
+	for _, ip := range ips {
+		if ip.To4() != nil {
+			addrs = append(addrs, net.JoinHostPort(ip.String(), port))
+		}
+	}
+	for _, ip := range ips {
+		if ip.To4() == nil {
+			addrs = append(addrs, net.JoinHostPort(ip.String(), port))
+		}
+	}
+	return addrs, nil
+}
+
+// fasthttpConnectDialer creates a dial function that tunnels through an HTTP proxy with CONNECT,
+// reaching the proxy over TLS when useTLS is set.
+//
 // The returned dial function can return the following errors:
 //   - types.ProxyDialError
-func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duration) fasthttp.DialFunc {
+func fasthttpConnectDialer(proxyURL *url.URL, useTLS bool) fasthttp.DialFuncWithTimeout {
+	defaultPort := "80"
+	if useTLS {
+		defaultPort = "443"
+	}
+
 	proxyAddr := proxyURL.Host
 	if proxyURL.Port() == "" {
-		proxyAddr = net.JoinHostPort(proxyURL.Hostname(), "443")
+		proxyAddr = net.JoinHostPort(proxyURL.Hostname(), defaultPort)
 	}

 	// Build Proxy-Authorization header if auth is present
@@ -222,10 +337,10 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio

 	proxyStr := proxyURL.String()

-	return func(addr string) (net.Conn, error) {
+	return func(addr string, timeout time.Duration) (net.Conn, error) {
 		// Establish TCP connection to proxy with timeout
 		start := time.Now()
-		conn, err := fasthttp.DialDualStackTimeout(proxyAddr, timeout)
+		conn, err := dialer.DialTimeout(proxyAddr, timeout)
 		if err != nil {
 			return nil, types.NewProxyDialError(proxyStr, err)
 		}
@@ -242,13 +357,15 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 			return nil, types.NewProxyDialError(proxyStr, err)
 		}

-		// Upgrade to TLS
-		tlsConn := tls.Client(conn, &tls.Config{
-			ServerName: proxyURL.Hostname(),
-		})
-		if err := tlsConn.Handshake(); err != nil {
-			tlsConn.Close() //nolint:errcheck,gosec
-			return nil, types.NewProxyDialError(proxyStr, err)
+		if useTLS {
+			tlsConn := tls.Client(conn, &tls.Config{
+				ServerName: proxyURL.Hostname(),
+			})
+			if err := tlsConn.Handshake(); err != nil {
+				tlsConn.Close() //nolint:errcheck,gosec
+				return nil, types.NewProxyDialError(proxyStr, err)
+			}
+			conn = tlsConn
 		}

 		// Build and send CONNECT request
@@ -262,35 +379,35 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 			connectReq.Header.Set("Proxy-Authorization", proxyAuth)
 		}

-		if err := connectReq.Write(tlsConn); err != nil {
-			tlsConn.Close() //nolint:errcheck,gosec
+		if err := connectReq.Write(conn); err != nil {
+			conn.Close() //nolint:errcheck,gosec
 			return nil, types.NewProxyDialError(proxyStr, err)
 		}

 		// Read response using buffered reader, but return wrapped connection
 		// to preserve any buffered data
-		bufReader := bufio.NewReader(tlsConn)
+		bufReader := bufio.NewReader(conn)
 		resp, err := http.ReadResponse(bufReader, connectReq)
 		if err != nil {
-			tlsConn.Close() //nolint:errcheck,gosec
+			conn.Close() //nolint:errcheck,gosec
 			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 		resp.Body.Close() //nolint:errcheck,gosec

 		if resp.StatusCode != http.StatusOK {
-			tlsConn.Close() //nolint:errcheck,gosec
+			conn.Close() //nolint:errcheck,gosec
 			return nil, types.NewProxyDialError(proxyStr, types.NewProxyConnectError(resp.Status))
 		}

 		// Clear deadline for the tunneled connection
-		if err := tlsConn.SetDeadline(time.Time{}); err != nil {
-			tlsConn.Close() //nolint:errcheck,gosec
+		if err := conn.SetDeadline(time.Time{}); err != nil {
+			conn.Close() //nolint:errcheck,gosec
 			return nil, types.NewProxyDialError(proxyStr, err)
 		}

 		// Return wrapped connection that uses the buffered reader
 		// to avoid losing any data that was read ahead
-		return &bufferedConn{Conn: tlsConn, reader: bufReader}, nil
+		return &bufferedConn{Conn: conn, reader: bufReader}, nil
 	}
 }

@@ -306,18 +423,10 @@ func (c *bufferedConn) Read(b []byte) (int, error) {
 	return c.reader.Read(b)
 }

-func NewHostClientGenerator(clients ...*fasthttp.HostClient) HostClientGenerator {
-	switch len(clients) {
-	case 0:
-		hostClient := &fasthttp.HostClient{}
-		return func() *fasthttp.HostClient {
-			return hostClient
-		}
-	case 1:
-		return func() *fasthttp.HostClient {
-			return clients[0]
-		}
-	default:
-		return utilsSlice.RandomCycle(nil, clients...)
+func newClientIndexGenerator(count int) clientIndexGenerator {
+	indexes := make([]int, count)
+	for i := range indexes {
+		indexes[i] = i
 	}
+	return utilsSlice.RandomCycle(nil, indexes...)
 }
@@ -58,8 +58,7 @@ func (fc *FileCache) GetOrLoad(source string) (*CachedFile, error) {

 	file := &CachedFile{Content: content, Filename: filename}

-	// LoadOrStore handles race condition - if another goroutine
-	// cached it first, we get theirs (no duplicate storage)
+	// If another goroutine cached it first, LoadOrStore returns theirs
 	actual, _ := fc.cache.LoadOrStore(source, file)
 	return actual.(*CachedFile), nil
 }
@@ -94,10 +93,18 @@ func (fc *FileCache) fetchURL(url string) ([]byte, string, error) {
 		return nil, "", types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
 	}

-	content, err := io.ReadAll(resp.Body)
+	// A declared size above the limit is refused before anything is read.
+	if resp.ContentLength > types.RemoteFetchLimit {
+		return nil, "", types.NewHTTPFetchError(url, types.ErrRemoteFileTooLarge)
+	}
+
+	content, err := io.ReadAll(io.LimitReader(resp.Body, types.RemoteFetchLimit+1))
 	if err != nil {
 		return nil, "", types.NewHTTPFetchError(url, err)
 	}
+	if len(content) > types.RemoteFetchLimit {
+		return nil, "", types.NewHTTPFetchError(url, types.ErrRemoteFileTooLarge)
+	}

 	// Extract filename from URL path
 	filename := path.Base(url)
@@ -1,6 +1,7 @@
 package sarin

 import (
+	"math"
 	"math/rand/v2"
 	"time"
 )
@@ -19,3 +20,17 @@ func firstOrEmpty(values []string) string {
 	}
 	return values[0]
 }
+
+func safeUintToInt(u uint) int {
+	if u > math.MaxInt {
+		return math.MaxInt
+	}
+	return int(u)
+}
+
+func safeUint64ToInt(u uint64) int {
+	if u > math.MaxInt {
+		return math.MaxInt
+	}
+	return int(u)
+}
@@ -191,18 +191,7 @@ func applyRequestDataToFastHTTP(reqData *script.RequestData, req *fasthttp.Reque
 	}

 	if len(reqData.Cookies) > 0 {
-		var sb strings.Builder
-		for k, values := range reqData.Cookies {
-			for _, v := range values {
-				if sb.Len() > 0 {
-					sb.WriteString("; ")
-				}
-				sb.WriteString(k)
-				sb.WriteByte('=')
-				sb.WriteString(v)
-			}
-		}
-		req.Header.Add("Cookie", sb.String())
+		req.Header.Add("Cookie", cookieHeaderValue(reqData.Cookies))
 	}

 	if scheme == "https" {
@@ -210,6 +199,22 @@ func applyRequestDataToFastHTTP(reqData *script.RequestData, req *fasthttp.Reque
 	}
 }

+// cookieHeaderValue joins cookies into a single Cookie header value ("a=1; b=2").
+func cookieHeaderValue(cookies map[string][]string) string {
+	var sb strings.Builder
+	for k, values := range cookies {
+		for _, v := range values {
+			if sb.Len() > 0 {
+				sb.WriteString("; ")
+			}
+			sb.WriteString(k)
+			sb.WriteByte('=')
+			sb.WriteString(v)
+		}
+	}
+	return sb.String()
+}
+
 func NewMethodGeneratorFunc(localRand *rand.Rand, methods []string, lazyRoot func() *template.Template) (requestDataGenerator, bool) {
 	methodGenerator, isDynamic := buildStringSliceGenerator(localRand, methods, lazyRoot)

@@ -17,6 +17,14 @@ import (
 const DefaultResponseDurationAccuracy uint32 = 1
 const DefaultResponseColumnMaxWidth = 50

+// maxResponseKeys bounds the distinct response keys, so error messages carrying
+// per-request text cannot grow the map without limit. It stays above the number of
+// HTTP status codes, so real ones are never merged.
+const maxResponseKeys = 1000
+
+// otherResponseKey collects everything that arrives after the key limit is reached.
+const otherResponseKey = "other"
+
 // Duration wraps time.Duration to provide consistent JSON/YAML marshaling as human-readable strings.
 type Duration time.Duration

@@ -95,6 +103,11 @@ func (data *SarinResponseData) Add(responseKey string, responseTime time.Duratio
 	defer data.Unlock()

 	response, ok := data.Responses[responseKey]
+	if !ok && len(data.Responses) >= maxResponseKeys {
+		responseKey = otherResponseKey
+		response, ok = data.Responses[responseKey]
+	}
+
 	if !ok {
 		data.Responses[responseKey] = &Response{
 			durations: map[time.Duration]uint64{
@@ -199,10 +199,15 @@ type sarin struct {
 	logError       bool
 	logFile        string

-	hostClients []*fasthttp.HostClient
-	responses   *SarinResponseData
-	fileCache   *FileCache
-	scriptChain *script.Chain
+	// jobsCtx ends the run, either when the duration expires or on Ctrl+C.
+	jobsCtx    context.Context //nolint:containedctx
+	jobsCancel context.CancelFunc
+
+	hostClients       []*fasthttp.HostClient
+	scriptHTTPClients []*scriptHTTPClient
+	responses         *SarinResponseData
+	fileCache         *FileCache
+	scriptChain       *script.Chain
 }

 // NewSarin creates a new sarin instance for load testing.
@@ -220,6 +225,7 @@ func NewSarin(
 	totalDuration *time.Duration,
 	showProgress bool,
 	skipCertVerify bool,
+	maxResponseBody uint64,
 	params types.Params,
 	headers types.Headers,
 	cookies types.Cookies,
@@ -248,7 +254,12 @@ func NewSarin(
 		}
 	}

-	hostClients, err := newHostClients(ctx, timeout, proxies, workers, requestURL, skipCertVerify)
+	proxyURLs := make([]url.URL, len(proxies))
+	for i, proxy := range proxies {
+		proxyURLs[i] = url.URL(proxy)
+	}
+
+	hostClients, err := NewHostClients(ctx, timeout, proxyURLs, workers, requestURL, skipCertVerify, maxResponseBody)
 	if err != nil {
 		return nil, err
 	}
@@ -266,28 +277,42 @@ func NewSarin(

 	scriptChain := script.NewChain(luaSources, jsSources)

+	jobsCtx, jobsCancel := context.WithCancel(ctx)
+
+	var scriptHTTPClients []*scriptHTTPClient
+	if !scriptChain.IsEmpty() {
+		scriptHTTPClients, err = newScriptHTTPClients(jobsCtx, proxyURLs, workers, maxResponseBody)
+		if err != nil {
+			jobsCancel()
+			return nil, err
+		}
+	}
+
 	srn := &sarin{
-		workers:        workers,
-		requestURL:     requestURL,
-		methods:        methods,
-		params:         params,
-		headers:        headers,
-		cookies:        cookies,
-		bodies:         bodies,
-		totalRequests:  totalRequests,
-		totalDuration:  totalDuration,
-		timeout:        timeout,
-		showProgress:   showProgress,
-		skipCertVerify: skipCertVerify,
-		values:         values,
-		collectStats:   collectStats,
-		dryRun:         dryRun,
-		logInfo:        logInfo,
-		logError:       logError,
-		logFile:        logFile,
-		hostClients:    hostClients,
-		fileCache:      NewFileCache(time.Second * 10),
-		scriptChain:    scriptChain,
+		jobsCtx:           jobsCtx,
+		jobsCancel:        jobsCancel,
+		workers:           workers,
+		requestURL:        requestURL,
+		methods:           methods,
+		params:            params,
+		headers:           headers,
+		cookies:           cookies,
+		bodies:            bodies,
+		totalRequests:     totalRequests,
+		totalDuration:     totalDuration,
+		timeout:           timeout,
+		showProgress:      showProgress,
+		skipCertVerify:    skipCertVerify,
+		values:            values,
+		collectStats:      collectStats,
+		dryRun:            dryRun,
+		logInfo:           logInfo,
+		logError:          logError,
+		logFile:           logFile,
+		hostClients:       hostClients,
+		scriptHTTPClients: scriptHTTPClients,
+		fileCache:         NewFileCache(time.Second * 30),
+		scriptChain:       scriptChain,
 	}

 	if collectStats {
@@ -301,9 +326,7 @@ func (s sarin) GetResponses() *SarinResponseData {
 	return s.responses
 }

-func (s sarin) Start(ctx context.Context, stopCtrl *StopController) {
-	jobsCtx, jobsCancel := context.WithCancel(ctx)
-
+func (s sarin) Start(stopCtrl *StopController) {
 	var workersWG sync.WaitGroup
 	jobsCh := make(chan struct{}, max(s.workers, 1))

@@ -364,18 +387,17 @@ func (s sarin) Start(ctx context.Context, stopCtrl *StopController) {
 	}

 	// Start workers
-	s.startWorkers(&workersWG, jobsCh, s.hostClients, &counter, sendLog, sendRespLog)
+	s.startWorkers(&workersWG, jobsCh, &counter, sendLog, sendRespLog)

 	if runTUI {
-		//nolint:contextcheck // streamCtx must remain active until all workers complete to ensure all collected data is streamed
 		go s.streamProgress(streamCtx, stopCtrl, streamCh, totalRequests, &counter, tuiLogChannel, showProgressBar)
 	}

 	// Setup duration-based cancellation
-	s.setupDurationTimeout(ctx, jobsCancel)
+	s.setupDurationTimeout()
 	// Distribute jobs to workers.
 	// This blocks until all jobs are sent or the context is canceled.
-	s.sendJobs(jobsCtx, jobsCh)
+	s.sendJobs(jobsCh)

 	// Close the jobs channel so workers stop after completing their current job
 	close(jobsCh)
@@ -394,7 +416,7 @@ func (s sarin) Start(ctx context.Context, stopCtrl *StopController) {
 }

 // newWriterLog builds the loggers that write formatted lines to w (a log file or
-// stderr). sendLog stays general (it filters by each log's level); sendRespLog
+// stderr). sendLog stays general (it filters by each log's level), while sendRespLog
 // only ever emits info, so its decision is baked once into a no-op when off.
 func (s sarin) newWriterLog(w io.Writer) (runtimeLogger, respLogger) {
 	// log.Logger serializes writes with its own mutex, so concurrent workers
@@ -432,65 +454,39 @@ func (s sarin) newChannelLog(ch chan<- runtimeLog) (runtimeLogger, respLogger) {
 	return sendLog, sendRespLog
 }

-// newHostClients initializes HTTP clients for the given configuration.
-// It can return the following errors:
-// - types.ProxyDialError
-func newHostClients(
-	ctx context.Context,
-	timeout time.Duration,
-	proxies types.Proxies,
-	workers uint,
-	requestURL *url.URL,
-	skipCertVerify bool,
-) ([]*fasthttp.HostClient, error) {
-	proxiesRaw := make([]url.URL, len(proxies))
-	for i, proxy := range proxies {
-		proxiesRaw[i] = url.URL(proxy)
-	}
-
-	return NewHostClients(
-		ctx,
-		timeout,
-		proxiesRaw,
-		workers,
-		requestURL,
-		skipCertVerify,
-	)
-}
-
-func (s sarin) startWorkers(wg *sync.WaitGroup, jobs <-chan struct{}, hostClients []*fasthttp.HostClient, counter *atomic.Uint64, sendLog runtimeLogger, sendRespLog respLogger) {
+func (s sarin) startWorkers(wg *sync.WaitGroup, jobs <-chan struct{}, counter *atomic.Uint64, sendLog runtimeLogger, sendRespLog respLogger) {
 	for range max(s.workers, 1) {
 		wg.Go(func() {
-			s.Worker(jobs, NewHostClientGenerator(hostClients...), counter, sendLog, sendRespLog)
+			s.Worker(jobs, counter, sendLog, sendRespLog)
 		})
 	}
 }

-func (s sarin) setupDurationTimeout(ctx context.Context, cancel context.CancelFunc) {
+func (s sarin) setupDurationTimeout() {
 	if s.totalDuration != nil {
 		go func() {
 			timer := time.NewTimer(*s.totalDuration)
 			defer timer.Stop()
 			select {
 			case <-timer.C:
-				cancel()
-			case <-ctx.Done():
+				s.jobsCancel()
+			case <-s.jobsCtx.Done():
 				// Context cancelled, cleanup
 			}
 		}()
 	}
 }

-func (s sarin) sendJobs(ctx context.Context, jobs chan<- struct{}) {
+func (s sarin) sendJobs(jobs chan<- struct{}) {
 	if s.totalRequests != nil && *s.totalRequests > 0 {
 		for range *s.totalRequests {
-			if ctx.Err() != nil {
+			if s.jobsCtx.Err() != nil {
 				break
 			}
 			jobs <- struct{}{}
 		}
 	} else {
-		for ctx.Err() == nil {
+		for s.jobsCtx.Err() == nil {
 			jobs <- struct{}{}
 		}
 	}
@@ -0,0 +1,210 @@
+package sarin
+
+import (
+	"compress/zlib"
+	"context"
+	"crypto/tls"
+	"errors"
+	"net"
+	"net/url"
+	"time"
+
+	"github.com/valyala/fasthttp"
+	"go.aykhans.me/sarin/internal/script"
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// scriptHTTPDefaultTimeout is the default timeout for script requests.
+const scriptHTTPDefaultTimeout = 10 * time.Second
+
+// scriptHTTPClient sends scripts' http.* requests. It is safe for concurrent use.
+type scriptHTTPClient struct {
+	ctx             context.Context //nolint:containedctx
+	client          *fasthttp.Client
+	insecureClient  *fasthttp.Client
+	defaultTimeout  time.Duration
+	maxResponseBody int
+}
+
+var _ script.HTTPDoer = (*scriptHTTPClient)(nil)
+
+// newScriptHTTPClients creates one client per proxy, in the same order as NewHostClients.
+// It can return the following errors:
+//   - types.ProxyDialError
+func newScriptHTTPClients(ctx context.Context, proxies []url.URL, maxConns uint, maxResponseBody uint64) ([]*scriptHTTPClient, error) {
+	if len(proxies) == 0 {
+		return []*scriptHTTPClient{newScriptHTTPClient(ctx, dialer.DialTimeout, scriptHTTPDefaultTimeout, maxConns, maxResponseBody)}, nil
+	}
+
+	clients := make([]*scriptHTTPClient, 0, len(proxies))
+	for _, proxy := range proxies {
+		dial, err := NewProxyDialFuncWithTimeout(ctx, &proxy)
+		if err != nil {
+			return nil, types.NewProxyDialError(proxy.String(), err)
+		}
+		clients = append(clients, newScriptHTTPClient(ctx, dial, scriptHTTPDefaultTimeout, maxConns, maxResponseBody))
+	}
+	return clients, nil
+}
+
+func newScriptHTTPClient(
+	ctx context.Context,
+	dial fasthttp.DialFuncWithTimeout,
+	defaultTimeout time.Duration,
+	maxConns uint,
+	maxResponseBody uint64,
+) *scriptHTTPClient {
+	newClient := func(insecure bool) *fasthttp.Client {
+		// Read and write timeouts stay unset because they would cap the per-call timeout,
+		// so dialWithDeadline is what bounds the TLS handshake.
+		return &fasthttp.Client{
+			DialTimeout:               dialWithDeadline(dial),
+			MaxConnsPerHost:           safeUintToInt(maxConns),
+			MaxResponseBodySize:       safeUint64ToInt(maxResponseBody),
+			MaxIdemponentCallAttempts: 1,
+			ReadBufferSize:            64 << 10, // 64 KiB
+			TLSConfig: &tls.Config{
+				InsecureSkipVerify: insecure, //nolint:gosec
+			},
+			DisableHeaderNamesNormalizing: true,
+			DisablePathNormalizing:        true,
+			NoDefaultUserAgentHeader:      true,
+		}
+	}
+
+	return &scriptHTTPClient{
+		ctx:             ctx,
+		client:          newClient(false),
+		insecureClient:  newClient(true),
+		defaultTimeout:  defaultTimeout,
+		maxResponseBody: safeUint64ToInt(maxResponseBody),
+	}
+}
+
+// dialWithDeadline dials within the request timeout and keeps it as the connection deadline,
+// so the TLS handshake fasthttp runs on the first write cannot outlive the request.
+func dialWithDeadline(dial fasthttp.DialFuncWithTimeout) fasthttp.DialFuncWithTimeout {
+	return func(addr string, timeout time.Duration) (net.Conn, error) {
+		if timeout <= 0 {
+			timeout = scriptHTTPDefaultTimeout
+		}
+		deadline := time.Now().Add(timeout)
+
+		conn, err := dial(addr, timeout)
+		if err != nil {
+			return nil, err
+		}
+		if err := conn.SetDeadline(deadline); err != nil {
+			conn.Close() //nolint:errcheck,gosec
+			return nil, err
+		}
+		return conn, nil
+	}
+}
+
+// Do sends the request. With maxRedirects the timeout applies to each hop.
+// It can return the following errors:
+//   - types.ScriptHTTPRequestError
+func (c *scriptHTTPClient) Do(r *script.HTTPRequest) (*script.HTTPResponse, error) {
+	if err := c.ctx.Err(); err != nil {
+		return nil, types.NewScriptHTTPRequestError(r.Method, r.URL, err)
+	}
+
+	parsedURL, err := url.Parse(r.URL)
+	if err != nil {
+		return nil, types.NewScriptHTTPRequestError(r.Method, r.URL, err)
+	}
+	if (parsedURL.Scheme != "http" && parsedURL.Scheme != "https") || parsedURL.Host == "" {
+		return nil, types.NewScriptHTTPRequestError(r.Method, r.URL, types.ErrScriptHTTPURLInvalid)
+	}
+
+	req := fasthttp.AcquireRequest()
+	resp := fasthttp.AcquireResponse()
+	defer fasthttp.ReleaseRequest(req)
+	defer fasthttp.ReleaseResponse(resp)
+
+	req.SetRequestURI(r.URL)
+	req.Header.SetMethod(r.Method)
+	for key, values := range r.Headers {
+		for _, value := range values {
+			req.Header.Add(key, value)
+		}
+	}
+
+	if len(r.Params) > 0 {
+		args := req.URI().QueryArgs()
+		for key, values := range r.Params {
+			for _, value := range values {
+				args.Add(key, value)
+			}
+		}
+	}
+
+	if len(r.Cookies) > 0 {
+		req.Header.Add("Cookie", cookieHeaderValue(r.Cookies))
+	}
+
+	req.SetBodyString(r.Body)
+
+	timeout := r.Timeout
+	if timeout <= 0 {
+		timeout = c.defaultTimeout
+	}
+
+	client := c.client
+	if r.Insecure {
+		client = c.insecureClient
+	}
+
+	req.SetTimeout(timeout)
+	if r.MaxRedirects > 0 {
+		err = client.DoRedirects(req, resp, r.MaxRedirects)
+	} else {
+		err = client.Do(req, resp)
+	}
+	if err != nil {
+		if errors.Is(err, fasthttp.ErrBodyTooLarge) {
+			err = types.ErrResponseBodyTooLarge
+		}
+		return nil, types.NewScriptHTTPRequestError(r.Method, r.URL, err)
+	}
+
+	body, err := decodeResponseBody(resp, c.maxResponseBody)
+	if err != nil {
+		return nil, types.NewScriptHTTPRequestError(r.Method, r.URL, err)
+	}
+
+	return &script.HTTPResponse{
+		Status:  resp.StatusCode(),
+		Headers: collectRespHeaders(resp),
+		Body:    string(body),
+	}, nil
+}
+
+// decodeResponseBody decodes a body the server compressed, since scripts read it as text.
+// The limit covers the decoded size, which is what a compression bomb inflates to.
+// An empty body is left alone because the decoders reject zero bytes, and a HEAD or a 304
+// carries the encoding with no body. A coding fasthttp does not decode, and the bare
+// deflate stream some servers send without a zlib header, are left raw for the script.
+func decodeResponseBody(resp *fasthttp.Response, maxResponseBody int) ([]byte, error) {
+	if len(resp.Header.ContentEncoding()) == 0 || len(resp.Body()) == 0 {
+		return resp.Body(), nil
+	}
+
+	decoded, err := resp.BodyUncompressedWithLimit(maxResponseBody)
+	if errors.Is(err, fasthttp.ErrContentEncodingUnsupported) || errors.Is(err, zlib.ErrHeader) {
+		return resp.Body(), nil
+	}
+	if err != nil {
+		if errors.Is(err, fasthttp.ErrBodyTooLarge) {
+			err = types.ErrResponseBodyTooLarge
+		}
+		return nil, err
+	}
+
+	// Neither header describes the decoded body, so both go, as net/http does on a gunzip.
+	resp.Header.SetContentEncoding("")
+	resp.Header.Del(fasthttp.HeaderContentLength)
+
+	return decoded, nil
+}
@@ -97,7 +97,7 @@ func NewDefaultTemplateFuncMap(randSource rand.Source, fileCache *FileCache) tem
 			return string(data), nil
 		},
 		// json_Object builds a JSON object from interleaved key-value pairs and returns it
-		// as a JSON string. Keys must be strings; values may be any JSON-encodable type.
+		// as a JSON string. Keys must be strings, values may be any JSON-encodable type.
 		// Usage: {{ json_Object "name" "Alice" "age" 30 }}
 		"json_Object": func(pairs ...any) (string, error) {
 			if len(pairs)%2 != 0 {
@@ -751,7 +751,7 @@ func NewDefaultBodyTemplateFuncMap(

 				switch {
 				case strings.HasPrefix(val, "@@"):
-					// Escaped @ - send as literal string without first @
+					// Escaped @: send the string without the first @
 					if err := writer.WriteField(key, val[1:]); err != nil {
 						return "", err
 					}
@@ -1,12 +1,16 @@
 package sarin

 import (
+	"errors"
+	"net"
 	"strconv"
+	"strings"
 	"sync/atomic"
 	"time"

 	"github.com/valyala/fasthttp"
 	"go.aykhans.me/sarin/internal/script"
+	"go.aykhans.me/sarin/internal/types"
 )

 const dryRunResponseKey = "dry-run"
@@ -29,9 +33,36 @@ func statusCodeToString(code int) string {
 	return strconv.Itoa(code)
 }

+// responseErrorKey drops the local address, whose port would mint a stats entry per connection.
+func responseErrorKey(err error) string {
+	if errors.Is(err, fasthttp.ErrBodyTooLarge) {
+		return types.ErrResponseBodyTooLarge.Error()
+	}
+
+	message := err.Error()
+
+	var opErr *net.OpError
+	if !errors.As(err, &opErr) || opErr.Source == nil {
+		return message
+	}
+
+	withoutSource := *opErr
+	withoutSource.Source = nil
+	return strings.Replace(message, opErr.Error(), withoutSource.Error(), 1)
+}
+
+// requestErrorKey keys a generation failure by its source instead of its message.
+// A script picks its own URLs and error texts, so the message would mint a new
+// stats entry per request. The full message still goes to the runtime log.
+func requestErrorKey(err error) string {
+	if chainErr, ok := errors.AsType[types.ScriptChainError](err); ok {
+		return chainErr.EngineType + " script[" + strconv.Itoa(chainErr.Index) + "] error"
+	}
+	return "request generation error"
+}
+
 func (s sarin) Worker(
 	jobs <-chan struct{},
-	hostClientGenerator HostClientGenerator,
 	counter *atomic.Uint64,
 	sendLog runtimeLogger,
 	sendRespLog respLogger,
@@ -53,6 +84,9 @@ func (s sarin) Worker(
 		defer scriptTransformer.Close()
 	}

+	nextClientIndex := newClientIndexGenerator(len(s.hostClients))
+	bindScriptProxy := s.newScriptProxyBinder(scriptTransformer)
+
 	requestGenerator, isDynamic := NewRequestGenerator(
 		s.methods, s.requestURL, s.params, s.headers, s.cookies, s.bodies, s.values, s.fileCache, scriptTransformer,
 	)
@@ -60,34 +94,45 @@ func (s sarin) Worker(
 	if s.dryRun {
 		switch {
 		case s.collectStats && isDynamic:
-			s.workerDryRunStatsWithDynamic(jobs, req, requestGenerator, counter, sendLog)
+			s.workerDryRunStatsWithDynamic(jobs, req, requestGenerator, nextClientIndex, bindScriptProxy, counter, sendLog)
 		case s.collectStats && !isDynamic:
 			s.workerDryRunStatsWithStatic(jobs, req, requestGenerator, counter, sendLog)
 		case !s.collectStats && isDynamic:
-			s.workerDryRunNoStatsWithDynamic(jobs, req, requestGenerator, counter, sendLog)
+			s.workerDryRunNoStatsWithDynamic(jobs, req, requestGenerator, nextClientIndex, bindScriptProxy, counter, sendLog)
 		default:
 			s.workerDryRunNoStatsWithStatic(jobs, req, requestGenerator, counter, sendLog)
 		}
 	} else {
 		switch {
 		case s.collectStats && isDynamic:
-			s.workerStatsWithDynamic(jobs, req, resp, requestGenerator, hostClientGenerator, counter, sendLog, sendRespLog)
+			s.workerStatsWithDynamic(jobs, req, resp, requestGenerator, nextClientIndex, bindScriptProxy, counter, sendLog, sendRespLog)
 		case s.collectStats && !isDynamic:
-			s.workerStatsWithStatic(jobs, req, resp, requestGenerator, hostClientGenerator, counter, sendLog, sendRespLog)
+			s.workerStatsWithStatic(jobs, req, resp, requestGenerator, nextClientIndex, counter, sendLog, sendRespLog)
 		case !s.collectStats && isDynamic:
-			s.workerNoStatsWithDynamic(jobs, req, resp, requestGenerator, hostClientGenerator, counter, sendLog, sendRespLog)
+			s.workerNoStatsWithDynamic(jobs, req, resp, requestGenerator, nextClientIndex, bindScriptProxy, counter, sendLog, sendRespLog)
 		default:
-			s.workerNoStatsWithStatic(jobs, req, resp, requestGenerator, hostClientGenerator, counter, sendLog, sendRespLog)
+			s.workerNoStatsWithStatic(jobs, req, resp, requestGenerator, nextClientIndex, counter, sendLog, sendRespLog)
 		}
 	}
 }

+// newScriptProxyBinder returns a func that sends the scripts' http.* calls through proxy i.
+func (s sarin) newScriptProxyBinder(scriptTransformer *script.Transformer) func(i int) {
+	if scriptTransformer == nil || scriptTransformer.IsEmpty() {
+		return func(int) {}
+	}
+	return func(i int) {
+		scriptTransformer.SetHTTPDoer(s.scriptHTTPClients[i])
+	}
+}
+
 func (s sarin) workerStatsWithDynamic(
 	jobs <-chan struct{},
 	req *fasthttp.Request,
 	resp *fasthttp.Response,
 	requestGenerator RequestGenerator,
-	hostClientGenerator HostClientGenerator,
+	nextClientIndex clientIndexGenerator,
+	bindScriptProxy func(int),
 	counter *atomic.Uint64,
 	sendLog runtimeLogger,
 	sendRespLog respLogger,
@@ -95,19 +140,22 @@ func (s sarin) workerStatsWithDynamic(
 	for range jobs {
 		req.Reset()

+		// Bound before generation so scripts' http.* calls use the main request's proxy.
+		clientIndex := nextClientIndex()
+		bindScriptProxy(clientIndex)
 		if err := requestGenerator(req); err != nil {
-			s.responses.Add(err.Error(), 0)
+			s.responses.Add(requestErrorKey(err), 0)
 			sendLog(runtimeLogLevelError, err.Error())
 			counter.Add(1)
 			continue
 		}

 		startTime := time.Now()
-		err := hostClientGenerator().DoTimeout(req, resp, s.timeout)
+		err := s.hostClients[clientIndex].DoTimeout(req, resp, s.timeout)
 		respDuration := time.Since(startTime)

 		if err != nil {
-			s.responses.Add(err.Error(), respDuration)
+			s.responses.Add(responseErrorKey(err), respDuration)
 		} else {
 			s.responses.Add(statusCodeToString(resp.StatusCode()), respDuration)
 			sendRespLog(respDuration, resp)
@@ -121,15 +169,16 @@ func (s sarin) workerStatsWithStatic(
 	req *fasthttp.Request,
 	resp *fasthttp.Response,
 	requestGenerator RequestGenerator,
-	hostClientGenerator HostClientGenerator,
+	nextClientIndex clientIndexGenerator,
 	counter *atomic.Uint64,
 	sendLog runtimeLogger,
 	sendRespLog respLogger,
 ) {
 	if err := requestGenerator(req); err != nil {
-		// Static request generation failed - record all jobs as errors
+		// Static request generation failed, so record all jobs as errors
+		errorKey := requestErrorKey(err)
 		for range jobs {
-			s.responses.Add(err.Error(), 0)
+			s.responses.Add(errorKey, 0)
 			sendLog(runtimeLogLevelError, err.Error())
 			counter.Add(1)
 		}
@@ -138,10 +187,10 @@ func (s sarin) workerStatsWithStatic(

 	for range jobs {
 		startTime := time.Now()
-		err := hostClientGenerator().DoTimeout(req, resp, s.timeout)
+		err := s.hostClients[nextClientIndex()].DoTimeout(req, resp, s.timeout)
 		respDuration := time.Since(startTime)
 		if err != nil {
-			s.responses.Add(err.Error(), respDuration)
+			s.responses.Add(responseErrorKey(err), respDuration)
 		} else {
 			s.responses.Add(statusCodeToString(resp.StatusCode()), respDuration)
 			sendRespLog(respDuration, resp)
@@ -155,20 +204,24 @@ func (s sarin) workerNoStatsWithDynamic(
 	req *fasthttp.Request,
 	resp *fasthttp.Response,
 	requestGenerator RequestGenerator,
-	hostClientGenerator HostClientGenerator,
+	nextClientIndex clientIndexGenerator,
+	bindScriptProxy func(int),
 	counter *atomic.Uint64,
 	sendLog runtimeLogger,
 	sendRespLog respLogger,
 ) {
 	for range jobs {
 		req.Reset()
+		// Bound before generation so scripts' http.* calls use the main request's proxy.
+		clientIndex := nextClientIndex()
+		bindScriptProxy(clientIndex)
 		if err := requestGenerator(req); err != nil {
 			sendLog(runtimeLogLevelError, err.Error())
 			counter.Add(1)
 			continue
 		}
 		startTime := time.Now()
-		err := hostClientGenerator().DoTimeout(req, resp, s.timeout)
+		err := s.hostClients[clientIndex].DoTimeout(req, resp, s.timeout)
 		if err == nil {
 			sendRespLog(time.Since(startTime), resp)
 		}
@@ -181,7 +234,7 @@ func (s sarin) workerNoStatsWithStatic(
 	req *fasthttp.Request,
 	resp *fasthttp.Response,
 	requestGenerator RequestGenerator,
-	hostClientGenerator HostClientGenerator,
+	nextClientIndex clientIndexGenerator,
 	counter *atomic.Uint64,
 	sendLog runtimeLogger,
 	sendRespLog respLogger,
@@ -189,7 +242,7 @@ func (s sarin) workerNoStatsWithStatic(
 	if err := requestGenerator(req); err != nil {
 		sendLog(runtimeLogLevelError, err.Error())

-		// Static request generation failed - just count the jobs without sending
+		// Static request generation failed, so just count the jobs without sending
 		for range jobs {
 			counter.Add(1)
 		}
@@ -198,7 +251,7 @@ func (s sarin) workerNoStatsWithStatic(

 	for range jobs {
 		startTime := time.Now()
-		err := hostClientGenerator().DoTimeout(req, resp, s.timeout)
+		err := s.hostClients[nextClientIndex()].DoTimeout(req, resp, s.timeout)
 		if err == nil {
 			sendRespLog(time.Since(startTime), resp)
 		}
@@ -210,14 +263,18 @@ func (s sarin) workerDryRunStatsWithDynamic(
 	jobs <-chan struct{},
 	req *fasthttp.Request,
 	requestGenerator RequestGenerator,
+	nextClientIndex clientIndexGenerator,
+	bindScriptProxy func(int),
 	counter *atomic.Uint64,
 	sendLog runtimeLogger,
 ) {
 	for range jobs {
 		req.Reset()
+		// Nothing is sent, this only binds a proxy for the scripts' http.* calls.
+		bindScriptProxy(nextClientIndex())
 		startTime := time.Now()
 		if err := requestGenerator(req); err != nil {
-			s.responses.Add(err.Error(), time.Since(startTime))
+			s.responses.Add(requestErrorKey(err), time.Since(startTime))
 			sendLog(runtimeLogLevelError, err.Error())
 			counter.Add(1)
 			continue
@@ -235,9 +292,10 @@ func (s sarin) workerDryRunStatsWithStatic(
 	sendLog runtimeLogger,
 ) {
 	if err := requestGenerator(req); err != nil {
-		// Static request generation failed - record all jobs as errors
+		// Static request generation failed, so record all jobs as errors
+		errorKey := requestErrorKey(err)
 		for range jobs {
-			s.responses.Add(err.Error(), 0)
+			s.responses.Add(errorKey, 0)
 			sendLog(runtimeLogLevelError, err.Error())
 			counter.Add(1)
 		}
@@ -254,11 +312,15 @@ func (s sarin) workerDryRunNoStatsWithDynamic(
 	jobs <-chan struct{},
 	req *fasthttp.Request,
 	requestGenerator RequestGenerator,
+	nextClientIndex clientIndexGenerator,
+	bindScriptProxy func(int),
 	counter *atomic.Uint64,
 	sendLog runtimeLogger,
 ) {
 	for range jobs {
 		req.Reset()
+		// Nothing is sent, this only binds a proxy for the scripts' http.* calls.
+		bindScriptProxy(nextClientIndex())
 		if err := requestGenerator(req); err != nil {
 			sendLog(runtimeLogLevelError, err.Error())
 		}
@@ -91,6 +91,16 @@ func (t *Transformer) Transform(req *RequestData) error {
 	return nil
 }

+// SetHTTPDoer sets the doer that the scripts' http.* calls are sent through.
+func (t *Transformer) SetHTTPDoer(doer HTTPDoer) {
+	for _, engine := range t.luaEngines {
+		engine.SetHTTPDoer(doer)
+	}
+	for _, engine := range t.jsEngines {
+		engine.SetHTTPDoer(doer)
+	}
+}
+
 // Close releases all engine resources.
 func (t *Transformer) Close() {
 	for _, engine := range t.luaEngines {
@@ -0,0 +1,133 @@
+package script
+
+import (
+	"math"
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// HTTPRequest is a request made by a script through the http.* functions.
+type HTTPRequest struct {
+	Method       string
+	URL          string
+	Headers      map[string][]string
+	Params       map[string][]string
+	Cookies      map[string][]string
+	Body         string
+	Timeout      time.Duration // zero means the doer's default timeout
+	Insecure     bool
+	MaxRedirects int
+}
+
+// HTTPResponse is the response returned to a script by the http.* functions.
+type HTTPResponse struct {
+	Status  int
+	Headers map[string][]string
+	Body    string
+}
+
+// Header returns the first value of the named header, matching the name case-insensitively.
+func (r *HTTPResponse) Header(name string) (string, bool) {
+	for key, values := range r.Headers {
+		if strings.EqualFold(key, name) && len(values) > 0 {
+			return values[0], true
+		}
+	}
+	return "", false
+}
+
+// Cookie returns the value of the named cookie from the Set-Cookie headers.
+func (r *HTTPResponse) Cookie(name string) (string, bool) {
+	for key, values := range r.Headers {
+		if !strings.EqualFold(key, "Set-Cookie") {
+			continue
+		}
+		for _, line := range values {
+			if cookie, err := http.ParseSetCookie(line); err == nil && cookie.Name == name {
+				return cookie.Value, true
+			}
+		}
+	}
+	return "", false
+}
+
+// HTTPDoer sends the HTTP requests that scripts make.
+type HTTPDoer interface {
+	Do(req *HTTPRequest) (*HTTPResponse, error)
+}
+
+// httpDefaultMethod is used when the options set no method.
+const httpDefaultMethod = http.MethodGet
+
+// httpMaxRedirectsLimit is the highest maxRedirects a script may ask for.
+const httpMaxRedirectsLimit = 100
+
+// Option keys accepted by the http function.
+const (
+	httpOptionMethod       = "method"
+	httpOptionHeaders      = "headers"
+	httpOptionParams       = "params"
+	httpOptionCookies      = "cookies"
+	httpOptionBody         = "body"
+	httpOptionTimeout      = "timeout"
+	httpOptionInsecure     = "insecure"
+	httpOptionMaxRedirects = "maxRedirects"
+)
+
+// httpBridge connects http to the doer. Calls only work while transform runs.
+type httpBridge struct {
+	doer   HTTPDoer
+	active bool
+}
+
+// It can return the following errors:
+//   - types.ErrScriptHTTPOutsideTransform
+//   - types.ErrScriptHTTPUnavailable
+//   - types.ErrScriptHTTPMethodEmpty
+//   - any error returned by the doer
+func (b *httpBridge) do(req *HTTPRequest) (*HTTPResponse, error) {
+	if !b.active {
+		return nil, types.ErrScriptHTTPOutsideTransform
+	}
+	if b.doer == nil {
+		return nil, types.ErrScriptHTTPUnavailable
+	}
+	if req.Method == "" {
+		return nil, types.ErrScriptHTTPMethodEmpty
+	}
+	return b.doer.Do(req)
+}
+
+// parseHTTPMaxRedirects checks that the redirect limit is a non-negative whole number.
+// It can return the following errors:
+//   - types.ScriptHTTPOptionError
+func parseHTTPMaxRedirects(count float64) (int, error) {
+	if math.IsNaN(count) || count < 0 || count > httpMaxRedirectsLimit || count != math.Trunc(count) {
+		return 0, types.NewScriptHTTPOptionError(
+			httpOptionMaxRedirects,
+			types.NewScriptTypeError(
+				"a whole number between 0 and "+strconv.Itoa(httpMaxRedirectsLimit),
+				strconv.FormatFloat(count, 'g', -1, 64),
+			),
+		)
+	}
+	return int(count), nil
+}
+
+// parseHTTPTimeout parses a Go duration string such as "500ms" or "2s".
+// It can return the following errors:
+//   - types.ScriptHTTPOptionError
+func parseHTTPTimeout(value string) (time.Duration, error) {
+	timeout, err := time.ParseDuration(value)
+	if err != nil {
+		return 0, types.NewScriptHTTPOptionError(httpOptionTimeout, err)
+	}
+	if timeout <= 0 {
+		return 0, types.NewScriptHTTPOptionError(httpOptionTimeout, types.NewScriptTypeError("a duration greater than 0", value))
+	}
+	return timeout, nil
+}
@@ -2,6 +2,9 @@ package script

 import (
 	"errors"
+	"fmt"
+	"reflect"
+	"strconv"

 	"github.com/dop251/goja"
 	"go.aykhans.me/sarin/internal/types"
@@ -11,8 +14,15 @@ import (
 type JsEngine struct {
 	runtime   *goja.Runtime
 	transform goja.Callable
+	jsonParse goja.Callable
+	// errorConstructor is the built-in Error, captured before the script can replace it.
+	errorConstructor goja.Value
+	http             httpBridge
 }

+// promiseType is what a transform returns when it is declared async or as a generator.
+var promiseType = reflect.TypeFor[*goja.Promise]()
+
 // NewJsEngine creates a new JavaScript script engine with the given script content.
 // The script must define a global `transform` function that takes a request object
 // and returns the modified request object.
@@ -29,11 +39,16 @@ type JsEngine struct {
 //   - types.ScriptExecutionError
 func NewJsEngine(scriptContent string) (*JsEngine, error) {
 	vm := goja.New()
+	engine := &JsEngine{runtime: vm}
+
+	// Register the globals before running the script so its functions can use them
+	if err := engine.registerHTTP(); err != nil {
+		return nil, types.NewScriptExecutionError("JavaScript", err)
+	}

 	// Execute the script to define the transform function
-	_, err := vm.RunString(scriptContent)
-	if err != nil {
-		return nil, types.NewScriptExecutionError("JavaScript", err)
+	if _, err := vm.RunString(scriptContent); err != nil {
+		return nil, types.NewScriptExecutionError("JavaScript", engine.exceptionError(err))
 	}

 	// Get the transform function
@@ -47,23 +62,43 @@ func NewJsEngine(scriptContent string) (*JsEngine, error) {
 		return nil, types.NewScriptExecutionError("JavaScript", errors.New("'transform' must be a function"))
 	}

-	return &JsEngine{
-		runtime:   vm,
-		transform: transform,
-	}, nil
+	// A generator returns an iterator, which is an ordinary object to Go, so the
+	// call cannot tell it apart and the declaration is the only thing left to read.
+	if isGeneratorFunction(vm, transformVal) {
+		return nil, types.ErrScriptTransformAsync
+	}
+
+	engine.transform = transform
+
+	return engine, nil
+}
+
+// SetHTTPDoer sets the doer that the script's http.* calls are sent through.
+func (e *JsEngine) SetHTTPDoer(doer HTTPDoer) {
+	e.http.doer = doer
 }

 // Transform executes the JavaScript transform function with the given request data.
 // It can return the following errors:
 //   - types.ScriptExecutionError
 func (e *JsEngine) Transform(req *RequestData) error {
+	e.http.active = true
+	defer func() { e.http.active = false }()
+
 	// Convert RequestData to JavaScript object
 	reqObj := e.requestDataToObject(req)

 	// Call transform(req)
 	result, err := e.transform(goja.Undefined(), reqObj)
 	if err != nil {
-		return types.NewScriptExecutionError("JavaScript", err)
+		return types.NewScriptExecutionError("JavaScript", e.exceptionError(err))
+	}
+
+	// An async or generator transform returns a promise nothing ever awaits, and a promise
+	// carries none of the request fields, so the request would go out untransformed.
+	// ExportType reads the type alone, while Export would run every getter on the result.
+	if result.ExportType() == promiseType {
+		return types.ErrScriptTransformAsync
 	}

 	// Update RequestData from the returned object
@@ -79,6 +114,8 @@ func (e *JsEngine) Close() {
 	// goja doesn't have an explicit close method, but we can help GC
 	e.runtime = nil
 	e.transform = nil
+	e.jsonParse = nil
+	e.errorConstructor = nil
 }

 // requestDataToObject converts RequestData to a goja Value (JavaScript object).
@@ -124,37 +161,70 @@ func (e *JsEngine) objectToRequestData(val goja.Value, req *RequestData) error {
 		return types.ErrScriptTransformReturnObject
 	}

-	// Method
-	if v := obj.Get("method"); v != nil && !goja.IsUndefined(v) {
-		req.Method = v.String()
+	// Reading a field runs the script's own getters and toString, which throw as a Go
+	// panic outside goja's execution context, so the whole read runs inside Try.
+	var fieldErr error
+	if thrown := e.runtime.Try(func() {
+		// Method
+		if v := obj.Get("method"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+			req.Method = v.String()
+		}
+
+		// Path
+		if v := obj.Get("path"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+			req.Path = v.String()
+		}
+
+		// Body
+		if v := obj.Get("body"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+			req.Body = v.String()
+		}
+
+		// Headers
+		if v := obj.Get("headers"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+			values, err := objectField("headers", v)
+			if err != nil {
+				fieldErr = err
+				return
+			}
+			req.Headers = e.objectToStringSliceMap(values)
+		}
+
+		// Params
+		if v := obj.Get("params"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+			values, err := objectField("params", v)
+			if err != nil {
+				fieldErr = err
+				return
+			}
+			req.Params = e.objectToStringSliceMap(values)
+		}
+
+		// Cookies
+		if v := obj.Get("cookies"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+			values, err := objectField("cookies", v)
+			if err != nil {
+				fieldErr = err
+				return
+			}
+			req.Cookies = e.objectToStringSliceMap(values)
+		}
+	}); thrown != nil {
+		return e.exceptionError(thrown)
 	}

-	// Path
-	if v := obj.Get("path"); v != nil && !goja.IsUndefined(v) {
-		req.Path = v.String()
-	}
+	return fieldErr
+}

-	// Body
-	if v := obj.Get("body"); v != nil && !goja.IsUndefined(v) {
-		req.Body = v.String()
+// objectField rejects anything that is not a plain object, since reading keys off an
+// array or a boxed string yields headers named after indexes and off a Date yields none.
+// A Map, a Set and a typed array still pass, so the field is emptied rather than refused.
+func objectField(name string, value goja.Value) (*goja.Object, error) {
+	obj, ok := value.(*goja.Object)
+	if !ok || obj.ClassName() != "Object" {
+		return nil, fmt.Errorf("%s must be an object", name)
 	}
-
-	// Headers
-	if v := obj.Get("headers"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
-		req.Headers = e.objectToStringSliceMap(v.ToObject(e.runtime))
-	}
-
-	// Params
-	if v := obj.Get("params"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
-		req.Params = e.objectToStringSliceMap(v.ToObject(e.runtime))
-	}
-
-	// Cookies
-	if v := obj.Get("cookies"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
-		req.Cookies = e.objectToStringSliceMap(v.ToObject(e.runtime))
-	}
-
-	return nil
+	return obj, nil
 }

 // stringSliceToArray converts a Go []string to a JavaScript array.
@@ -176,23 +246,78 @@ func (e *JsEngine) objectToStringSliceMap(obj *goja.Object) map[string][]string
 	result := make(map[string][]string)
 	for _, key := range obj.Keys() {
 		v := obj.Get(key)
-		if v == nil || goja.IsUndefined(v) || goja.IsNull(v) {
-			continue
-		}

 		// Check if it's an array
-		if arr, ok := v.Export().([]any); ok {
-			var values []string
-			for _, item := range arr {
-				if s, ok := item.(string); ok {
-					values = append(values, s)
+		if arr, ok := v.(*goja.Object); ok && arr.ClassName() == "Array" {
+			length := int(arr.Get("length").ToInteger())
+			values := make([]string, 0, length)
+			for i := range length {
+				if text, ok := jsStringValue(arr.Get(strconv.Itoa(i))); ok {
+					values = append(values, text)
 				}
 			}
 			result[key] = values
-		} else {
-			// Single value - wrap in slice
-			result[key] = []string{v.String()}
+			continue
+		}
+
+		// Single value, wrap it in a slice
+		if text, ok := jsStringValue(v); ok {
+			result[key] = []string{text}
 		}
 	}
 	return result
 }
+
+// exceptionError keeps the thrown value's message without goja's stack frame.
+// Stringifying runs through Runtime.Try because a thrown value with no usable
+// toString makes goja panic outside its own execution context.
+func (e *JsEngine) exceptionError(err error) error {
+	var exception *goja.Exception
+	if !errors.As(err, &exception) {
+		return err
+	}
+
+	var message string
+	if thrown := e.runtime.Try(func() { message = exception.Value().String() }); thrown != nil {
+		message = "unprintable thrown value"
+	}
+	return errors.New(message)
+}
+
+// jsStringValue renders a JavaScript primitive the way the script would see it.
+// Objects, arrays and functions have no useful text form, so they are skipped.
+func jsStringValue(v goja.Value) (string, bool) {
+	if v == nil || goja.IsUndefined(v) || goja.IsNull(v) {
+		return "", false
+	}
+	if _, isObject := v.(*goja.Object); isObject {
+		return "", false
+	}
+	return v.String(), true
+}
+
+// isGeneratorFunction reports whether a function is declared with a star.
+// The name comes from the script's own prototype chain, so a script can falsify it either way.
+// Reading it can run a script getter, so the whole walk stays inside Try.
+func isGeneratorFunction(runtime *goja.Runtime, value goja.Value) bool {
+	var name string
+	thrown := runtime.Try(func() {
+		obj := value.ToObject(runtime)
+		if obj == nil {
+			return
+		}
+		constructor := obj.Get("constructor")
+		if constructor == nil || goja.IsUndefined(constructor) || goja.IsNull(constructor) {
+			return
+		}
+		constructorObj := constructor.ToObject(runtime)
+		if constructorObj == nil {
+			return
+		}
+		if value := constructorObj.Get("name"); value != nil {
+			name = value.String()
+		}
+	})
+
+	return thrown == nil && name == "GeneratorFunction"
+}
@@ -0,0 +1,220 @@
+package script
+
+import (
+	"errors"
+
+	"github.com/dop251/goja"
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// registerHTTP defines the global http function.
+func (e *JsEngine) registerHTTP() error {
+	rt := e.runtime
+
+	jsonParse, ok := goja.AssertFunction(rt.Get("JSON").ToObject(rt).Get("parse"))
+	if !ok {
+		return errors.New("JSON.parse is not available")
+	}
+	e.jsonParse = jsonParse
+	e.errorConstructor = rt.Get("Error")
+
+	return rt.Set("http", func(call goja.FunctionCall) goja.Value {
+		return e.httpCall(call.Argument(0), call.Argument(1))
+	})
+}
+
+func (e *JsEngine) httpCall(urlValue, optsValue goja.Value) goja.Value {
+	rt := e.runtime
+
+	url, ok := urlValue.Export().(string)
+	if !ok {
+		panic(rt.NewTypeError("http: url must be a string, got %s", jsTypeName(urlValue)))
+	}
+	req := &HTTPRequest{Method: httpDefaultMethod, URL: url}
+
+	if !goja.IsUndefined(optsValue) && !goja.IsNull(optsValue) {
+		opts, ok := optsValue.(*goja.Object)
+		if !ok {
+			panic(rt.NewTypeError("http: options must be an object, got %s", jsTypeName(optsValue)))
+		}
+		if err := e.parseHTTPOptions(opts, req); err != nil {
+			e.throw(err)
+		}
+	}
+
+	resp, err := e.http.do(req)
+	if err != nil {
+		e.throw(err)
+	}
+
+	return e.httpResponseToObject(resp)
+}
+
+// parseHTTPOptions fills req from a JavaScript options object.
+// It can return the following errors:
+//   - types.ScriptHTTPOptionError
+func (e *JsEngine) parseHTTPOptions(opts *goja.Object, req *HTTPRequest) error {
+	for _, name := range opts.Keys() {
+		value := opts.Get(name)
+		if goja.IsUndefined(value) {
+			continue
+		}
+
+		switch name {
+		case httpOptionMethod:
+			method, ok := value.Export().(string)
+			if !ok {
+				return types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("string", jsTypeName(value)))
+			}
+
+			req.Method = method
+		case httpOptionHeaders, httpOptionParams, httpOptionCookies:
+			object, ok := value.(*goja.Object)
+			if !ok {
+				return types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("object", jsTypeName(value)))
+			}
+
+			values := e.objectToStringSliceMap(object)
+			switch name {
+			case httpOptionHeaders:
+				req.Headers = values
+			case httpOptionParams:
+				req.Params = values
+			default:
+				req.Cookies = values
+			}
+		case httpOptionBody:
+			body, ok := value.Export().(string)
+			if !ok {
+				return types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("string", jsTypeName(value)))
+			}
+
+			req.Body = body
+		case httpOptionTimeout:
+			timeout, ok := value.Export().(string)
+			if !ok {
+				return types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("duration string", jsTypeName(value)))
+			}
+
+			var err error
+			if req.Timeout, err = parseHTTPTimeout(timeout); err != nil {
+				return err
+			}
+		case httpOptionInsecure:
+			flag, ok := value.Export().(bool)
+			if !ok {
+				return types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("boolean", jsTypeName(value)))
+			}
+
+			req.Insecure = flag
+		case httpOptionMaxRedirects:
+			count, ok := numberValue(value)
+			if !ok {
+				return types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("number", jsTypeName(value)))
+			}
+
+			var err error
+			if req.MaxRedirects, err = parseHTTPMaxRedirects(count); err != nil {
+				return err
+			}
+		default:
+			return types.NewScriptHTTPOptionError(name, types.ErrScriptHTTPUnknownOption)
+		}
+	}
+	return nil
+}
+
+// httpResponseToObject converts a response to a JavaScript object.
+func (e *JsEngine) httpResponseToObject(resp *HTTPResponse) *goja.Object {
+	rt := e.runtime
+	obj := rt.NewObject()
+
+	headers := rt.NewObject()
+	for key, values := range resp.Headers {
+		_ = headers.Set(key, e.stringSliceToArray(values))
+	}
+
+	lookup := func(find func(string) (string, bool)) func(goja.FunctionCall) goja.Value {
+		return func(call goja.FunctionCall) goja.Value {
+			name, ok := call.Argument(0).Export().(string)
+			if !ok {
+				panic(rt.NewTypeError("name must be a string, got %s", jsTypeName(call.Argument(0))))
+			}
+			if value, ok := find(name); ok {
+				return rt.ToValue(value)
+			}
+			return goja.Null()
+		}
+	}
+
+	_ = obj.Set("status", resp.Status)
+	_ = obj.Set("headers", headers)
+	_ = obj.Set("body", resp.Body)
+	_ = obj.Set("header", lookup(resp.Header))
+	_ = obj.Set("cookie", lookup(resp.Cookie))
+	_ = obj.Set("json", func(goja.FunctionCall) goja.Value {
+		value, err := e.jsonParse(goja.Undefined(), rt.ToValue(resp.Body))
+		if err != nil {
+			panic(err)
+		}
+		return value
+	})
+	// body is a JavaScript string, so bytes that are not valid UTF-8 reach the script
+	// only through this.
+	_ = obj.Set("bytes", func(goja.FunctionCall) goja.Value {
+		buffer := rt.NewArrayBuffer([]byte(resp.Body))
+		value, err := rt.New(rt.Get("Uint8Array").ToObject(rt), rt.ToValue(buffer))
+		if err != nil {
+			panic(err)
+		}
+		return value
+	})
+
+	return obj
+}
+
+// jsTypeName returns a short type name for error messages.
+func jsTypeName(value goja.Value) string {
+	switch {
+	case value == nil || goja.IsUndefined(value):
+		return "undefined"
+	case goja.IsNull(value):
+		return "null"
+	}
+	switch value.Export().(type) {
+	case string:
+		return "string"
+	case bool:
+		return "boolean"
+	case int64, float64:
+		return "number"
+	case []any:
+		return "array"
+	default:
+		if _, ok := goja.AssertFunction(value); ok {
+			return "function"
+		}
+		return "object"
+	}
+}
+
+// throw raises err as a plain JavaScript Error instead of a GoError.
+func (e *JsEngine) throw(err error) {
+	errorValue, newErr := e.runtime.New(e.errorConstructor, e.runtime.ToValue(err.Error()))
+	if newErr != nil {
+		panic(e.runtime.NewGoError(err))
+	}
+	panic(errorValue)
+}
+
+// numberValue reports whether value is a JavaScript number, and returns it.
+func numberValue(value goja.Value) (float64, bool) {
+	switch number := value.Export().(type) {
+	case int64:
+		return float64(number), true
+	case float64:
+		return number, true
+	default:
+		return 0, false
+	}
+}
@@ -11,6 +11,7 @@ import (
 type LuaEngine struct {
 	state     *lua.LState
 	transform *lua.LFunction
+	http      httpBridge
 }

 // NewLuaEngine creates a new Lua script engine with the given script content.
@@ -29,6 +30,11 @@ type LuaEngine struct {
 //   - types.ScriptExecutionError
 func NewLuaEngine(scriptContent string) (*LuaEngine, error) {
 	L := lua.NewState()
+	engine := &LuaEngine{state: L}
+
+	// Register the globals before running the script so its functions can use them
+	engine.registerHTTP()
+	registerLuaJSON(L)

 	// Execute the script to define the transform function
 	if err := L.DoString(scriptContent); err != nil {
@@ -42,17 +48,23 @@ func NewLuaEngine(scriptContent string) (*LuaEngine, error) {
 		L.Close()
 		return nil, types.ErrScriptTransformMissing
 	}
+	engine.transform = transform.(*lua.LFunction)

-	return &LuaEngine{
-		state:     L,
-		transform: transform.(*lua.LFunction),
-	}, nil
+	return engine, nil
+}
+
+// SetHTTPDoer sets the doer that the script's http.* calls are sent through.
+func (e *LuaEngine) SetHTTPDoer(doer HTTPDoer) {
+	e.http.doer = doer
 }

 // Transform executes the Lua transform function with the given request data.
 // It can return the following errors:
 //   - types.ScriptExecutionError
 func (e *LuaEngine) Transform(req *RequestData) error {
+	e.http.active = true
+	defer func() { e.http.active = false }()
+
 	// Convert RequestData to Lua table
 	reqTable := e.requestDataToTable(req)

@@ -93,39 +105,23 @@ func (e *LuaEngine) requestDataToTable(req *RequestData) *lua.LTable {
 	t.RawSetString("path", lua.LString(req.Path))
 	t.RawSetString("body", lua.LString(req.Body))

-	// Headers (map[string][]string -> table of arrays)
-	headers := L.NewTable()
-	for k, values := range req.Headers {
-		arr := L.NewTable()
+	t.RawSetString("headers", stringSliceMapToTable(L, req.Headers))
+	t.RawSetString("params", stringSliceMapToTable(L, req.Params))
+	t.RawSetString("cookies", stringSliceMapToTable(L, req.Cookies))
+
+	return t
+}
+
+// stringSliceMapToTable converts a Go map[string][]string to a Lua table of arrays.
+func stringSliceMapToTable(state *lua.LState, m map[string][]string) *lua.LTable {
+	t := state.NewTable()
+	for k, values := range m {
+		arr := state.NewTable()
 		for _, v := range values {
 			arr.Append(lua.LString(v))
 		}
-		headers.RawSetString(k, arr)
+		t.RawSetString(k, arr)
 	}
-	t.RawSetString("headers", headers)
-
-	// Params (map[string][]string -> table of arrays)
-	params := L.NewTable()
-	for k, values := range req.Params {
-		arr := L.NewTable()
-		for _, v := range values {
-			arr.Append(lua.LString(v))
-		}
-		params.RawSetString(k, arr)
-	}
-	t.RawSetString("params", params)
-
-	// Cookies (map[string][]string -> table of arrays)
-	cookies := L.NewTable()
-	for k, values := range req.Cookies {
-		arr := L.NewTable()
-		for _, v := range values {
-			arr.Append(lua.LString(v))
-		}
-		cookies.RawSetString(k, arr)
-	}
-	t.RawSetString("cookies", cookies)
-
 	return t
 }

@@ -172,20 +168,33 @@ func (e *LuaEngine) tableToStringSliceMap(t *lua.LTable) map[string][]string {
 		}
 		key := string(k.(lua.LString))

-		switch v.Type() {
-		case lua.LTString:
-			// Single string value
-			result[key] = []string{string(v.(lua.LString))}
-		case lua.LTTable:
-			// Array of strings
-			var values []string
-			v.(*lua.LTable).ForEach(func(_, item lua.LValue) {
-				if item.Type() == lua.LTString {
-					values = append(values, string(item.(lua.LString)))
+		if v.Type() == lua.LTTable {
+			// Array of values, keys outside 1..n are skipped
+			arr := v.(*lua.LTable)
+			length := arr.Len()
+			values := make([]string, 0, length)
+			for i := 1; i <= length; i++ {
+				if text, ok := luaScalarString(arr.RawGetInt(i)); ok {
+					values = append(values, text)
 				}
-			})
+			}
 			result[key] = values
+			return
+		}
+
+		if text, ok := luaScalarString(v); ok {
+			result[key] = []string{text}
 		}
 	})
 	return result
 }
+
+// luaScalarString renders strings, numbers and booleans as text.
+func luaScalarString(v lua.LValue) (string, bool) {
+	switch v.Type() {
+	case lua.LTString, lua.LTNumber, lua.LTBool:
+		return v.String(), true
+	default:
+		return "", false
+	}
+}
@@ -0,0 +1,142 @@
+package script
+
+import (
+	lua "github.com/yuin/gopher-lua"
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// registerHTTP defines the global http function.
+func (e *LuaEngine) registerHTTP() {
+	L := e.state
+	L.SetGlobal("http", L.NewFunction(func(L *lua.LState) int {
+		return e.httpCall(L, 1)
+	}))
+}
+
+// httpCall sends the request whose URL is at stack position urlIndex and pushes the response.
+func (e *LuaEngine) httpCall(state *lua.LState, urlIndex int) int {
+	req := &HTTPRequest{
+		Method: httpDefaultMethod,
+		URL:    state.CheckString(urlIndex),
+	}
+
+	switch opts := state.Get(urlIndex + 1).(type) {
+	case *lua.LNilType:
+	case *lua.LTable:
+		if err := e.parseHTTPOptions(opts, req); err != nil {
+			state.RaiseError("%s", err.Error())
+		}
+	default:
+		state.ArgError(urlIndex+1, "options must be a table")
+	}
+
+	resp, err := e.http.do(req)
+	if err != nil {
+		state.RaiseError("%s", err.Error())
+	}
+
+	state.Push(httpResponseToTable(state, resp))
+	return 1
+}
+
+// parseHTTPOptions fills req from a Lua options table.
+// It can return the following errors:
+//   - types.ScriptHTTPOptionError
+func (e *LuaEngine) parseHTTPOptions(opts *lua.LTable, req *HTTPRequest) error {
+	var err error
+	opts.ForEach(func(k, v lua.LValue) {
+		if err != nil {
+			return
+		}
+
+		key, ok := k.(lua.LString)
+		if !ok {
+			err = types.NewScriptHTTPOptionError(k.String(), types.ErrScriptHTTPUnknownOption)
+			return
+		}
+
+		switch name := string(key); name {
+		case httpOptionMethod:
+			method, ok := v.(lua.LString)
+			if !ok {
+				err = types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("string", v.Type().String()))
+				return
+			}
+			req.Method = string(method)
+		case httpOptionHeaders, httpOptionParams, httpOptionCookies:
+			table, ok := v.(*lua.LTable)
+			if !ok {
+				err = types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("table", v.Type().String()))
+				return
+			}
+			values := e.tableToStringSliceMap(table)
+			switch name {
+			case httpOptionHeaders:
+				req.Headers = values
+			case httpOptionParams:
+				req.Params = values
+			default:
+				req.Cookies = values
+			}
+		case httpOptionBody:
+			body, ok := v.(lua.LString)
+			if !ok {
+				err = types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("string", v.Type().String()))
+				return
+			}
+			req.Body = string(body)
+		case httpOptionTimeout:
+			timeout, ok := v.(lua.LString)
+			if !ok {
+				err = types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("duration string", v.Type().String()))
+				return
+			}
+			req.Timeout, err = parseHTTPTimeout(string(timeout))
+		case httpOptionInsecure:
+			flag, ok := v.(lua.LBool)
+			if !ok {
+				err = types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("boolean", v.Type().String()))
+				return
+			}
+			req.Insecure = bool(flag)
+		case httpOptionMaxRedirects:
+			count, ok := v.(lua.LNumber)
+			if !ok {
+				err = types.NewScriptHTTPOptionError(name, types.NewScriptTypeError("number", v.Type().String()))
+				return
+			}
+			req.MaxRedirects, err = parseHTTPMaxRedirects(float64(count))
+		default:
+			err = types.NewScriptHTTPOptionError(name, types.ErrScriptHTTPUnknownOption)
+		}
+	})
+	return err
+}
+
+// httpResponseToTable converts a response to a Lua table.
+func httpResponseToTable(state *lua.LState, resp *HTTPResponse) *lua.LTable {
+	t := state.NewTable()
+	t.RawSetString("status", lua.LNumber(resp.Status))
+	t.RawSetString("headers", stringSliceMapToTable(state, resp.Headers))
+	t.RawSetString("body", lua.LString(resp.Body))
+
+	lookup := func(find func(string) (string, bool)) *lua.LFunction {
+		return state.NewFunction(func(L *lua.LState) int {
+			// Last argument, so both res:header() and res.header() work.
+			top := L.GetTop()
+			if top == 0 {
+				L.ArgError(1, "name expected")
+			}
+			if value, ok := find(L.CheckString(top)); ok {
+				L.Push(lua.LString(value))
+			} else {
+				L.Push(lua.LNil)
+			}
+			return 1
+		})
+	}
+	t.RawSetString("header", lookup(resp.Header))
+	t.RawSetString("cookie", lookup(resp.Cookie))
+
+	return t
+}
@@ -0,0 +1,159 @@
+package script
+
+import (
+	"bytes"
+	"encoding/json"
+	"math"
+	"strings"
+
+	lua "github.com/yuin/gopher-lua"
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// registerLuaJSON defines the global json table with encode and decode.
+func registerLuaJSON(state *lua.LState) {
+	module := state.NewTable()
+	module.RawSetString("encode", state.NewFunction(luaJSONEncode))
+	module.RawSetString("decode", state.NewFunction(luaJSONDecode))
+	state.SetGlobal("json", module)
+}
+
+func luaJSONEncode(state *lua.LState) int {
+	value, err := luaToGo(state.CheckAny(1), make(map[*lua.LTable]bool))
+	if err != nil {
+		state.RaiseError("%s", types.NewScriptJSONError("encode", err).Error())
+	}
+
+	var buf bytes.Buffer
+	encoder := json.NewEncoder(&buf)
+	encoder.SetEscapeHTML(false)
+	if err := encoder.Encode(value); err != nil {
+		state.RaiseError("%s", types.NewScriptJSONError("encode", err).Error())
+	}
+
+	state.Push(lua.LString(strings.TrimSuffix(buf.String(), "\n")))
+	return 1
+}
+
+func luaJSONDecode(state *lua.LState) int {
+	var value any
+	if err := json.Unmarshal([]byte(state.CheckString(1)), &value); err != nil {
+		state.RaiseError("%s", types.NewScriptJSONError("decode", err).Error())
+	}
+
+	state.Push(goToLua(state, value))
+	return 1
+}
+
+// luaJSONMaxDepth matches the depth encoding/json accepts, so a table a script
+// decoded always encodes again, and it stays far below what overflows the Go stack.
+const luaJSONMaxDepth = 10000
+
+// luaToGo converts a Lua value to a JSON-encodable Go value.
+// It can return the following errors:
+//   - types.ErrScriptJSONCycle
+//   - types.ErrScriptJSONDepth
+//   - types.ScriptTypeError
+func luaToGo(value lua.LValue, visiting map[*lua.LTable]bool) (any, error) {
+	switch v := value.(type) {
+	case *lua.LNilType:
+		return nil, nil
+	case lua.LBool:
+		return bool(v), nil
+	case lua.LNumber:
+		return float64(v), nil
+	case lua.LString:
+		return string(v), nil
+	case *lua.LTable:
+		if visiting[v] {
+			return nil, types.ErrScriptJSONCycle
+		}
+		// visiting holds the tables on the current path, so its size is the depth.
+		// A table millions of levels deep overflows the Go stack, which is a fatal
+		// error no pcall can catch, so the descent is bounded well before that.
+		if len(visiting) >= luaJSONMaxDepth {
+			return nil, types.ErrScriptJSONDepth
+		}
+		visiting[v] = true
+		defer delete(visiting, v)
+
+		if length, ok := luaArrayLength(v); ok {
+			array := make([]any, length)
+			for i := range length {
+				item, err := luaToGo(v.RawGetInt(i+1), visiting)
+				if err != nil {
+					return nil, err
+				}
+				array[i] = item
+			}
+			return array, nil
+		}
+
+		object := make(map[string]any)
+		var err error
+		v.ForEach(func(key, item lua.LValue) {
+			if err != nil {
+				return
+			}
+			switch key.(type) {
+			case lua.LString, lua.LNumber:
+			default:
+				err = types.NewScriptTypeError("string or number object key", key.Type().String())
+				return
+			}
+			object[key.String()], err = luaToGo(item, visiting)
+		})
+		if err != nil {
+			return nil, err
+		}
+		return object, nil
+	default:
+		return nil, types.NewScriptTypeError("nil, boolean, number, string or table", value.Type().String())
+	}
+}
+
+// luaArrayLength reports whether t is a non-empty sequence with keys exactly 1..n.
+func luaArrayLength(t *lua.LTable) (int, bool) {
+	count := 0
+	isArray := true
+	maxKey := 0.0
+	t.ForEach(func(key, _ lua.LValue) {
+		count++
+		number, ok := key.(lua.LNumber)
+		if !ok || float64(number) < 1 || float64(number) != math.Trunc(float64(number)) {
+			isArray = false
+			return
+		}
+		maxKey = max(maxKey, float64(number))
+	})
+	if !isArray || count == 0 || maxKey != float64(count) {
+		return 0, false
+	}
+	return count, true
+}
+
+// goToLua converts a value produced by encoding/json to a Lua value.
+func goToLua(state *lua.LState, value any) lua.LValue {
+	switch v := value.(type) {
+	case bool:
+		return lua.LBool(v)
+	case float64:
+		return lua.LNumber(v)
+	case string:
+		return lua.LString(v)
+	case []any:
+		t := state.CreateTable(len(v), 0)
+		for i, item := range v {
+			t.RawSetInt(i+1, goToLua(state, item))
+		}
+		return t
+	case map[string]any:
+		t := state.CreateTable(0, len(v))
+		for key, item := range v {
+			t.RawSetString(key, goToLua(state, item))
+		}
+		return t
+	default:
+		return lua.LNil
+	}
+}
@@ -31,6 +31,9 @@ type Engine interface {
 	// The script should modify the RequestData and return it.
 	Transform(req *RequestData) error

+	// SetHTTPDoer sets the doer that the script's http.* calls are sent through.
+	SetHTTPDoer(doer HTTPDoer)
+
 	// Close releases any resources held by the engine.
 	Close()
 }
@@ -69,7 +72,7 @@ func LoadSource(ctx context.Context, source string, engineType EngineType) (*Sou

 	switch {
 	case strings.HasPrefix(source, "@@"):
-		// Escaped @ - it's an inline script starting with literal @
+		// Escaped @: an inline script starting with a literal @
 		content = source[1:] // Remove first @, keep the rest
 	case strings.HasPrefix(source, "@"):
 		// File or URL reference
@@ -125,7 +128,7 @@ func ValidateScript(ctx context.Context, source string, engineType EngineType) e
 		return err
 	}

-	// Try to create an engine - this validates syntax and transform function
+	// Creating an engine validates the syntax and the transform function
 	var engine Engine
 	switch engineType {
 	case EngineTypeLua:
@@ -140,7 +143,7 @@ func ValidateScript(ctx context.Context, source string, engineType EngineType) e
 		return err
 	}

-	// Clean up the engine - we only needed it for validation
+	// The engine was only needed for validation
 	engine.Close()
 	return nil
 }
@@ -168,10 +171,18 @@ func fetchURL(ctx context.Context, url string) (string, error) {
 		return "", types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
 	}

-	data, err := io.ReadAll(resp.Body)
+	// A declared size above the limit is refused before anything is read.
+	if resp.ContentLength > types.RemoteFetchLimit {
+		return "", types.NewHTTPFetchError(url, types.ErrRemoteFileTooLarge)
+	}
+
+	data, err := io.ReadAll(io.LimitReader(resp.Body, types.RemoteFetchLimit+1))
 	if err != nil {
 		return "", types.NewHTTPFetchError(url, err)
 	}
+	if len(data) > types.RemoteFetchLimit {
+		return "", types.NewHTTPFetchError(url, types.ErrRemoteFileTooLarge)
+	}

 	return string(data), nil
 }
@@ -0,0 +1,87 @@
+package types
+
+import (
+	"math"
+	"strconv"
+	"strings"
+)
+
+// ByteSizeLimit is the largest size any option may ask for. A limit is also the most
+// memory one response may take, so it stays well under what a machine can hold.
+const ByteSizeLimit = 1 << 30 // 1 GiB
+
+// RemoteFetchLimit bounds a file sarin downloads for itself, such as a config file or a script.
+const RemoteFetchLimit = 64 << 20 // 64 MiB
+
+// byteSizeUnits run from the largest scale down, so formatting picks the largest unit
+// that fits. A bare "K" is out: the common libraries disagree on whether it counts
+// 1000 or 1024 bytes.
+var byteSizeUnits = []struct {
+	suffix string
+	scale  uint64
+}{
+	{"TiB", 1 << 40},
+	{"TB", 1000 * 1000 * 1000 * 1000},
+	{"GiB", 1 << 30},
+	{"GB", 1000 * 1000 * 1000},
+	{"MiB", 1 << 20},
+	{"MB", 1000 * 1000},
+	{"KiB", 1 << 10},
+	{"KB", 1000},
+	{"B", 1},
+}
+
+// ParseByteSize reads a byte count, with or without a unit suffix.
+// It can return the following errors:
+//   - ErrByteSizeInvalid
+func ParseByteSize(value string) (uint64, error) {
+	text := strings.TrimSpace(value)
+
+	// The suffix is split off by index, so no case mapping can shift the digits.
+	digitsEnd := len(text)
+	for digitsEnd > 0 && isASCIILetter(text[digitsEnd-1]) {
+		digitsEnd--
+	}
+
+	scale := uint64(1)
+	if suffix := text[digitsEnd:]; suffix != "" {
+		unitScale, ok := byteSizeScale(suffix)
+		if !ok {
+			return 0, ErrByteSizeInvalid
+		}
+		scale = unitScale
+	}
+
+	size, err := strconv.ParseUint(strings.TrimSpace(text[:digitsEnd]), 10, 64)
+	if err != nil {
+		return 0, ErrByteSizeInvalid
+	}
+	if size > math.MaxUint64/scale {
+		return 0, ErrByteSizeInvalid
+	}
+	return size * scale, nil
+}
+
+// FormatByteSize renders a byte count so that ParseByteSize reads back the same value.
+func FormatByteSize(size uint64) string {
+	for _, unit := range byteSizeUnits {
+		if unit.scale > 1 && size >= unit.scale && size%unit.scale == 0 {
+			return strconv.FormatUint(size/unit.scale, 10) + unit.suffix
+		}
+	}
+	return strconv.FormatUint(size, 10)
+}
+
+// byteSizeScale looks up a unit suffix, ignoring case.
+func byteSizeScale(suffix string) (uint64, bool) {
+	for _, unit := range byteSizeUnits {
+		if strings.EqualFold(suffix, unit.suffix) {
+			return unit.scale, true
+		}
+	}
+	return 0, false
+}
+
+func isASCIILetter(char byte) bool {
+	return (char >= 'a' && char <= 'z') || (char >= 'A' && char <= 'Z')
+}
@@ -10,6 +10,17 @@ import (

 var (
 	errNoError = errors.New("no error (internal)")
+
+	ErrByteSizeInvalid = errors.New(
+		"invalid value for size, expected a whole number of bytes or a whole number with a unit: " +
+			"B, KB, MB, GB, TB (1000 based) or KiB, MiB, GiB, TiB (1024 based), e.g. '1048576', '10MiB', '2MB'",
+	)
+
+	ErrRemoteFileTooLarge = errors.New("remote file exceeds " + FormatByteSize(RemoteFetchLimit))
+
+	ErrResponseBodyTooLarge = errors.New("response body exceeds the body limit")
+
+	ErrYAMLMultipleDocuments = errors.New("expected a single YAML document, but found more than one")
 )

 type FieldParseError struct {
@@ -297,6 +308,27 @@ func (e CLIUnexpectedArgsError) Error() string {

 // ======================================== Config File ========================================

+// ConfigFileError names the file a parse error came from, which nested configFile chains lose.
+type ConfigFileError struct {
+	Path  string
+	error error
+}
+
+func NewConfigFileError(path string, err error) ConfigFileError {
+	if err == nil {
+		err = errNoError
+	}
+	return ConfigFileError{path, err}
+}
+
+func (e ConfigFileError) Error() string {
+	return e.error.Error()
+}
+
+func (e ConfigFileError) Unwrap() error {
+	return e.error
+}
+
 type ConfigFileReadError struct {
 	error error
 }
@@ -318,6 +350,8 @@ func (e ConfigFileReadError) Unwrap() error {

 // ======================================== Proxy ========================================

+var ErrProxyNoContextDialer = errors.New("SOCKS5 dialer does not support timeouts")
+
 type ProxyUnsupportedSchemeError struct {
 	Scheme string
 }
@@ -400,9 +434,91 @@ var (
 	ErrScriptSourceEmpty           = errors.New("script source cannot be empty after @")
 	ErrScriptTransformMissing      = errors.New("script must define a global 'transform' function")
 	ErrScriptTransformReturnObject = errors.New("transform function must return an object")
+	ErrScriptTransformAsync        = errors.New("transform function must be synchronous, async and generator functions are not supported")
 	ErrScriptURLNoHost             = errors.New("script URL must have a host")
+	ErrScriptHTTPOutsideTransform  = errors.New("http functions can only be called while transform is running")
+	ErrScriptHTTPUnavailable       = errors.New("http client is not available")
+	ErrScriptHTTPMethodEmpty       = errors.New("http method cannot be empty")
+	ErrScriptHTTPURLInvalid        = errors.New("URL must be an absolute http or https URL")
+	ErrScriptHTTPUnknownOption     = errors.New("unknown option")
+	ErrScriptJSONCycle             = errors.New("cannot encode a table that contains itself")
+	ErrScriptJSONDepth             = errors.New("cannot encode a table nested too deeply")
 )

+type ScriptTypeError struct {
+	Expected string
+	Got      string
+}
+
+func NewScriptTypeError(expected, got string) ScriptTypeError {
+	return ScriptTypeError{expected, got}
+}
+
+func (e ScriptTypeError) Error() string {
+	return fmt.Sprintf("expected %s, got %s", e.Expected, e.Got)
+}
+
+type ScriptHTTPOptionError struct {
+	Option string
+	Err    error
+}
+
+func NewScriptHTTPOptionError(option string, err error) ScriptHTTPOptionError {
+	if err == nil {
+		err = errNoError
+	}
+	return ScriptHTTPOptionError{option, err}
+}
+
+func (e ScriptHTTPOptionError) Error() string {
+	return fmt.Sprintf("http option %q: %v", e.Option, e.Err)
+}
+
+func (e ScriptHTTPOptionError) Unwrap() error {
+	return e.Err
+}
+
+type ScriptHTTPRequestError struct {
+	Method string
+	URL    string
+	Err    error
+}
+
+func NewScriptHTTPRequestError(method, url string, err error) ScriptHTTPRequestError {
+	if err == nil {
+		err = errNoError
+	}
+	return ScriptHTTPRequestError{method, url, err}
+}
+
+func (e ScriptHTTPRequestError) Error() string {
+	return fmt.Sprintf("http %s %s: %v", e.Method, e.URL, e.Err)
+}
+
+func (e ScriptHTTPRequestError) Unwrap() error {
+	return e.Err
+}
+
+type ScriptJSONError struct {
+	Op  string
+	Err error
+}
+
+func NewScriptJSONError(op string, err error) ScriptJSONError {
+	if err == nil {
+		err = errNoError
+	}
+	return ScriptJSONError{op, err}
+}
+
+func (e ScriptJSONError) Error() string {
+	return fmt.Sprintf("json.%s: %v", e.Op, e.Err)
+}
+
+func (e ScriptJSONError) Unwrap() error {
+	return e.Err
+}
+
 type ScriptLoadError struct {
 	Source string
 	Err    error
@@ -8,11 +8,11 @@

 (buildGoModule.override { go = go_1_27; }) (finalAttrs: {
  pname = "sarin";
-  version = "1.4.4"; # bump per release
+  version = "1.5.0"; # bump per release

  src = lib.cleanSource ../.;

-  vendorHash = "sha256-77wMBIhsXUwZ97iAaJivB7qOuQhBZ1LJphnRWfUTjJo=";
+  vendorHash = "sha256-SpMOMmDa7llS8wbCDf6QJbWtZ/s8RgzMvVP1xrY934s=";

  subPackages = [ "cmd/cli" ];
Author	SHA1	Message	Date
aykhans	a58ef60316	Merge pull request #232 from aykhans/feat/script-http feat: add http function to script engines	2026-10-04 22:12:38 +04:00
aykhans	5b11ef2933	Merge remote-tracking branch 'origin/main' into feat/script-http # Conflicts: # nix/package.nix	2026-10-04 22:10:02 +04:00
aykhans	3e01ce47cc	nix: update vendorHash	2026-10-04 22:04:01 +04:00
aykhans	95f55e1056	nix: bump package version to 1.5.0	2026-10-04 21:59:36 +04:00
aykhans	824f03452e	chore(deps): update dependencies	2026-10-04 21:58:53 +04:00
aykhans	b7c589de53	fix(client): prefer IPv4 and fall back to IPv6 instead of dialing both	2026-10-04 21:50:18 +04:00
aykhans	23b3d7b1cc	fix: dial dual-stack only for IPv6-only hosts	2026-10-04 15:51:50 +04:00
aykhans	ab3b458f15	docs: fix statements that did not match the code	2026-10-02 02:05:49 +04:00
aykhans	51e8f2a57f	fix(script): match the main request default for the script http timeout	2026-10-02 01:13:25 +04:00
aykhans	a1e4c15285	docs: say what the dry run duration measures	2026-10-02 00:12:50 +04:00
aykhans	bea48eeab0	docs: say that a dry run still sends a script's own requests	2026-10-01 22:43:24 +04:00
aykhans	cd0fa453c2	fix(script): bound how deep json.encode walks a Lua table	2026-10-01 22:13:29 +04:00
aykhans	847c4b39ce	fix(script): decode a compressed http() response body	2026-10-01 21:17:43 +04:00
aykhans	9e773a4e6c	fix(script): reject a non-object headers, params or cookies in JavaScript	2026-09-30 21:18:50 +04:00
aykhans	bcf2ea3446	fix(script): reject async/generator JS transforms and catch throwing getters	2026-09-30 19:53:37 +04:00
aykhans	e85a6d4613	docs: drop the redirect login example, it cannot work (#235 )	2026-09-30 12:59:16 +04:00
aykhans	4a14d57d15	fix(config): reject a hidden YAML document and drop Go types from errors	2026-09-30 12:29:00 +04:00
aykhans	633f548e7b	fix(config): report YAML config errors more clearly	2026-09-30 01:07:28 +04:00
aykhans	68de415645	fix(config): reject unknown fields in a YAML config file	2026-09-30 00:14:51 +04:00
aykhans	fb8443fd25	chore: read response headers into a 64KiB buffer instead of 4KiB	2026-09-29 23:40:57 +04:00
aykhans	e0f6d4aeba	fix: send an idempotent request once instead of retrying it five times	2026-09-29 23:22:51 +04:00
aykhans	17dfa16051	feat(script): add res.bytes() so JavaScript can read a binary body	2026-09-29 23:22:51 +04:00
aykhans	a327c4978b	fix: report one error text when a response body is too large	2026-09-29 23:06:22 +04:00
aykhans	9d6e4e6bf8	chore(template): raise the remote file timeout to 30s	2026-09-29 22:59:42 +04:00
aykhans	9c7b7c26a0	refactor(script): drop the per-call maxBodySize and its client cache	2026-09-29 10:58:25 +04:00
aykhans	ef5907dc0c	fix: cap body limits at 1GiB and key script clients by the exact size	2026-09-29 00:55:04 +04:00
aykhans	1956eb1c44	fix(script): enforce script body limits without streaming Streaming accepted a chunked body cut at a chunk boundary as complete, so the limit now comes from the client that reads the body, picked per requested size. Script requests keep their own default, maxBodySize accepts sizes like 10MiB, and files sarin downloads for itself are capped at 64MiB.	2026-09-29 00:41:48 +04:00
aykhans	f1d91e3d77	feat: limit response body sizes fasthttp allocates the body from the declared Content-Length before reading it, so a server could exhaust memory by declaring a size it never sends. The new max response body config bounds every response, defaults to 10MiB and accepts sizes like 10MiB or 2MB. Scripts get the same as a maxBodySize option on http().	2026-09-28 22:49:39 +04:00
aykhans	412a02fea3	chore: bump golangci-lint to v2.14.0	2026-09-27 01:05:07 +04:00
aykhans	8da05bb10c	fix(stats): key generation failures by source and bound the key count	2026-09-27 01:03:04 +04:00
aykhans	c32949c8e0	fix(script): convert non-string header, param and cookie values to text	2026-09-26 01:17:49 +04:00
aykhans	a30c862936	fix(script): bind script HTTP requests to the run context	2026-09-25 22:34:20 +04:00
aykhans	5d91141f80	fix(script): stringify thrown JavaScript values through Runtime.Try	2026-09-24 22:34:59 +04:00
aykhans	878ca6dfbd	fix(script): bound maxRedirects to 100 and reject out of range values	2026-09-24 19:08:58 +04:00
aykhans	9b0e7c4304	fix: dial IPv6 hosts by enabling dual-stack and trying every resolved address	2026-09-23 01:03:42 +04:00
aykhans	d4c7dcc42c	fix: apply per-request timeouts to script HTTP proxy dials	2026-09-23 00:45:44 +04:00
aykhans	00226b0eb7	fix: size script HTTP client connection pool to worker count	2026-09-22 19:29:51 +04:00
aykhans	a60287b78b	docs: document http() and Lua json in scripts	2026-09-20 23:30:16 +04:00
aykhans	529420ee5a	feat(script): replace followRedirects with maxRedirects http option	2026-09-20 23:18:21 +04:00
aykhans	8b437b487c	feat(script): add http() and json to Lua and JS scripts	2026-09-20 23:02:51 +04:00
aykhans	5ca9cda2b9	docs(cli): show config-file flag type as []string in help text	2026-09-19 00:14:48 +04:00
aykhans	6b03ba9579	refactor(sarin): extract cookie header building into cookieHeaderValue	2026-09-18 22:39:56 +04:00