refactor(script): drop the per-call maxBodySize and its client cache

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.

.github/workflows/lint.yaml

@@ -24,4 +24,4 @@ jobs:
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
-          version: v2.13.2
+          version: v2.14.0

README.md

@@ -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:

Taskfile.yaml

@@ -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:

cmd/cli/main.go

@@ -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:

docs/configuration.md

@@ -15,7 +15,7 @@ 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     |
@@ -23,6 +23,7 @@ Use `-s` or `--show-config` to see the final merged configuration before sending
 | [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                 |
@@ -168,6 +169,18 @@ 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 `body size exceeds the given 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.
+
 ## Concurrency
 
 Number of concurrent workers. Must be between 1 and 100,000,000. Defaults to `1`.
@@ -456,7 +469,9 @@ 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 +536,9 @@ 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 +571,101 @@ 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"`) | `30s`   | 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                           |
+
+In Lua, call the helpers as `res:header("name")` or `res.header("name")`.
+
+**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.
+- **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;
+}
+```

docs/examples.md

@@ -1087,3 +1087,154 @@ 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;
+    }
+```
+
+**Follow redirects and use a slower timeout for a login endpoint:**
+
+```sh
+sarin -U http://example.com/api/me -r 200 -c 5 \
+  -lua 'function transform(req)
+    local res = http("http://example.com/login", {
+      method = "POST",
+      maxRedirects = 5,
+      timeout = "10s",
+    })
+    req.cookies["session"] = res:cookie("session")
+    return req
+  end'
+```
+
+**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;
+  }'
+```

docs/templating.md

@@ -521,7 +521,7 @@ 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"`                  |

internal/config/bytesize.go

@@ -0,0 +1,49 @@
+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 accepts both a YAML integer and a string with a unit.
+func (size *byteSize) UnmarshalYAML(node *yaml.Node) error {
+	if node.Kind != yaml.ScalarNode {
+		return fmt.Errorf("expected a size, but got %s", node.Tag)
+	}
+
+	if node.Tag == "!!int" {
+		var parsed uint64
+		if err := node.Decode(&parsed); err != nil {
+			return types.ErrByteSizeInvalid
+		}
+		*size = byteSize(parsed)
+		return nil
+	}
+
+	return size.Set(node.Value)
+}

internal/config/cli.go

@@ -23,7 +23,7 @@ Flags:
     -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)
+    -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)
@@ -43,6 +43,7 @@ Flags:
     -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)`
@@ -106,6 +107,7 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 		proxies         = stringSliceArg{}
 		values          = stringSliceArg{}
 		timeout         time.Duration
+		maxResponseBody byteSize
 		insecure        bool
 		luaScripts      = stringSliceArg{}
 		jsScripts       = stringSliceArg{}
@@ -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,
 	)
 }

internal/config/config.go

@@ -30,6 +30,7 @@ var Defaults = struct {
 	UserAgent       string
 	Method          string
 	RequestTimeout  time.Duration
+	MaxResponseBody uint64
 	Concurrency     uint
 	ShowConfig      bool
 	Progress        ConfigProgressType
@@ -41,6 +42,7 @@ var Defaults = struct {
 	UserAgent:       "Sarin/" + version.Version,
 	Method:          "GET",
 	RequestTimeout:  time.Second * 10,
+	MaxResponseBody: 10 << 20, // 10 MiB
 	Concurrency:     1,
 	ShowConfig:      false,
 	Progress:        ConfigProgressTypeBar,
@@ -87,6 +89,7 @@ type Config struct {
 	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"`
@@ -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")))
 	}
@@ -715,7 +735,7 @@ 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 +745,7 @@ 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 +764,7 @@ 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 +842,7 @@ 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)

internal/config/env.go

@@ -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 {

internal/config/file.go

@@ -79,10 +79,18 @@ 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
 }
@@ -211,6 +219,7 @@ type configYAML struct {
 	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"`
@@ -290,6 +299,9 @@ 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...)

internal/config/template_validator.go

@@ -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)
 

internal/sarin/client.go

@@ -5,27 +5,19 @@ 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
-	}
-	return int(u)
-}
 
 // NewHostClients creates a list of fasthttp.HostClient instances for the given proxies.
 // If no proxies are provided, a single client without a proxy is returned.
@@ -38,6 +30,7 @@ func NewHostClients(
 	maxConns uint,
 	requestURL *url.URL,
 	skipVerify bool,
+	maxResponseBody uint64,
 ) ([]*fasthttp.HostClient, error) {
 	isTLS := requestURL.Scheme == "https"
 
@@ -66,6 +59,7 @@ func NewHostClients(
 				MaxConnDuration:               timeout,
 				WriteTimeout:                  timeout,
 				ReadTimeout:                   timeout,
+				MaxResponseBodySize:           safeUint64ToInt(maxResponseBody),
 				DisableHeaderNamesNormalizing: true,
 				DisablePathNormalizing:        true,
 				NoDefaultUserAgentHeader:      true,
@@ -79,6 +73,7 @@ func NewHostClients(
 	client := &fasthttp.HostClient{
 		MaxConns:      safeUintToInt(maxConns),
 		IsTLS:         isTLS,
+		DialDualStack: true,
 		TLSConfig: &tls.Config{
 			InsecureSkipVerify: skipVerify, //nolint:gosec
 		},
@@ -87,6 +82,7 @@ func NewHostClients(
 		MaxConnDuration:               timeout,
 		WriteTimeout:                  timeout,
 		ReadTimeout:                   timeout,
+		MaxResponseBodySize:           safeUint64ToInt(maxResponseBody),
 		DisableHeaderNamesNormalizing: true,
 		DisablePathNormalizing:        true,
 		NoDefaultUserAgentHeader:      true,
@@ -94,40 +90,45 @@ 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 (
+	dial, err := NewProxyDialFuncWithTimeout(ctx, proxyURL)
-		dialer fasthttp.DialFunc
+	if err != nil {
-		err    error
+		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)
+		return fasthttpSocksDialer(ctx, proxyURL, true)
-		if err != nil {
-			return nil, err
-		}
 	case "socks5h":
-		dialer, err = fasthttpSocksDialerDualStackTimeout(ctx, proxyURL, timeout, false)
+		return fasthttpSocksDialer(ctx, proxyURL, false)
-		if err != nil {
-			return nil, err
-		}
 	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 +150,92 @@ 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 nil, types.ErrProxyNoContextDialer
-		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 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))
 		}
 
-			// Use the first resolved IP
+		// Try every resolved address, so one unreachable family does not fail the dial.
-			addr = net.JoinHostPort(ips[0].String(), port)
+		var lastErr error
-		}
+		for _, candidate := range addrs {
-
-		// 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", candidate)
-
+			dialCancel()
-		conn, err := contextDialer.DialContext(dialCtx, "tcp", addr)
+			if err == nil {
-		if err != nil {
-			return nil, types.NewProxyDialError(proxyStr, err)
-		}
 				return conn, nil
+			}
+			lastErr = err
+		}
+		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
@@ -222,7 +249,7 @@ 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)
@@ -242,7 +269,7 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 
-		// Upgrade to TLS
+		if useTLS {
 			tlsConn := tls.Client(conn, &tls.Config{
 				ServerName: proxyURL.Hostname(),
 			})
@@ -250,6 +277,8 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 				tlsConn.Close() //nolint:errcheck,gosec
 				return nil, types.NewProxyDialError(proxyStr, err)
 			}
+			conn = tlsConn
+		}
 
 		// Build and send CONNECT request
 		connectReq := &http.Request{
@@ -262,35 +291,35 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 			connectReq.Header.Set("Proxy-Authorization", proxyAuth)
 		}
 
-		if err := connectReq.Write(tlsConn); err != nil {
+		if err := connectReq.Write(conn); err != nil {
-			tlsConn.Close() //nolint:errcheck,gosec
+			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 {
+		if err := conn.SetDeadline(time.Time{}); err != nil {
-			tlsConn.Close() //nolint:errcheck,gosec
+			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 +335,10 @@ func (c *bufferedConn) Read(b []byte) (int, error) {
 	return c.reader.Read(b)
 }
 
-func NewHostClientGenerator(clients ...*fasthttp.HostClient) HostClientGenerator {
+func newClientIndexGenerator(count int) clientIndexGenerator {
-	switch len(clients) {
+	indexes := make([]int, count)
-	case 0:
+	for i := range indexes {
-		hostClient := &fasthttp.HostClient{}
+		indexes[i] = i
-		return func() *fasthttp.HostClient {
-			return hostClient
-		}
-	case 1:
-		return func() *fasthttp.HostClient {
-			return clients[0]
-		}
-	default:
-		return utilsSlice.RandomCycle(nil, clients...)
 	}
+	return utilsSlice.RandomCycle(nil, indexes...)
 }

internal/sarin/filecache.go

@@ -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
+	// If another goroutine cached it first, LoadOrStore returns theirs
-	// cached it first, we get theirs (no duplicate storage)
 	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)

internal/sarin/helpers.go

@@ -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)
+}

internal/sarin/request.go

@@ -191,8 +191,18 @@ func applyRequestDataToFastHTTP(reqData *script.RequestData, req *fasthttp.Reque
 	}
 
 	if len(reqData.Cookies) > 0 {
+		req.Header.Add("Cookie", cookieHeaderValue(reqData.Cookies))
+	}
+
+	if scheme == "https" {
+		req.URI().SetScheme("https")
+	}
+}
+
+// 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 reqData.Cookies {
+	for k, values := range cookies {
 		for _, v := range values {
 			if sb.Len() > 0 {
 				sb.WriteString("; ")
@@ -202,12 +212,7 @@ func applyRequestDataToFastHTTP(reqData *script.RequestData, req *fasthttp.Reque
 			sb.WriteString(v)
 		}
 	}
-		req.Header.Add("Cookie", sb.String())
+	return sb.String()
-	}
-
-	if scheme == "https" {
-		req.URI().SetScheme("https")
-	}
 }
 
 func NewMethodGeneratorFunc(localRand *rand.Rand, methods []string, lazyRoot func() *template.Template) (requestDataGenerator, bool) {

internal/sarin/response.go

@@ -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{

internal/sarin/runner.go

@@ -199,7 +199,12 @@ type sarin struct {
 	logError       bool
 	logFile        string
 
+	// 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
@@ -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
 	}
@@ -266,7 +277,20 @@ 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{
+		jobsCtx:           jobsCtx,
+		jobsCancel:        jobsCancel,
 		workers:           workers,
 		requestURL:        requestURL,
 		methods:           methods,
@@ -286,6 +310,7 @@ func NewSarin(
 		logError:          logError,
 		logFile:           logFile,
 		hostClients:       hostClients,
+		scriptHTTPClients: scriptHTTPClients,
 		fileCache:         NewFileCache(time.Second * 10),
 		scriptChain:       scriptChain,
 	}
@@ -301,9 +326,7 @@ func (s sarin) GetResponses() *SarinResponseData {
 	return s.responses
 }
 
-func (s sarin) Start(ctx context.Context, stopCtrl *StopController) {
+func (s sarin) Start(stopCtrl *StopController) {
-	jobsCtx, jobsCancel := context.WithCancel(ctx)
-
 	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.
+func (s sarin) startWorkers(wg *sync.WaitGroup, jobs <-chan struct{}, counter *atomic.Uint64, sendLog runtimeLogger, sendRespLog respLogger) {
-// 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) {
 	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()
+				s.jobsCancel()
-			case <-ctx.Done():
+			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{}{}
 		}
 	}

internal/sarin/scripthttp.go

@@ -0,0 +1,174 @@
+package sarin
+
+import (
+	"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, independent of -T.
+const scriptHTTPDefaultTimeout = 30 * 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
+}
+
+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, fasthttp.DialDualStackTimeout, 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),
+			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,
+	}
+}
+
+// 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.ErrScriptHTTPBodyTooLarge
+		}
+		return nil, types.NewScriptHTTPRequestError(r.Method, r.URL, err)
+	}
+
+	body := resp.Body()
+
+	return &script.HTTPResponse{
+		Status:  resp.StatusCode(),
+		Headers: collectRespHeaders(resp),
+		Body:    string(body),
+	}, nil
+}

internal/sarin/template.go

@@ -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
 					}

internal/sarin/worker.go

@@ -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 +33,32 @@ 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 {
+	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 +80,9 @@ 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 +90,45 @@ 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 +136,22 @@ 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 +165,16 @@ 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 +183,10 @@ 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 +200,24 @@ 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 +230,7 @@ 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 +238,7 @@ 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 +247,7 @@ 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 +259,18 @@ 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 +288,10 @@ 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 +308,15 @@ 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())
 		}

internal/script/chain.go

@@ -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 {

internal/script/http.go

@@ -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
+}

internal/script/js.go

@@ -2,6 +2,7 @@ package script
 
 import (
 	"errors"
+	"strconv"
 
 	"github.com/dop251/goja"
 	"go.aykhans.me/sarin/internal/types"
@@ -11,6 +12,10 @@ 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
 }
 
 // NewJsEngine creates a new JavaScript script engine with the given script content.
@@ -29,11 +34,17 @@ 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)
+		return nil, types.NewScriptExecutionError("JavaScript", engine.exceptionError(err))
 	}
 
 	// Get the transform function
@@ -47,23 +58,30 @@ func NewJsEngine(scriptContent string) (*JsEngine, error) {
 		return nil, types.NewScriptExecutionError("JavaScript", errors.New("'transform' must be a function"))
 	}
 
-	return &JsEngine{
+	engine.transform = transform
-		runtime:   vm,
+
-		transform: transform,
+	return engine, nil
-	}, 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))
 	}
 
 	// Update RequestData from the returned object
@@ -79,6 +97,8 @@ 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).
@@ -176,23 +196,52 @@ 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 {
+		if arr, ok := v.(*goja.Object); ok && arr.ClassName() == "Array" {
-			var values []string
+			length := int(arr.Get("length").ToInteger())
-			for _, item := range arr {
+			values := make([]string, 0, length)
-				if s, ok := item.(string); ok {
+			for i := range length {
-					values = append(values, s)
+				if text, ok := jsStringValue(arr.Get(strconv.Itoa(i))); ok {
+					values = append(values, text)
 				}
 			}
 			result[key] = values
-		} else {
+			continue
-			// Single value - wrap in slice
+		}
-			result[key] = []string{v.String()}
+
+		// 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
+}

internal/script/js_http.go

@@ -0,0 +1,210 @@
+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
+	})
+
+	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
+	}
+}

internal/script/lua.go

@@ -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{
+	return engine, nil
-		state:     L,
+}
-		transform: transform.(*lua.LFunction),
+
-	}, 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)
+	t.RawSetString("headers", stringSliceMapToTable(L, req.Headers))
-	headers := L.NewTable()
+	t.RawSetString("params", stringSliceMapToTable(L, req.Params))
-	for k, values := range req.Headers {
+	t.RawSetString("cookies", stringSliceMapToTable(L, req.Cookies))
-		arr := L.NewTable()
+
+	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() {
+		if v.Type() == lua.LTTable {
-		case lua.LTString:
+			// Array of values, keys outside 1..n are skipped
-			// Single string value
+			arr := v.(*lua.LTable)
-			result[key] = []string{string(v.(lua.LString))}
+			length := arr.Len()
-		case lua.LTTable:
+			values := make([]string, 0, length)
-			// Array of strings
+			for i := 1; i <= length; i++ {
-			var values []string
+				if text, ok := luaScalarString(arr.RawGetInt(i)); ok {
-			v.(*lua.LTable).ForEach(func(_, item lua.LValue) {
+					values = append(values, text)
-				if item.Type() == lua.LTString {
+				}
-					values = append(values, string(item.(lua.LString)))
 			}
-			})
 			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
+	}
+}

internal/script/lua_http.go

@@ -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
+}

internal/script/lua_json.go

@@ -0,0 +1,148 @@
+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
+}
+
+// luaToGo converts a Lua value to a JSON-encodable Go value.
+// It can return the following errors:
+//   - types.ErrScriptJSONCycle
+//   - 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[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
+	}
+}

internal/script/script.go

@@ -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
 }

internal/types/byte_size.go

@@ -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')
+}

internal/types/errors.go

@@ -10,6 +10,13 @@ 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))
 )
 
 type FieldParseError struct {
@@ -318,6 +325,8 @@ func (e ConfigFileReadError) Unwrap() error {
 
 // ======================================== Proxy ========================================
 
+var ErrProxyNoContextDialer = errors.New("SOCKS5 dialer does not support timeouts")
+
 type ProxyUnsupportedSchemeError struct {
 	Scheme string
 }
@@ -401,8 +410,89 @@ var (
 	ErrScriptTransformMissing      = errors.New("script must define a global 'transform' function")
 	ErrScriptTransformReturnObject = errors.New("transform function must return an object")
 	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")
+	ErrScriptHTTPBodyTooLarge      = errors.New("response body exceeds the body limit")
+	ErrScriptJSONCycle             = errors.New("cannot encode a table that contains itself")
 )
 
+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