Merge pull request #171 from aykhans/dependabot/go_modules/github.com/charmbracelet/bubbles-1.0.0

Bump github.com/charmbracelet/bubbles from 0.21.1 to 1.0.0

.github/workflows/lint.yaml

@@ -16,8 +16,12 @@ jobs:
       - uses: actions/checkout@v5
       - uses: actions/setup-go@v6
         with:
-          go-version: 1.25.5
+          go-version: 1.26.0
+      - name: go fix
+        run: |
+          go fix ./...
+          git diff --exit-code
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
-          version: v2.7.2
+          version: v2.9.0

.github/workflows/release.yaml

@@ -35,7 +35,7 @@ jobs:
         run: |
           echo "VERSION=$(git describe --tags --always)" >> $GITHUB_ENV
           echo "GIT_COMMIT=$(git rev-parse HEAD)" >> $GITHUB_ENV
-          echo "GO_VERSION=1.25.5" >> $GITHUB_ENV
+          echo "GO_VERSION=1.26.0" >> $GITHUB_ENV
 
       - name: Set up Go
         if: github.event_name == 'release' || inputs.build_binaries
@@ -53,12 +53,12 @@ jobs:
                    -X 'go.aykhans.me/sarin/internal/version.GoVersion=$(go version)' \
                    -s -w"
 
-          CGO_ENABLED=0 GOEXPERIMENT=greenteagc GOOS=linux   GOARCH=amd64 go build -ldflags "$LDFLAGS" -o ./sarin-linux-amd64 ./cmd/cli/main.go
-          CGO_ENABLED=0 GOEXPERIMENT=greenteagc GOOS=linux   GOARCH=arm64 go build -ldflags "$LDFLAGS" -o ./sarin-linux-arm64 ./cmd/cli/main.go
-          CGO_ENABLED=0 GOEXPERIMENT=greenteagc GOOS=darwin  GOARCH=amd64 go build -ldflags "$LDFLAGS" -o ./sarin-darwin-amd64 ./cmd/cli/main.go
-          CGO_ENABLED=0 GOEXPERIMENT=greenteagc GOOS=darwin  GOARCH=arm64 go build -ldflags "$LDFLAGS" -o ./sarin-darwin-arm64 ./cmd/cli/main.go
-          CGO_ENABLED=0 GOEXPERIMENT=greenteagc GOOS=windows GOARCH=amd64 go build -ldflags "$LDFLAGS" -o ./sarin-windows-amd64.exe ./cmd/cli/main.go
-          CGO_ENABLED=0 GOEXPERIMENT=greenteagc GOOS=windows GOARCH=arm64 go build -ldflags "$LDFLAGS" -o ./sarin-windows-arm64.exe ./cmd/cli/main.go
+          CGO_ENABLED=0 GOOS=linux   GOARCH=amd64 go build -ldflags "$LDFLAGS" -o ./sarin-linux-amd64 ./cmd/cli/main.go
+          CGO_ENABLED=0 GOOS=linux   GOARCH=arm64 go build -ldflags "$LDFLAGS" -o ./sarin-linux-arm64 ./cmd/cli/main.go
+          CGO_ENABLED=0 GOOS=darwin  GOARCH=amd64 go build -ldflags "$LDFLAGS" -o ./sarin-darwin-amd64 ./cmd/cli/main.go
+          CGO_ENABLED=0 GOOS=darwin  GOARCH=arm64 go build -ldflags "$LDFLAGS" -o ./sarin-darwin-arm64 ./cmd/cli/main.go
+          CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build -ldflags "$LDFLAGS" -o ./sarin-windows-amd64.exe ./cmd/cli/main.go
+          CGO_ENABLED=0 GOOS=windows GOARCH=arm64 go build -ldflags "$LDFLAGS" -o ./sarin-windows-arm64.exe ./cmd/cli/main.go
 
       - name: Upload Release Assets
         if: github.event_name == 'release' || inputs.build_binaries

.golangci.yaml

@@ -1,7 +1,7 @@
 version: "2"
 
 run:
-  go: "1.25"
+  go: "1.26"
   concurrency: 12
 
 linters:

Dockerfile

@@ -1,4 +1,4 @@
-ARG GO_VERSION=1.25.5
+ARG GO_VERSION=1.26.0
 
 FROM docker.io/library/golang:${GO_VERSION}-alpine AS builder
 
@@ -12,7 +12,7 @@ RUN --mount=type=bind,source=./go.mod,target=./go.mod \
     go mod download
 
 RUN --mount=type=bind,source=./,target=./ \
-    CGO_ENABLED=0 GOEXPERIMENT=greenteagc go build \
+    CGO_ENABLED=0 go build \
     -ldflags "-X 'go.aykhans.me/sarin/internal/version.Version=${VERSION}' \
     -X 'go.aykhans.me/sarin/internal/version.GitCommit=${GIT_COMMIT}' \
     -X 'go.aykhans.me/sarin/internal/version.BuildDate=$(date -u +%Y-%m-%dT%H:%M:%SZ)' \

README.md

@@ -22,13 +22,14 @@
 
 Sarin is designed for efficient HTTP load testing with minimal resource consumption. It prioritizes simplicity—features like templating add zero overhead when unused.
 
-| ✅ Supported                                               | ❌ Not Supported                  |
-| ---------------------------------------------------------- | --------------------------------- |
-| High-performance with low memory footprint                 | Detailed response body analysis   |
-| Long-running duration/count based tests                    | Extensive response statistics     |
-| Dynamic requests via 320+ template functions               | Web UI or complex TUI             |
-| Multiple proxy protocols<br>(HTTP, HTTPS, SOCKS5, SOCKS5H) | Scripting or multi-step scenarios |
-| Flexible config (CLI, ENV, YAML)                           | HTTP/2, HTTP/3, WebSocket, gRPC   |
+| ✅ Supported                                               | ❌ Not Supported                |
+| ---------------------------------------------------------- | ------------------------------- |
+| High-performance with low memory footprint                 | Detailed response body analysis |
+| Long-running duration/count based tests                    | Extensive response statistics   |
+| Dynamic requests via 320+ template functions               | Web UI or complex TUI           |
+| Request scripting with Lua and JavaScript                  | Distributed load testing        |
+| Multiple proxy protocols<br>(HTTP, HTTPS, SOCKS5, SOCKS5H) | HTTP/2, HTTP/3, WebSocket, gRPC |
+| Flexible config (CLI, ENV, YAML)                           | Plugins / extensions ecosystem  |
 
 ## Installation
 
@@ -56,12 +57,12 @@ Download the latest binaries from the [releases](https://github.com/aykhans/sari
 
 ### Building from Source
 
-Requires [Go 1.25+](https://golang.org/dl/).
+Requires [Go 1.26+](https://golang.org/dl/).
 
 ```sh
 git clone https://github.com/aykhans/sarin.git && cd sarin
 
-CGO_ENABLED=0 GOEXPERIMENT=greenteagc go build \
+CGO_ENABLED=0 go build \
     -ldflags "-X 'go.aykhans.me/sarin/internal/version.Version=dev' \
     -X 'go.aykhans.me/sarin/internal/version.GitCommit=$(git rev-parse HEAD)' \
     -X 'go.aykhans.me/sarin/internal/version.BuildDate=$(date -u +%Y-%m-%dT%H:%M:%SZ)' \

Taskfile.yaml

@@ -3,7 +3,7 @@ version: "3"
 
 vars:
   BIN_DIR: ./bin
-  GOLANGCI_LINT_VERSION: v2.7.2
+  GOLANGCI_LINT_VERSION: v2.9.0
   GOLANGCI: "{{.BIN_DIR}}/golangci-lint-{{.GOLANGCI_LINT_VERSION}}"
 
 tasks:
@@ -11,16 +11,22 @@ tasks:
     desc: Run fmt, tidy, and lint.
     cmds:
       - task: fmt
+      - task: fix
       - task: tidy
       - task: lint
 
   fmt:
-    desc: Run linters
+    desc: Run format
     deps:
       - install-golangci-lint
     cmds:
       - "{{.GOLANGCI}} fmt"
 
+  fix:
+    desc: Run go fix
+    cmds:
+      - go fix ./...
+
   tidy:
     desc: Run go mod tidy.
     cmds:
@@ -52,7 +58,7 @@ tasks:
     cmds:
       - rm -f {{.OUTPUT}}
       - >-
-        CGO_ENABLED=0 GOEXPERIMENT=greenteagc go build
+        CGO_ENABLED=0 go build
         -ldflags "-X 'go.aykhans.me/sarin/internal/version.Version=$(git describe --tags --always)'
         -X 'go.aykhans.me/sarin/internal/version.GitCommit=$(git rev-parse HEAD)'
         -X 'go.aykhans.me/sarin/internal/version.BuildDate=$(date -u +%Y-%m-%dT%H:%M:%SZ)'

cmd/cli/main.go

@@ -53,6 +53,7 @@ func main() {
 		combinedConfig.Cookies, combinedConfig.Bodies, combinedConfig.Proxies, combinedConfig.Values,
 		*combinedConfig.Output != config.ConfigOutputTypeNone,
 		*combinedConfig.DryRun,
+		combinedConfig.Lua, combinedConfig.Js,
 	)
 	_ = utilsErr.MustHandle(err,
 		utilsErr.OnType(func(err types.ProxyDialError) error {
@@ -60,6 +61,16 @@ func main() {
 			os.Exit(1)
 			return nil
 		}),
+		utilsErr.OnSentinel(types.ErrScriptEmpty, func(err error) error {
+			fmt.Fprintln(os.Stderr, config.StyleRed.Render("[SCRIPT] ")+err.Error())
+			os.Exit(1)
+			return nil
+		}),
+		utilsErr.OnType(func(err types.ScriptLoadError) error {
+			fmt.Fprintln(os.Stderr, config.StyleRed.Render("[SCRIPT] ")+err.Error())
+			os.Exit(1)
+			return nil
+		}),
 	)
 
 	srn.Start(ctx)

docs/configuration.md

@@ -36,6 +36,8 @@ Use `-s` or `--show-config` to see the final merged configuration before sending
 | [Cookies](#cookies)         | `cookies`<br>(object)               | `-cookie` / `-C`<br>(string / []string)      | `SARIN_COOKIE`<br>(string)       | -       | HTTP cookies                 |
 | [Proxy](#proxy)             | `proxy`<br>(string / []string)      | `-proxy` / `-X`<br>(string / []string)       | `SARIN_PROXY`<br>(string)        | -       | Proxy URL(s)                 |
 | [Values](#values)           | `values`<br>(string / []string)     | `-values` / `-V`<br>(string / []string)      | `SARIN_VALUES`<br>(string)       | -       | Template values (key=value)  |
+| [Lua](#lua)                 | `lua`<br>(string / []string)        | `-lua`<br>(string / []string)                | `SARIN_LUA`<br>(string)          | -       | Lua script(s)                |
+| [Js](#js)                   | `js`<br>(string / []string)         | `-js`<br>(string / []string)                 | `SARIN_JS`<br>(string)           | -       | JavaScript script(s)         |
 
 ---
 
@@ -374,3 +376,133 @@ values: |
 ```sh
 SARIN_VALUES="key1=value1"
 ```
+
+## Lua
+
+Lua script(s) for request transformation. Each script must define a global `transform` function that receives a request object and returns the modified request object. Scripts run after template rendering, before the request is sent.
+
+If multiple Lua scripts are provided, they are chained in order—the output of one becomes the input to the next. When both Lua and JavaScript scripts are specified, all Lua scripts run first, then all JavaScript scripts.
+
+**Script sources:**
+
+Scripts can be provided as:
+
+- **Inline script:** Direct script code
+- **File reference:** `@/path/to/script.lua` or `@./relative/path.lua`
+- **URL reference:** `@http://...` or `@https://...`
+- **Escaped `@`:** `@@...` for inline scripts that start with a literal `@`
+
+**The `transform` function:**
+
+```lua
+function transform(req)
+    -- req.method   (string)                    - HTTP method (e.g. "GET", "POST")
+    -- req.path     (string)                    - URL path (e.g. "/api/users")
+    -- req.body     (string)                    - Request body
+    -- req.headers  (table of string/arrays)    - HTTP headers (e.g. {["X-Key"] = "value"})
+    -- req.params   (table of string/arrays)    - Query parameters (e.g. {["id"] = "123"})
+    -- req.cookies  (table of string/arrays)    - Cookies (e.g. {["session"] = "abc"})
+
+    req.headers["X-Custom"] = "my-value"
+    return 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"}`).
+
+**YAML example:**
+
+```yaml
+lua: |
+    function transform(req)
+        req.headers["X-Custom"] = "my-value"
+        return req
+    end
+
+# OR
+
+lua:
+    - "@/path/to/script1.lua"
+    - "@/path/to/script2.lua"
+```
+
+**CLI example:**
+
+```sh
+-lua 'function transform(req) req.headers["X-Custom"] = "my-value" return req end'
+
+# OR
+
+-lua @/path/to/script1.lua -lua @/path/to/script2.lua
+```
+
+**ENV example:**
+
+```sh
+SARIN_LUA='function transform(req) req.headers["X-Custom"] = "my-value" return req end'
+```
+
+## Js
+
+JavaScript script(s) for request transformation. Each script must define a global `transform` function that receives a request object and returns the modified request object. Scripts run after template rendering, before the request is sent.
+
+If multiple JavaScript scripts are provided, they are chained in order—the output of one becomes the input to the next. When both Lua and JavaScript scripts are specified, all Lua scripts run first, then all JavaScript scripts.
+
+**Script sources:**
+
+Scripts can be provided as:
+
+- **Inline script:** Direct script code
+- **File reference:** `@/path/to/script.js` or `@./relative/path.js`
+- **URL reference:** `@http://...` or `@https://...`
+- **Escaped `@`:** `@@...` for inline scripts that start with a literal `@`
+
+**The `transform` function:**
+
+```javascript
+function transform(req) {
+    // req.method   (string)                    - HTTP method (e.g. "GET", "POST")
+    // req.path     (string)                    - URL path (e.g. "/api/users")
+    // req.body     (string)                    - Request body
+    // req.headers  (object of string/arrays)   - HTTP headers (e.g. {"X-Key": "value"})
+    // req.params   (object of string/arrays)   - Query parameters (e.g. {"id": "123"})
+    // req.cookies  (object of string/arrays)   - Cookies (e.g. {"session": "abc"})
+
+    req.headers["X-Custom"] = "my-value";
+    return req;
+}
+```
+
+> **Note:** Header, parameter, and cookie values can be a single string or an array for multiple values per key (e.g. `["val1", "val2"]`).
+
+**YAML example:**
+
+```yaml
+js: |
+    function transform(req) {
+        req.headers["X-Custom"] = "my-value";
+        return req;
+    }
+
+# OR
+
+js:
+    - "@/path/to/script1.js"
+    - "@/path/to/script2.js"
+```
+
+**CLI example:**
+
+```sh
+-js 'function transform(req) { req.headers["X-Custom"] = "my-value"; return req; }'
+
+# OR
+
+-js @/path/to/script1.js -js @/path/to/script2.js
+```
+
+**ENV example:**
+
+```sh
+SARIN_JS='function transform(req) { req.headers["X-Custom"] = "my-value"; return req; }'
+```

docs/examples.md

@@ -15,6 +15,7 @@ This guide provides practical examples for common Sarin use cases.
 - [Docker Usage](#docker-usage)
 - [Dry Run Mode](#dry-run-mode)
 - [Show Configuration](#show-configuration)
+- [Scripting](#scripting)
 
 ---
 
@@ -894,3 +895,124 @@ headers:
 ```
 
 </details>
+
+## Scripting
+
+Transform requests using Lua or JavaScript scripts. Scripts run after template rendering, before the request is sent.
+
+**Add a custom header with Lua:**
+
+```sh
+sarin -U http://example.com/api -r 1000 -c 10 \
+  -lua 'function transform(req) req.headers["X-Custom"] = "my-value" return req end'
+```
+
+<details>
+<summary>YAML equivalent</summary>
+
+```yaml
+url: http://example.com/api
+requests: 1000
+concurrency: 10
+lua: |
+    function transform(req)
+        req.headers["X-Custom"] = "my-value"
+        return req
+    end
+```
+
+</details>
+
+**Modify request body with JavaScript:**
+
+```sh
+sarin -U http://example.com/api/data -r 1000 -c 10 \
+  -M POST \
+  -H "Content-Type: application/json" \
+  -B '{"name": "test"}' \
+  -js 'function transform(req) { var body = JSON.parse(req.body); body.timestamp = Date.now(); req.body = JSON.stringify(body); return req; }'
+```
+
+<details>
+<summary>YAML equivalent</summary>
+
+```yaml
+url: http://example.com/api/data
+requests: 1000
+concurrency: 10
+method: POST
+headers:
+    Content-Type: application/json
+body: '{"name": "test"}'
+js: |
+    function transform(req) {
+        var body = JSON.parse(req.body);
+        body.timestamp = Date.now();
+        req.body = JSON.stringify(body);
+        return req;
+    }
+```
+
+</details>
+
+**Load script from a file:**
+
+```sh
+sarin -U http://example.com/api -r 1000 -c 10 \
+  -lua @./scripts/transform.lua
+```
+
+<details>
+<summary>YAML equivalent</summary>
+
+```yaml
+url: http://example.com/api
+requests: 1000
+concurrency: 10
+lua: "@./scripts/transform.lua"
+```
+
+</details>
+
+**Load script from a URL:**
+
+```sh
+sarin -U http://example.com/api -r 1000 -c 10 \
+  -js @https://example.com/scripts/transform.js
+```
+
+<details>
+<summary>YAML equivalent</summary>
+
+```yaml
+url: http://example.com/api
+requests: 1000
+concurrency: 10
+js: "@https://example.com/scripts/transform.js"
+```
+
+</details>
+
+**Chain multiple scripts (Lua runs first, then JavaScript):**
+
+```sh
+sarin -U http://example.com/api -r 1000 -c 10 \
+  -lua @./scripts/auth.lua \
+  -lua @./scripts/headers.lua \
+  -js @./scripts/body.js
+```
+
+<details>
+<summary>YAML equivalent</summary>
+
+```yaml
+url: http://example.com/api
+requests: 1000
+concurrency: 10
+lua:
+    - "@./scripts/auth.lua"
+    - "@./scripts/headers.lua"
+js: "@./scripts/body.js"
+```
+
+</details>

go.mod

@@ -1,19 +1,21 @@
 module go.aykhans.me/sarin
 
-go 1.25.5
+go 1.26.0
 
 require (
 	github.com/brianvoe/gofakeit/v7 v7.14.0
-	github.com/charmbracelet/bubbles v0.21.1
+	github.com/charmbracelet/bubbles v1.0.0
 	github.com/charmbracelet/bubbletea v1.3.10
 	github.com/charmbracelet/glamour v0.10.0
 	github.com/charmbracelet/lipgloss v1.1.1-0.20250404203927-76690c660834
 	github.com/charmbracelet/x/term v0.2.2
+	github.com/dop251/goja v0.0.0-20260106131823-651366fbe6e3
 	github.com/joho/godotenv v1.5.1
 	github.com/valyala/fasthttp v1.69.0
+	github.com/yuin/gopher-lua v1.1.1
 	go.aykhans.me/utils v1.0.7
 	go.yaml.in/yaml/v4 v4.0.0-rc.3
-	golang.org/x/net v0.49.0
+	golang.org/x/net v0.50.0
 )
 
 require (
@@ -23,7 +25,7 @@ require (
 	github.com/aymerick/douceur v0.2.0 // indirect
 	github.com/charmbracelet/colorprofile v0.4.1 // indirect
 	github.com/charmbracelet/harmonica v0.2.0 // indirect
-	github.com/charmbracelet/x/ansi v0.11.5 // indirect
+	github.com/charmbracelet/x/ansi v0.11.6 // indirect
 	github.com/charmbracelet/x/cellbuf v0.0.15 // indirect
 	github.com/charmbracelet/x/exp/slice v0.0.0-20260109001716-2fbdffcb221f // indirect
 	github.com/clipperhouse/displaywidth v0.9.0 // indirect
@@ -32,6 +34,8 @@ require (
 	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/dlclark/regexp2 v1.11.5 // indirect
 	github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f // indirect
+	github.com/go-sourcemap/sourcemap v2.1.3+incompatible // indirect
+	github.com/google/pprof v0.0.0-20230207041349-798e818bf904 // indirect
 	github.com/gorilla/css v1.0.1 // indirect
 	github.com/klauspost/compress v1.18.2 // indirect
 	github.com/lucasb-eyer/go-colorful v1.3.0 // indirect
@@ -49,7 +53,7 @@ require (
 	github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e // indirect
 	github.com/yuin/goldmark v1.7.16 // indirect
 	github.com/yuin/goldmark-emoji v1.0.6 // indirect
-	golang.org/x/sys v0.40.0 // indirect
-	golang.org/x/term v0.39.0 // indirect
-	golang.org/x/text v0.33.0 // indirect
+	golang.org/x/sys v0.41.0 // indirect
+	golang.org/x/term v0.40.0 // indirect
+	golang.org/x/text v0.34.0 // indirect
 )

go.sum

@@ -1,3 +1,5 @@
+github.com/Masterminds/semver/v3 v3.2.1 h1:RN9w6+7QoMeJVGyfmbcgs28Br8cvmnucEXnY0rYXWg0=
+github.com/Masterminds/semver/v3 v3.2.1/go.mod h1:qvl/7zhW3nngYb5+80sSMF+FG2BjYrf8m9wsX0PNOMQ=
 github.com/alecthomas/assert/v2 v2.11.0 h1:2Q9r3ki8+JYXvGsDyBXwH3LcJ+WK5D0gc5E8vS6K3D0=
 github.com/alecthomas/assert/v2 v2.11.0/go.mod h1:Bze95FyfUr7x34QZrjL+XP+0qgp/zg8yS+TtBj1WA3k=
 github.com/alecthomas/chroma/v2 v2.21.1 h1:FaSDrp6N+3pphkNKU6HPCiYLgm8dbe5UXIXcoBhZSWA=
@@ -14,8 +16,8 @@ github.com/aymerick/douceur v0.2.0 h1:Mv+mAeH1Q+n9Fr+oyamOlAkUNPWPlA8PPGR0QAaYuP
 github.com/aymerick/douceur v0.2.0/go.mod h1:wlT5vV2O3h55X9m7iVYN0TBM0NH/MmbLnd30/FjWUq4=
 github.com/brianvoe/gofakeit/v7 v7.14.0 h1:R8tmT/rTDJmD2ngpqBL9rAKydiL7Qr2u3CXPqRt59pk=
 github.com/brianvoe/gofakeit/v7 v7.14.0/go.mod h1:QXuPeBw164PJCzCUZVmgpgHJ3Llj49jSLVkKPMtxtxA=
-github.com/charmbracelet/bubbles v0.21.1 h1:nj0decPiixaZeL9diI4uzzQTkkz1kYY8+jgzCZXSmW0=
-github.com/charmbracelet/bubbles v0.21.1/go.mod h1:HHvIYRCpbkCJw2yo0vNX1O5loCwSr9/mWS8GYSg50Sk=
+github.com/charmbracelet/bubbles v1.0.0 h1:12J8/ak/uCZEMQ6KU7pcfwceyjLlWsDLAxB5fXonfvc=
+github.com/charmbracelet/bubbles v1.0.0/go.mod h1:9d/Zd5GdnauMI5ivUIVisuEm3ave1XwXtD1ckyV6r3E=
 github.com/charmbracelet/bubbletea v1.3.10 h1:otUDHWMMzQSB0Pkc87rm691KZ3SWa4KUlvF9nRvCICw=
 github.com/charmbracelet/bubbletea v1.3.10/go.mod h1:ORQfo0fk8U+po9VaNvnV95UPWA1BitP1E0N6xJPlHr4=
 github.com/charmbracelet/colorprofile v0.4.1 h1:a1lO03qTrSIRaK8c3JRxJDZOvhvIeSco3ej+ngLk1kk=
@@ -26,8 +28,8 @@ github.com/charmbracelet/harmonica v0.2.0 h1:8NxJWRWg/bzKqqEaaeFNipOu77YR5t8aSwG
 github.com/charmbracelet/harmonica v0.2.0/go.mod h1:KSri/1RMQOZLbw7AHqgcBycp8pgJnQMYYT8QZRqZ1Ao=
 github.com/charmbracelet/lipgloss v1.1.1-0.20250404203927-76690c660834 h1:ZR7e0ro+SZZiIZD7msJyA+NjkCNNavuiPBLgerbOziE=
 github.com/charmbracelet/lipgloss v1.1.1-0.20250404203927-76690c660834/go.mod h1:aKC/t2arECF6rNOnaKaVU6y4t4ZeHQzqfxedE/VkVhA=
-github.com/charmbracelet/x/ansi v0.11.5 h1:NBWeBpj/lJPE3Q5l+Lusa4+mH6v7487OP8K0r1IhRg4=
-github.com/charmbracelet/x/ansi v0.11.5/go.mod h1:2JNYLgQUsyqaiLovhU2Rv/pb8r6ydXKS3NIttu3VGZQ=
+github.com/charmbracelet/x/ansi v0.11.6 h1:GhV21SiDz/45W9AnV2R61xZMRri5NlLnl6CVF7ihZW8=
+github.com/charmbracelet/x/ansi v0.11.6/go.mod h1:2JNYLgQUsyqaiLovhU2Rv/pb8r6ydXKS3NIttu3VGZQ=
 github.com/charmbracelet/x/cellbuf v0.0.15 h1:ur3pZy0o6z/R7EylET877CBxaiE1Sp1GMxoFPAIztPI=
 github.com/charmbracelet/x/cellbuf v0.0.15/go.mod h1:J1YVbR7MUuEGIFPCaaZ96KDl5NoS0DAWkskup+mOY+Q=
 github.com/charmbracelet/x/exp/golden v0.0.0-20241011142426-46044092ad91 h1:payRxjMjKgx2PaCWLZ4p3ro9y97+TVLZNaRZgJwSVDQ=
@@ -46,8 +48,14 @@ github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/dlclark/regexp2 v1.11.5 h1:Q/sSnsKerHeCkc/jSTNq1oCm7KiVgUMZRDUoRu0JQZQ=
 github.com/dlclark/regexp2 v1.11.5/go.mod h1:DHkYz0B9wPfa6wondMfaivmHpzrQ3v9q8cnmRbL6yW8=
+github.com/dop251/goja v0.0.0-20260106131823-651366fbe6e3 h1:bVp3yUzvSAJzu9GqID+Z96P+eu5TKnIMJSV4QaZMauM=
+github.com/dop251/goja v0.0.0-20260106131823-651366fbe6e3/go.mod h1:MxLav0peU43GgvwVgNbLAj1s/bSGboKkhuULvq/7hx4=
 github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f h1:Y/CXytFA4m6baUTXGLOoWe4PQhGxaX0KpnayAqC48p4=
 github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f/go.mod h1:vw97MGsxSvLiUE2X8qFplwetxpGLQrlU1Q9AUEIzCaM=
+github.com/go-sourcemap/sourcemap v2.1.3+incompatible h1:W1iEw64niKVGogNgBN3ePyLFfuisuzeidWPMPWmECqU=
+github.com/go-sourcemap/sourcemap v2.1.3+incompatible/go.mod h1:F8jJfvm2KbVjc5NqelyYJmf/v5J0dwNLS2mL4sNA1Jg=
+github.com/google/pprof v0.0.0-20230207041349-798e818bf904 h1:4/hN5RUoecvl+RmJRE2YxKWtnnQls6rQjjW5oV7qg2U=
+github.com/google/pprof v0.0.0-20230207041349-798e818bf904/go.mod h1:uglQLonpP8qtYCYyzA+8c/9qtqgA3qsXGYqCPKARAFg=
 github.com/gorilla/css v1.0.1 h1:ntNaBIghp6JmvWnxbZKANoLyuXTPZ4cAMlo6RyhlbO8=
 github.com/gorilla/css v1.0.1/go.mod h1:BvnYkspnSzMmwRK+b8/xgNPLiIuNZr6vbZBTPQ2A3b0=
 github.com/hexops/gotextdiff v1.0.3 h1:gitA9+qJrrTCsiCl7+kh75nPqQt1cx4ZkudSTLoUqJM=
@@ -95,21 +103,25 @@ github.com/yuin/goldmark v1.7.16 h1:n+CJdUxaFMiDUNnWC3dMWCIQJSkxH4uz3ZwQBkAlVNE=
 github.com/yuin/goldmark v1.7.16/go.mod h1:ip/1k0VRfGynBgxOz0yCqHrbZXhcjxyuS66Brc7iBKg=
 github.com/yuin/goldmark-emoji v1.0.6 h1:QWfF2FYaXwL74tfGOW5izeiZepUDroDJfWubQI9HTHs=
 github.com/yuin/goldmark-emoji v1.0.6/go.mod h1:ukxJDKFpdFb5x0a5HqbdlcKtebh086iJpI31LTKmWuA=
+github.com/yuin/gopher-lua v1.1.1 h1:kYKnWBjvbNP4XLT3+bPEwAXJx262OhaHDWDVOPjL46M=
+github.com/yuin/gopher-lua v1.1.1/go.mod h1:GBR0iDaNXjAgGg9zfCvksxSRnQx76gclCIb7kdAd1Pw=
 go.aykhans.me/utils v1.0.7 h1:ClHXHlWmkjfFlD7+w5BQY29lKCEztxY/yCf543x4hZw=
 go.aykhans.me/utils v1.0.7/go.mod h1:0Jz8GlZLN35cCHLOLx39sazWwEe33bF6SYlSeqzEXoI=
 go.yaml.in/yaml/v4 v4.0.0-rc.3 h1:3h1fjsh1CTAPjW7q/EMe+C8shx5d8ctzZTrLcs/j8Go=
 go.yaml.in/yaml/v4 v4.0.0-rc.3/go.mod h1:aZqd9kCMsGL7AuUv/m/PvWLdg5sjJsZ4oHDEnfPPfY0=
 golang.org/x/exp v0.0.0-20231006140011-7918f672742d h1:jtJma62tbqLibJ5sFQz8bKtEM8rJBtfilJ2qTU199MI=
 golang.org/x/exp v0.0.0-20231006140011-7918f672742d/go.mod h1:ldy0pHrwJyGW56pPQzzkH36rKxoZW1tw7ZJpeKx+hdo=
-golang.org/x/net v0.49.0 h1:eeHFmOGUTtaaPSGNmjBKpbng9MulQsJURQUAfUwY++o=
-golang.org/x/net v0.49.0/go.mod h1:/ysNB2EvaqvesRkuLAyjI1ycPZlQHM3q01F02UY/MV8=
+golang.org/x/net v0.50.0 h1:ucWh9eiCGyDR3vtzso0WMQinm2Dnt8cFMuQa9K33J60=
+golang.org/x/net v0.50.0/go.mod h1:UgoSli3F/pBgdJBHCTc+tp3gmrU4XswgGRgtnwWTfyM=
 golang.org/x/sys v0.0.0-20210809222454-d867a43fc93e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.40.0 h1:DBZZqJ2Rkml6QMQsZywtnjnnGvHza6BTfYFWY9kjEWQ=
-golang.org/x/sys v0.40.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
-golang.org/x/term v0.39.0 h1:RclSuaJf32jOqZz74CkPA9qFuVTX7vhLlpfj/IGWlqY=
-golang.org/x/term v0.39.0/go.mod h1:yxzUCTP/U+FzoxfdKmLaA0RV1WgE0VY7hXBwKtY/4ww=
-golang.org/x/text v0.33.0 h1:B3njUFyqtHDUI5jMn1YIr5B0IE2U0qck04r6d4KPAxE=
-golang.org/x/text v0.33.0/go.mod h1:LuMebE6+rBincTi9+xWTY8TztLzKHc/9C1uBCG27+q8=
+golang.org/x/sys v0.41.0 h1:Ivj+2Cp/ylzLiEU89QhWblYnOE9zerudt9Ftecq2C6k=
+golang.org/x/sys v0.41.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+golang.org/x/term v0.40.0 h1:36e4zGLqU4yhjlmxEaagx2KuYbJq3EwY8K943ZsHcvg=
+golang.org/x/term v0.40.0/go.mod h1:w2P8uVp06p2iyKKuvXIm7N/y0UCRt3UfJTfZ7oOpglM=
+golang.org/x/text v0.34.0 h1:oL/Qq0Kdaqxa1KbNeMKwQq0reLCCaFtqu2eNuSeNHbk=
+golang.org/x/text v0.34.0/go.mod h1:homfLqTYRFyVYemLBFl5GgL/DWEiH5wcsQ5gSh1yziA=
+gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
+gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

internal/config/cli.go

@@ -10,27 +10,13 @@ import (
 
 	"go.aykhans.me/sarin/internal/types"
 	versionpkg "go.aykhans.me/sarin/internal/version"
-	"go.aykhans.me/utils/common"
 )
 
 const cliUsageText = `Usage:
   sarin [flags]
 
 Simple usage:
-  sarin -U https://example.com -d 1m
-
-Usage with all flags:
-  sarin -s -q -z -o json -f ./config.yaml -c 50 -r 100_000 -d 2m30s \
-    -U https://example.com \
-    -M POST \
-    -V "sharedUUID={{ fakeit_UUID }}" \
-    -B '{"product": "car"}' \
-    -P "id={{ .Values.sharedUUID }}" \
-    -H "User-Agent: {{ fakeit_UserAgent }}" -H "Accept: */*" \
-    -C "token={{ .Values.sharedUUID }}" \
-    -X "http://proxy.example.com" \
-    -T 3s \
-    -I
+  sarin -U https://example.com -r 1
 
 Flags:
   General Config:
@@ -55,7 +41,9 @@ 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)
-    -I, -insecure      bool       Skip SSL/TLS certificate verification (default %v)`
+    -I, -insecure      bool       Skip SSL/TLS certificate verification (default %v)
+        -lua           []string   Lua script for request transformation (inline or @file/@url)
+        -js            []string   JavaScript script for request transformation (inline or @file/@url)`
 
 var _ IParser = ConfigCLIParser{}
 
@@ -106,16 +94,18 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 		dryRun       bool
 
 		// Request config
-		urlInput string
-		methods  = stringSliceArg{}
-		bodies   = stringSliceArg{}
-		params   = stringSliceArg{}
-		headers  = stringSliceArg{}
-		cookies  = stringSliceArg{}
-		proxies  = stringSliceArg{}
-		values   = stringSliceArg{}
-		timeout  time.Duration
-		insecure bool
+		urlInput   string
+		methods    = stringSliceArg{}
+		bodies     = stringSliceArg{}
+		params     = stringSliceArg{}
+		headers    = stringSliceArg{}
+		cookies    = stringSliceArg{}
+		proxies    = stringSliceArg{}
+		values     = stringSliceArg{}
+		timeout    time.Duration
+		insecure   bool
+		luaScripts = stringSliceArg{}
+		jsScripts  = stringSliceArg{}
 	)
 
 	{
@@ -177,6 +167,10 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 
 		flagSet.BoolVar(&insecure, "insecure", false, "Skip SSL/TLS certificate verification")
 		flagSet.BoolVar(&insecure, "I", false, "Skip SSL/TLS certificate verification")
+
+		flagSet.Var(&luaScripts, "lua", "Lua script for request transformation (inline or @file/@url)")
+
+		flagSet.Var(&jsScripts, "js", "JavaScript script for request transformation (inline or @file/@url)")
 	}
 
 	// Parse the specific arguments provided to the parser, skipping the program name.
@@ -207,23 +201,23 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 		switch flagVar.Name {
 		// General config
 		case "show-config", "s":
-			config.ShowConfig = common.ToPtr(showConfig)
+			config.ShowConfig = new(showConfig)
 		case "config-file", "f":
 			for _, configFile := range configFiles {
 				config.Files = append(config.Files, *types.ParseConfigFile(configFile))
 			}
 		case "concurrency", "c":
-			config.Concurrency = common.ToPtr(concurrency)
+			config.Concurrency = new(concurrency)
 		case "requests", "r":
-			config.Requests = common.ToPtr(requestCount)
+			config.Requests = new(requestCount)
 		case "duration", "d":
-			config.Duration = common.ToPtr(duration)
+			config.Duration = new(duration)
 		case "quiet", "q":
-			config.Quiet = common.ToPtr(quiet)
+			config.Quiet = new(quiet)
 		case "output", "o":
-			config.Output = common.ToPtr(ConfigOutputType(output))
+			config.Output = new(ConfigOutputType(output))
 		case "dry-run", "z":
-			config.DryRun = common.ToPtr(dryRun)
+			config.DryRun = new(dryRun)
 
 		// Request config
 		case "url", "U":
@@ -256,9 +250,13 @@ func (parser ConfigCLIParser) Parse() (*Config, error) {
 		case "values", "V":
 			config.Values = append(config.Values, values...)
 		case "timeout", "T":
-			config.Timeout = common.ToPtr(timeout)
+			config.Timeout = new(timeout)
 		case "insecure", "I":
-			config.Insecure = common.ToPtr(insecure)
+			config.Insecure = new(insecure)
+		case "lua":
+			config.Lua = append(config.Lua, luaScripts...)
+		case "js":
+			config.Js = append(config.Js, jsScripts...)
 		}
 	})
 

internal/config/config.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"net/url"
@@ -16,6 +17,7 @@ import (
 	"github.com/charmbracelet/glamour/styles"
 	"github.com/charmbracelet/lipgloss"
 	"github.com/charmbracelet/x/term"
+	"go.aykhans.me/sarin/internal/script"
 	"go.aykhans.me/sarin/internal/types"
 	"go.aykhans.me/sarin/internal/version"
 	"go.aykhans.me/utils/common"
@@ -87,10 +89,8 @@ type Config struct {
 	Bodies      []string           `yaml:"bodies,omitempty"`
 	Proxies     types.Proxies      `yaml:"proxies,omitempty"`
 	Values      []string           `yaml:"values,omitempty"`
-}
-
-func NewConfig() *Config {
-	return &Config{}
+	Lua         []string           `yaml:"lua,omitempty"`
+	Js          []string           `yaml:"js,omitempty"`
 }
 
 func (config Config) MarshalYAML() (any, error) {
@@ -219,6 +219,8 @@ func (config Config) MarshalYAML() (any, error) {
 	}
 
 	addStringSlice(content, "values", config.Values, false)
+	addStringSlice(content, "lua", config.Lua, false)
+	addStringSlice(content, "js", config.Js, false)
 
 	return root, nil
 }
@@ -323,6 +325,12 @@ func (config *Config) Merge(newConfig *Config) {
 	if len(newConfig.Values) != 0 {
 		config.Values = append(config.Values, newConfig.Values...)
 	}
+	if len(newConfig.Lua) != 0 {
+		config.Lua = append(config.Lua, newConfig.Lua...)
+	}
+	if len(newConfig.Js) != 0 {
+		config.Js = append(config.Js, newConfig.Js...)
+	}
 }
 
 func (config *Config) SetDefaults() {
@@ -348,26 +356,26 @@ func (config *Config) SetDefaults() {
 		config.Timeout = &Defaults.RequestTimeout
 	}
 	if config.Concurrency == nil {
-		config.Concurrency = common.ToPtr(Defaults.Concurrency)
+		config.Concurrency = new(Defaults.Concurrency)
 	}
 	if config.ShowConfig == nil {
-		config.ShowConfig = common.ToPtr(Defaults.ShowConfig)
+		config.ShowConfig = new(Defaults.ShowConfig)
 	}
 	if config.Quiet == nil {
-		config.Quiet = common.ToPtr(Defaults.Quiet)
+		config.Quiet = new(Defaults.Quiet)
 	}
 	if config.Insecure == nil {
-		config.Insecure = common.ToPtr(Defaults.Insecure)
+		config.Insecure = new(Defaults.Insecure)
 	}
 	if config.DryRun == nil {
-		config.DryRun = common.ToPtr(Defaults.DryRun)
+		config.DryRun = new(Defaults.DryRun)
 	}
 	if !config.Headers.Has("User-Agent") {
 		config.Headers = append(config.Headers, types.Header{Key: "User-Agent", Value: []string{Defaults.UserAgent}})
 	}
 
 	if config.Output == nil {
-		config.Output = common.ToPtr(Defaults.Output)
+		config.Output = new(Defaults.Output)
 	}
 }
 
@@ -465,6 +473,44 @@ func (config Config) Validate() error {
 		}
 	}
 
+	// Create a context with timeout for script validation (loading from URLs)
+	scriptCtx, scriptCancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer scriptCancel()
+
+	for i, scriptSrc := range config.Lua {
+		if err := validateScriptSource(scriptSrc); err != nil {
+			validationErrors = append(
+				validationErrors,
+				types.NewFieldValidationError(fmt.Sprintf("Lua[%d]", i), scriptSrc, err),
+			)
+			continue
+		}
+		// Validate script syntax
+		if err := script.ValidateScript(scriptCtx, scriptSrc, script.EngineTypeLua); err != nil {
+			validationErrors = append(
+				validationErrors,
+				types.NewFieldValidationError(fmt.Sprintf("Lua[%d]", i), scriptSrc, err),
+			)
+		}
+	}
+
+	for i, scriptSrc := range config.Js {
+		if err := validateScriptSource(scriptSrc); err != nil {
+			validationErrors = append(
+				validationErrors,
+				types.NewFieldValidationError(fmt.Sprintf("Js[%d]", i), scriptSrc, err),
+			)
+			continue
+		}
+		// Validate script syntax
+		if err := script.ValidateScript(scriptCtx, scriptSrc, script.EngineTypeJavaScript); err != nil {
+			validationErrors = append(
+				validationErrors,
+				types.NewFieldValidationError(fmt.Sprintf("Js[%d]", i), scriptSrc, err),
+			)
+		}
+	}
+
 	templateErrors := ValidateTemplates(&config)
 	validationErrors = append(validationErrors, templateErrors...)
 
@@ -582,6 +628,57 @@ func parseConfigFile(configFile types.ConfigFile, maxDepth int) (*Config, error)
 	return fileConfig, nil
 }
 
+// validateScriptSource validates a script source string.
+// Scripts can be:
+//   - Inline script: any string not starting with "@"
+//   - Escaped "@": strings starting with "@@" (literal "@" at start)
+//   - File reference: "@/path/to/file" or "@./relative/path"
+//   - URL reference: "@http://..." or "@https://..."
+//
+// It can return the following errors:
+//   - types.ErrScriptEmpty
+//   - types.ErrScriptSourceEmpty
+//   - types.ErrScriptURLNoHost
+//   - types.URLParseError
+func validateScriptSource(script string) error {
+	// Empty script is invalid
+	if script == "" {
+		return types.ErrScriptEmpty
+	}
+
+	// Not a file/URL reference - it's an inline script
+	if !strings.HasPrefix(script, "@") {
+		return nil
+	}
+
+	// Escaped @ - it's an inline script starting with literal @
+	if strings.HasPrefix(script, "@@") {
+		return nil
+	}
+
+	// It's a file or URL reference - validate the source
+	source := script[1:] // Remove the @ prefix
+
+	if source == "" {
+		return types.ErrScriptSourceEmpty
+	}
+
+	// Check if it's a http(s) URL
+	if strings.HasPrefix(source, "http://") || strings.HasPrefix(source, "https://") {
+		parsedURL, err := url.Parse(source)
+		if err != nil {
+			return types.NewURLParseError(source, err)
+		}
+		if parsedURL.Host == "" {
+			return types.ErrScriptURLNoHost
+		}
+		return nil
+	}
+
+	// It's a file path - basic validation (not empty, checked above)
+	return nil
+}
+
 func printParseErrors(parserName string, errors ...types.FieldParseError) {
 	for _, fieldErr := range errors {
 		if fieldErr.Value == "" {

internal/config/env.go

@@ -7,7 +7,6 @@ import (
 	"time"
 
 	"go.aykhans.me/sarin/internal/types"
-	"go.aykhans.me/utils/common"
 	utilsParse "go.aykhans.me/utils/parser"
 )
 
@@ -67,7 +66,7 @@ func (parser ConfigENVParser) Parse() (*Config, error) {
 	}
 
 	if output := parser.getEnv("OUTPUT"); output != "" {
-		config.Output = common.ToPtr(ConfigOutputType(output))
+		config.Output = new(ConfigOutputType(output))
 	}
 
 	if insecure := parser.getEnv("INSECURE"); insecure != "" {
@@ -216,6 +215,14 @@ func (parser ConfigENVParser) Parse() (*Config, error) {
 		config.Values = []string{values}
 	}
 
+	if lua := parser.getEnv("LUA"); lua != "" {
+		config.Lua = []string{lua}
+	}
+
+	if js := parser.getEnv("JS"); js != "" {
+		config.Js = []string{js}
+	}
+
 	if len(fieldParseErrors) > 0 {
 		return nil, types.NewFieldParseErrors(fieldParseErrors)
 	}

internal/config/file.go

@@ -12,7 +12,6 @@ import (
 	"time"
 
 	"go.aykhans.me/sarin/internal/types"
-	"go.aykhans.me/utils/common"
 	"go.yaml.in/yaml/v4"
 )
 
@@ -49,6 +48,10 @@ func (parser ConfigFileParser) Parse() (*Config, error) {
 }
 
 // fetchFile retrieves file contents from a local path or HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.FileReadError
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func fetchFile(ctx context.Context, src string) ([]byte, error) {
 	if strings.HasPrefix(src, "http://") || strings.HasPrefix(src, "https://") {
 		return fetchHTTP(ctx, src)
@@ -57,25 +60,28 @@ func fetchFile(ctx context.Context, src string) ([]byte, error) {
 }
 
 // fetchHTTP downloads file contents from an HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func fetchHTTP(ctx context.Context, url string) ([]byte, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
-		return nil, fmt.Errorf("failed to create request: %w", err)
+		return nil, types.NewHTTPFetchError(url, err)
 	}
 
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
-		return nil, fmt.Errorf("failed to fetch file: %w", err)
+		return nil, types.NewHTTPFetchError(url, err)
 	}
 	defer resp.Body.Close() //nolint:errcheck
 
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("failed to fetch file: HTTP %d %s", resp.StatusCode, resp.Status)
+		return nil, types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
 	}
 
 	data, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read response body: %w", err)
+		return nil, types.NewHTTPFetchError(url, err)
 	}
 
 	return data, nil
@@ -83,19 +89,21 @@ func fetchHTTP(ctx context.Context, url string) ([]byte, error) {
 
 // fetchLocal reads file contents from the local filesystem.
 // It resolves relative paths from the current working directory.
+// It can return the following errors:
+//   - types.FileReadError
 func fetchLocal(src string) ([]byte, error) {
 	path := src
 	if !filepath.IsAbs(src) {
 		pwd, err := os.Getwd()
 		if err != nil {
-			return nil, fmt.Errorf("failed to get working directory: %w", err)
+			return nil, types.NewFileReadError(src, err)
 		}
 		path = filepath.Join(pwd, src)
 	}
 
 	data, err := os.ReadFile(path) //nolint:gosec
 	if err != nil {
-		return nil, fmt.Errorf("failed to read file: %w", err)
+		return nil, types.NewFileReadError(path, err)
 	}
 
 	return data, nil
@@ -202,6 +210,8 @@ type configYAML struct {
 	Bodies       stringOrSliceField `yaml:"body"`
 	Proxies      stringOrSliceField `yaml:"proxy"`
 	Values       stringOrSliceField `yaml:"values"`
+	Lua          stringOrSliceField `yaml:"lua"`
+	Js           stringOrSliceField `yaml:"js"`
 }
 
 // ParseYAML parses YAML config file arguments into a Config object.
@@ -230,7 +240,7 @@ func (parser ConfigFileParser) ParseYAML(data []byte) (*Config, error) {
 	config.Quiet = parsedData.Quiet
 
 	if parsedData.Output != nil {
-		config.Output = common.ToPtr(ConfigOutputType(*parsedData.Output))
+		config.Output = new(ConfigOutputType(*parsedData.Output))
 	}
 
 	config.Insecure = parsedData.Insecure
@@ -246,6 +256,8 @@ func (parser ConfigFileParser) ParseYAML(data []byte) (*Config, error) {
 	}
 	config.Bodies = append(config.Bodies, parsedData.Bodies...)
 	config.Values = append(config.Values, parsedData.Values...)
+	config.Lua = append(config.Lua, parsedData.Lua...)
+	config.Js = append(config.Js, parsedData.Js...)
 
 	if len(parsedData.ConfigFiles) > 0 {
 		for _, configFile := range parsedData.ConfigFiles {

internal/config/template_validator.go

@@ -8,6 +8,8 @@ import (
 	"go.aykhans.me/sarin/internal/types"
 )
 
+// It can return the following errors:
+//   - types.TemplateParseError
 func validateTemplateString(value string, funcMap template.FuncMap) error {
 	if value == "" {
 		return nil
@@ -15,7 +17,7 @@ func validateTemplateString(value string, funcMap template.FuncMap) error {
 
 	_, err := template.New("").Funcs(funcMap).Parse(value)
 	if err != nil {
-		return fmt.Errorf("template parse error: %w", err)
+		return types.NewTemplateParseError(err)
 	}
 
 	return nil

internal/sarin/client.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/base64"
-	"errors"
 	"math"
 	"net"
 	"net/http"
@@ -95,6 +94,9 @@ func NewHostClients(
 	return []*fasthttp.HostClient{client}, nil
 }
 
+// NewProxyDialFunc creates a dial function for the given proxy URL.
+// It can return the following errors:
+//   - types.ProxyUnsupportedSchemeError
 func NewProxyDialFunc(ctx context.Context, proxyURL *url.URL, timeout time.Duration) (fasthttp.DialFunc, error) {
 	var (
 		dialer fasthttp.DialFunc
@@ -117,16 +119,14 @@ func NewProxyDialFunc(ctx context.Context, proxyURL *url.URL, timeout time.Durat
 	case "https":
 		dialer = fasthttpHTTPSDialerDualStackTimeout(proxyURL, timeout)
 	default:
-		return nil, errors.New("unsupported proxy scheme")
-	}
-
-	if dialer == nil {
-		return nil, errors.New("internal error: proxy dialer is nil")
+		return nil, types.NewProxyUnsupportedSchemeError(proxyURL.Scheme)
 	}
 
 	return dialer, nil
 }
 
+// The returned dial function can return the following errors:
+//   - types.ProxyDialError
 func fasthttpSocksDialerDualStackTimeout(ctx context.Context, proxyURL *url.URL, timeout time.Duration, resolveLocally bool) (fasthttp.DialFunc, error) {
 	netDialer := &net.Dialer{}
 
@@ -147,12 +147,18 @@ func fasthttpSocksDialerDualStackTimeout(ctx context.Context, proxyURL *url.URL,
 		return nil, err
 	}
 
+	proxyStr := proxyURL.String()
+
 	// Assert to ContextDialer for timeout support
 	contextDialer, ok := socksDialer.(proxy.ContextDialer)
 	if !ok {
 		// Fallback without timeout (should not happen with net.Dialer)
 		return func(addr string) (net.Conn, error) {
-			return socksDialer.Dial("tcp", addr)
+			conn, err := socksDialer.Dial("tcp", addr)
+			if err != nil {
+				return nil, types.NewProxyDialError(proxyStr, err)
+			}
+			return conn, nil
 		}, nil
 	}
 
@@ -163,7 +169,7 @@ func fasthttpSocksDialerDualStackTimeout(ctx context.Context, proxyURL *url.URL,
 		if resolveLocally {
 			host, port, err := net.SplitHostPort(addr)
 			if err != nil {
-				return nil, err
+				return nil, types.NewProxyDialError(proxyStr, err)
 			}
 
 			// Cap DNS resolution to half the timeout to reserve time for dial
@@ -171,10 +177,10 @@ func fasthttpSocksDialerDualStackTimeout(ctx context.Context, proxyURL *url.URL,
 			ips, err := net.DefaultResolver.LookupIP(dnsCtx, "ip", host)
 			dnsCancel()
 			if err != nil {
-				return nil, err
+				return nil, types.NewProxyDialError(proxyStr, err)
 			}
 			if len(ips) == 0 {
-				return nil, errors.New("no IP addresses found for host: " + host)
+				return nil, types.NewProxyDialError(proxyStr, types.NewProxyResolveError(host))
 			}
 
 			// Use the first resolved IP
@@ -184,16 +190,22 @@ func fasthttpSocksDialerDualStackTimeout(ctx context.Context, proxyURL *url.URL,
 		// Use remaining time for dial
 		remaining := time.Until(deadline)
 		if remaining <= 0 {
-			return nil, context.DeadlineExceeded
+			return nil, types.NewProxyDialError(proxyStr, context.DeadlineExceeded)
 		}
 
 		dialCtx, dialCancel := context.WithTimeout(ctx, remaining)
 		defer dialCancel()
 
-		return contextDialer.DialContext(dialCtx, "tcp", addr)
+		conn, err := contextDialer.DialContext(dialCtx, "tcp", addr)
+		if err != nil {
+			return nil, types.NewProxyDialError(proxyStr, err)
+		}
+		return conn, nil
 	}, nil
 }
 
+// The returned dial function can return the following errors:
+//   - types.ProxyDialError
 func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duration) fasthttp.DialFunc {
 	proxyAddr := proxyURL.Host
 	if proxyURL.Port() == "" {
@@ -209,24 +221,26 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 		proxyAuth = "Basic " + base64.StdEncoding.EncodeToString([]byte(credentials))
 	}
 
+	proxyStr := proxyURL.String()
+
 	return func(addr string) (net.Conn, error) {
 		// Establish TCP connection to proxy with timeout
 		start := time.Now()
 		conn, err := fasthttp.DialDualStackTimeout(proxyAddr, timeout)
 		if err != nil {
-			return nil, err
+			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 
 		remaining := timeout - time.Since(start)
 		if remaining <= 0 {
 			conn.Close() //nolint:errcheck,gosec
-			return nil, context.DeadlineExceeded
+			return nil, types.NewProxyDialError(proxyStr, context.DeadlineExceeded)
 		}
 
 		// Set deadline for the TLS handshake and CONNECT request
 		if err := conn.SetDeadline(time.Now().Add(remaining)); err != nil {
 			conn.Close() //nolint:errcheck,gosec
-			return nil, err
+			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 
 		// Upgrade to TLS
@@ -235,7 +249,7 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 		})
 		if err := tlsConn.Handshake(); err != nil {
 			tlsConn.Close() //nolint:errcheck,gosec
-			return nil, err
+			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 
 		// Build and send CONNECT request
@@ -251,7 +265,7 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 
 		if err := connectReq.Write(tlsConn); err != nil {
 			tlsConn.Close() //nolint:errcheck,gosec
-			return nil, err
+			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 
 		// Read response using buffered reader, but return wrapped connection
@@ -260,19 +274,19 @@ func fasthttpHTTPSDialerDualStackTimeout(proxyURL *url.URL, timeout time.Duratio
 		resp, err := http.ReadResponse(bufReader, connectReq)
 		if err != nil {
 			tlsConn.Close() //nolint:errcheck,gosec
-			return nil, err
+			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 		resp.Body.Close() //nolint:errcheck,gosec
 
 		if resp.StatusCode != http.StatusOK {
 			tlsConn.Close() //nolint:errcheck,gosec
-			return nil, errors.New("proxy CONNECT failed: " + resp.Status)
+			return nil, types.NewProxyDialError(proxyStr, types.NewProxyConnectError(resp.Status))
 		}
 
 		// Clear deadline for the tunneled connection
 		if err := tlsConn.SetDeadline(time.Time{}); err != nil {
 			tlsConn.Close() //nolint:errcheck,gosec
-			return nil, err
+			return nil, types.NewProxyDialError(proxyStr, err)
 		}
 
 		// Return wrapped connection that uses the buffered reader

internal/sarin/filecache.go

@@ -1,7 +1,6 @@
 package sarin
 
 import (
-	"fmt"
 	"io"
 	"net/http"
 	"os"
@@ -10,6 +9,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"go.aykhans.me/sarin/internal/types"
 )
 
 // CachedFile holds the cached content and metadata of a file.
@@ -31,6 +32,10 @@ func NewFileCache(requestTimeout time.Duration) *FileCache {
 
 // GetOrLoad retrieves a file from cache or loads it using the provided source.
 // The source can be a local file path or an HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.FileReadError
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func (fc *FileCache) GetOrLoad(source string) (*CachedFile, error) {
 	if val, ok := fc.cache.Load(source); ok {
 		return val.(*CachedFile), nil
@@ -59,14 +64,21 @@ func (fc *FileCache) GetOrLoad(source string) (*CachedFile, error) {
 	return actual.(*CachedFile), nil
 }
 
+// readLocalFile reads a file from the local filesystem and returns its content and filename.
+// It can return the following errors:
+//   - types.FileReadError
 func (fc *FileCache) readLocalFile(filePath string) ([]byte, string, error) {
 	content, err := os.ReadFile(filePath) //nolint:gosec
 	if err != nil {
-		return nil, "", fmt.Errorf("failed to read file %s: %w", filePath, err)
+		return nil, "", types.NewFileReadError(filePath, err)
 	}
 	return content, filepath.Base(filePath), nil
 }
 
+// fetchURL downloads file contents from an HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func (fc *FileCache) fetchURL(url string) ([]byte, string, error) {
 	client := &http.Client{
 		Timeout: fc.requestTimeout,
@@ -74,17 +86,17 @@ func (fc *FileCache) fetchURL(url string) ([]byte, string, error) {
 
 	resp, err := client.Get(url)
 	if err != nil {
-		return nil, "", fmt.Errorf("failed to fetch URL %s: %w", url, err)
+		return nil, "", types.NewHTTPFetchError(url, err)
 	}
 	defer resp.Body.Close() //nolint:errcheck
 
 	if resp.StatusCode != http.StatusOK {
-		return nil, "", fmt.Errorf("failed to fetch URL %s: HTTP %d", url, resp.StatusCode)
+		return nil, "", types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
 	}
 
 	content, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return nil, "", fmt.Errorf("failed to read response body from %s: %w", url, err)
+		return nil, "", types.NewHTTPFetchError(url, err)
 	}
 
 	// Extract filename from URL path

internal/sarin/request.go

@@ -2,7 +2,6 @@ package sarin
 
 import (
 	"bytes"
-	"fmt"
 	"maps"
 	"math/rand/v2"
 	"net/url"
@@ -11,13 +10,14 @@ import (
 
 	"github.com/joho/godotenv"
 	"github.com/valyala/fasthttp"
+	"go.aykhans.me/sarin/internal/script"
 	"go.aykhans.me/sarin/internal/types"
 	utilsSlice "go.aykhans.me/utils/slice"
 )
 
 type RequestGenerator func(*fasthttp.Request) error
 
-type RequestGeneratorWithData func(*fasthttp.Request, any) error
+type requestDataGenerator func(*script.RequestData, any) error
 
 type valuesData struct {
 	Values map[string]string
@@ -26,6 +26,9 @@ type valuesData struct {
 // NewRequestGenerator creates a new RequestGenerator function that generates HTTP requests
 // with the specified configuration. The returned RequestGenerator is NOT safe for concurrent
 // use by multiple goroutines.
+//
+// Note: Scripts must be validated before calling this function (e.g., in NewSarin).
+// The caller is responsible for managing the scriptTransformer lifecycle.
 func NewRequestGenerator(
 	methods []string,
 	requestURL *url.URL,
@@ -35,6 +38,7 @@ func NewRequestGenerator(
 	bodies []string,
 	values []string,
 	fileCache *FileCache,
+	scriptTransformer *script.Transformer,
 ) (RequestGenerator, bool) {
 	randSource := NewDefaultRandSource()
 	//nolint:gosec // G404: Using non-cryptographic rand for load testing, not security
@@ -53,13 +57,24 @@ func NewRequestGenerator(
 
 	valuesGenerator := NewValuesGeneratorFunc(values, templateFuncMap)
 
+	hasScripts := scriptTransformer != nil && !scriptTransformer.IsEmpty()
+
+	host := requestURL.Host
+	scheme := requestURL.Scheme
+
+	reqData := &script.RequestData{
+		Headers: make(map[string][]string),
+		Params:  make(map[string][]string),
+		Cookies: make(map[string][]string),
+	}
+
 	var (
 		data valuesData
 		path string
 		err  error
 	)
 	return func(req *fasthttp.Request) error {
-			req.Header.SetHost(requestURL.Host)
+			resetRequestData(reqData)
 
 			data, err = valuesGenerator()
 			if err != nil {
@@ -70,87 +85,135 @@ func NewRequestGenerator(
 			if err != nil {
 				return err
 			}
-			req.SetRequestURI(path)
+			reqData.Path = path
 
-			if err = methodGenerator(req, data); err != nil {
+			if err = methodGenerator(reqData, data); err != nil {
 				return err
 			}
 
 			bodyTemplateFuncMapData.ClearFormDataContenType()
-			if err = bodyGenerator(req, data); err != nil {
+			if err = bodyGenerator(reqData, data); err != nil {
 				return err
 			}
 
-			if err = headersGenerator(req, data); err != nil {
+			if err = headersGenerator(reqData, data); err != nil {
 				return err
 			}
 			if bodyTemplateFuncMapData.GetFormDataContenType() != "" {
-				req.Header.Add("Content-Type", bodyTemplateFuncMapData.GetFormDataContenType())
+				reqData.Headers["Content-Type"] = append(reqData.Headers["Content-Type"], bodyTemplateFuncMapData.GetFormDataContenType())
 			}
 
-			if err = paramsGenerator(req, data); err != nil {
+			if err = paramsGenerator(reqData, data); err != nil {
 				return err
 			}
-			if err = cookiesGenerator(req, data); err != nil {
+			if err = cookiesGenerator(reqData, data); err != nil {
 				return err
 			}
 
-			if requestURL.Scheme == "https" {
-				req.URI().SetScheme("https")
+			if hasScripts {
+				if err = scriptTransformer.Transform(reqData); err != nil {
+					return err
+				}
 			}
+
+			applyRequestDataToFastHTTP(reqData, req, host, scheme)
+
 			return nil
 		}, isPathGeneratorDynamic ||
 			isMethodGeneratorDynamic ||
 			isParamsGeneratorDynamic ||
 			isHeadersGeneratorDynamic ||
 			isCookiesGeneratorDynamic ||
-			isBodyGeneratorDynamic
+			isBodyGeneratorDynamic ||
+			hasScripts
 }
 
-func NewMethodGeneratorFunc(localRand *rand.Rand, methods []string, templateFunctions template.FuncMap) (RequestGeneratorWithData, bool) {
+func resetRequestData(reqData *script.RequestData) {
+	reqData.Method = ""
+	reqData.Path = ""
+	reqData.Body = ""
+	clear(reqData.Headers)
+	clear(reqData.Params)
+	clear(reqData.Cookies)
+}
+
+func applyRequestDataToFastHTTP(reqData *script.RequestData, req *fasthttp.Request, host, scheme string) {
+	req.Header.SetHost(host)
+	req.SetRequestURI(reqData.Path)
+	req.Header.SetMethod(reqData.Method)
+	req.SetBody([]byte(reqData.Body))
+
+	for k, values := range reqData.Headers {
+		for _, v := range values {
+			req.Header.Add(k, v)
+		}
+	}
+
+	for k, values := range reqData.Params {
+		for _, v := range values {
+			req.URI().QueryArgs().Add(k, v)
+		}
+	}
+
+	if len(reqData.Cookies) > 0 {
+		cookieStrings := make([]string, 0, len(reqData.Cookies))
+		for k, values := range reqData.Cookies {
+			for _, v := range values {
+				cookieStrings = append(cookieStrings, k+"="+v)
+			}
+		}
+		req.Header.Add("Cookie", strings.Join(cookieStrings, "; "))
+	}
+
+	if scheme == "https" {
+		req.URI().SetScheme("https")
+	}
+}
+
+func NewMethodGeneratorFunc(localRand *rand.Rand, methods []string, templateFunctions template.FuncMap) (requestDataGenerator, bool) {
 	methodGenerator, isDynamic := buildStringSliceGenerator(localRand, methods, templateFunctions)
 
 	var (
 		method string
 		err    error
 	)
-	return func(req *fasthttp.Request, data any) error {
+	return func(reqData *script.RequestData, data any) error {
 		method, err = methodGenerator()(data)
 		if err != nil {
 			return err
 		}
 
-		req.Header.SetMethod(method)
+		reqData.Method = method
 		return nil
 	}, isDynamic
 }
 
-func NewBodyGeneratorFunc(localRand *rand.Rand, bodies []string, templateFunctions template.FuncMap) (RequestGeneratorWithData, bool) {
+func NewBodyGeneratorFunc(localRand *rand.Rand, bodies []string, templateFunctions template.FuncMap) (requestDataGenerator, bool) {
 	bodyGenerator, isDynamic := buildStringSliceGenerator(localRand, bodies, templateFunctions)
 
 	var (
 		body string
 		err  error
 	)
-	return func(req *fasthttp.Request, data any) error {
+	return func(reqData *script.RequestData, data any) error {
 		body, err = bodyGenerator()(data)
 		if err != nil {
 			return err
 		}
 
-		req.SetBody([]byte(body))
+		reqData.Body = body
 		return nil
 	}, isDynamic
 }
 
-func NewParamsGeneratorFunc(localRand *rand.Rand, params types.Params, templateFunctions template.FuncMap) (RequestGeneratorWithData, bool) {
+func NewParamsGeneratorFunc(localRand *rand.Rand, params types.Params, templateFunctions template.FuncMap) (requestDataGenerator, bool) {
 	generators, isDynamic := buildKeyValueGenerators(localRand, params, templateFunctions)
 
 	var (
 		key, value string
 		err        error
 	)
-	return func(req *fasthttp.Request, data any) error {
+	return func(reqData *script.RequestData, data any) error {
 		for _, gen := range generators {
 			key, err = gen.Key(data)
 			if err != nil {
@@ -162,20 +225,20 @@ func NewParamsGeneratorFunc(localRand *rand.Rand, params types.Params, templateF
 				return err
 			}
 
-			req.URI().QueryArgs().Add(key, value)
+			reqData.Params[key] = append(reqData.Params[key], value)
 		}
 		return nil
 	}, isDynamic
 }
 
-func NewHeadersGeneratorFunc(localRand *rand.Rand, headers types.Headers, templateFunctions template.FuncMap) (RequestGeneratorWithData, bool) {
+func NewHeadersGeneratorFunc(localRand *rand.Rand, headers types.Headers, templateFunctions template.FuncMap) (requestDataGenerator, bool) {
 	generators, isDynamic := buildKeyValueGenerators(localRand, headers, templateFunctions)
 
 	var (
 		key, value string
 		err        error
 	)
-	return func(req *fasthttp.Request, data any) error {
+	return func(reqData *script.RequestData, data any) error {
 		for _, gen := range generators {
 			key, err = gen.Key(data)
 			if err != nil {
@@ -187,41 +250,33 @@ func NewHeadersGeneratorFunc(localRand *rand.Rand, headers types.Headers, templa
 				return err
 			}
 
-			req.Header.Add(key, value)
+			reqData.Headers[key] = append(reqData.Headers[key], value)
 		}
 		return nil
 	}, isDynamic
 }
 
-func NewCookiesGeneratorFunc(localRand *rand.Rand, cookies types.Cookies, templateFunctions template.FuncMap) (RequestGeneratorWithData, bool) {
+func NewCookiesGeneratorFunc(localRand *rand.Rand, cookies types.Cookies, templateFunctions template.FuncMap) (requestDataGenerator, bool) {
 	generators, isDynamic := buildKeyValueGenerators(localRand, cookies, templateFunctions)
 
 	var (
 		key, value string
 		err        error
 	)
-	if len(generators) > 0 {
-		return func(req *fasthttp.Request, data any) error {
-			cookieStrings := make([]string, 0, len(generators))
-			for _, gen := range generators {
-				key, err = gen.Key(data)
-				if err != nil {
-					return err
-				}
-
-				value, err = gen.Value()(data)
-				if err != nil {
-					return err
-				}
-
-				cookieStrings = append(cookieStrings, key+"="+value)
+	return func(reqData *script.RequestData, data any) error {
+		for _, gen := range generators {
+			key, err = gen.Key(data)
+			if err != nil {
+				return err
 			}
-			req.Header.Add("Cookie", strings.Join(cookieStrings, "; "))
-			return nil
-		}, isDynamic
-	}
 
-	return func(req *fasthttp.Request, data any) error {
+			value, err = gen.Value()(data)
+			if err != nil {
+				return err
+			}
+
+			reqData.Cookies[key] = append(reqData.Cookies[key], value)
+		}
 		return nil
 	}, isDynamic
 }
@@ -243,12 +298,12 @@ func NewValuesGeneratorFunc(values []string, templateFunctions template.FuncMap)
 		for _, generator := range generators {
 			rendered, err = generator(nil)
 			if err != nil {
-				return valuesData{}, fmt.Errorf("values rendering: %w", err)
+				return valuesData{}, types.NewTemplateRenderError(err)
 			}
 
 			data, err = godotenv.Unmarshal(rendered)
 			if err != nil {
-				return valuesData{}, fmt.Errorf("values rendering: %w", err)
+				return valuesData{}, types.NewTemplateRenderError(err)
 			}
 
 			maps.Copy(result, data)
@@ -265,7 +320,7 @@ func createTemplateFunc(value string, templateFunctions template.FuncMap) (func(
 		return func(data any) (string, error) {
 			var buf bytes.Buffer
 			if err = tmpl.Execute(&buf, data); err != nil {
-				return "", fmt.Errorf("template rendering: %w", err)
+				return "", types.NewTemplateRenderError(err)
 			}
 			return buf.String(), nil
 		}, true

internal/sarin/sarin.go

@@ -14,6 +14,7 @@ import (
 	tea "github.com/charmbracelet/bubbletea"
 	"github.com/charmbracelet/lipgloss"
 	"github.com/valyala/fasthttp"
+	"go.aykhans.me/sarin/internal/script"
 	"go.aykhans.me/sarin/internal/types"
 )
 
@@ -52,11 +53,14 @@ type sarin struct {
 	hostClients []*fasthttp.HostClient
 	responses   *SarinResponseData
 	fileCache   *FileCache
+	scriptChain *script.Chain
 }
 
 // NewSarin creates a new sarin instance for load testing.
 // It can return the following errors:
-// - types.ProxyDialError
+//   - types.ProxyDialError
+//   - types.ErrScriptEmpty
+//   - types.ScriptLoadError
 func NewSarin(
 	ctx context.Context,
 	methods []string,
@@ -75,6 +79,8 @@ func NewSarin(
 	values []string,
 	collectStats bool,
 	dryRun bool,
+	luaScripts []string,
+	jsScripts []string,
 ) (*sarin, error) {
 	if workers == 0 {
 		workers = 1
@@ -85,6 +91,19 @@ func NewSarin(
 		return nil, err
 	}
 
+	// Load script sources
+	luaSources, err := script.LoadSources(ctx, luaScripts, script.EngineTypeLua)
+	if err != nil {
+		return nil, err
+	}
+
+	jsSources, err := script.LoadSources(ctx, jsScripts, script.EngineTypeJavaScript)
+	if err != nil {
+		return nil, err
+	}
+
+	scriptChain := script.NewChain(luaSources, jsSources)
+
 	srn := &sarin{
 		workers:        workers,
 		requestURL:     requestURL,
@@ -103,6 +122,7 @@ func NewSarin(
 		dryRun:         dryRun,
 		hostClients:    hostClients,
 		fileCache:      NewFileCache(time.Second * 10),
+		scriptChain:    scriptChain,
 	}
 
 	if collectStats {
@@ -193,7 +213,21 @@ func (q sarin) Worker(
 	defer fasthttp.ReleaseRequest(req)
 	defer fasthttp.ReleaseResponse(resp)
 
-	requestGenerator, isDynamic := NewRequestGenerator(q.methods, q.requestURL, q.params, q.headers, q.cookies, q.bodies, q.values, q.fileCache)
+	// Create script transformer for this worker (engines are not thread-safe)
+	// Scripts are pre-validated in NewSarin, so this should not fail
+	var scriptTransformer *script.Transformer
+	if !q.scriptChain.IsEmpty() {
+		var err error
+		scriptTransformer, err = q.scriptChain.NewTransformer()
+		if err != nil {
+			panic(err)
+		}
+		defer scriptTransformer.Close()
+	}
+
+	requestGenerator, isDynamic := NewRequestGenerator(
+		q.methods, q.requestURL, q.params, q.headers, q.cookies, q.bodies, q.values, q.fileCache, scriptTransformer,
+	)
 
 	if q.dryRun {
 		switch {

internal/sarin/template.go

@@ -3,7 +3,6 @@ package sarin
 import (
 	"bytes"
 	"encoding/base64"
-	"errors"
 	"math/rand/v2"
 	"mime/multipart"
 	"strings"
@@ -12,6 +11,7 @@ import (
 	"time"
 
 	"github.com/brianvoe/gofakeit/v7"
+	"go.aykhans.me/sarin/internal/types"
 )
 
 func NewDefaultTemplateFuncMap(randSource rand.Source, fileCache *FileCache) template.FuncMap {
@@ -90,7 +90,7 @@ func NewDefaultTemplateFuncMap(randSource rand.Source, fileCache *FileCache) tem
 		//        {{ file_Base64 "https://example.com/image.png" }}
 		"file_Base64": func(source string) (string, error) {
 			if fileCache == nil {
-				return "", errors.New("file cache is not initialized")
+				return "", types.ErrFileCacheNotInitialized
 			}
 			cached, err := fileCache.GetOrLoad(source)
 			if err != nil {
@@ -582,7 +582,7 @@ func NewDefaultBodyTemplateFuncMap(
 		//   {{ body_FormData "name" "John" "avatar" "@/path/to/photo.jpg" "doc" "@https://example.com/file.pdf" }}
 		funcMap["body_FormData"] = func(pairs ...string) (string, error) {
 			if len(pairs)%2 != 0 {
-				return "", errors.New("body_FormData requires an even number of arguments (key-value pairs)")
+				return "", types.ErrFormDataOddArgs
 			}
 
 			var multipartData bytes.Buffer
@@ -602,7 +602,7 @@ func NewDefaultBodyTemplateFuncMap(
 				case strings.HasPrefix(val, "@"):
 					// File (local path or remote URL)
 					if fileCache == nil {
-						return "", errors.New("file cache is not initialized")
+						return "", types.ErrFileCacheNotInitialized
 					}
 					source := val[1:]
 					cached, err := fileCache.GetOrLoad(source)

internal/script/chain.go

@@ -0,0 +1,107 @@
+package script
+
+import (
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// Chain holds the loaded script sources and can create engine instances.
+// The sources are loaded once, but engines are created per-worker since they're not thread-safe.
+type Chain struct {
+	luaSources []*Source
+	jsSources  []*Source
+}
+
+// NewChain creates a new script chain from loaded sources.
+// Lua scripts run first, then JavaScript scripts, in the order provided.
+func NewChain(luaSources, jsSources []*Source) *Chain {
+	return &Chain{
+		luaSources: luaSources,
+		jsSources:  jsSources,
+	}
+}
+
+// IsEmpty returns true if there are no scripts to execute.
+func (c *Chain) IsEmpty() bool {
+	return len(c.luaSources) == 0 && len(c.jsSources) == 0
+}
+
+// Transformer holds instantiated script engines for a single worker.
+// It is NOT safe for concurrent use.
+type Transformer struct {
+	luaEngines []*LuaEngine
+	jsEngines  []*JsEngine
+}
+
+// NewTransformer creates engine instances from the chain's sources.
+// Call this once per worker goroutine.
+// It can return the following errors:
+//   - types.ScriptChainError
+func (c *Chain) NewTransformer() (*Transformer, error) {
+	if c.IsEmpty() {
+		return &Transformer{}, nil
+	}
+
+	t := &Transformer{
+		luaEngines: make([]*LuaEngine, 0, len(c.luaSources)),
+		jsEngines:  make([]*JsEngine, 0, len(c.jsSources)),
+	}
+
+	// Create Lua engines
+	for i, src := range c.luaSources {
+		engine, err := NewLuaEngine(src.Content)
+		if err != nil {
+			t.Close() // Clean up already created engines
+			return nil, types.NewScriptChainError("lua", i, err)
+		}
+		t.luaEngines = append(t.luaEngines, engine)
+	}
+
+	// Create JS engines
+	for i, src := range c.jsSources {
+		engine, err := NewJsEngine(src.Content)
+		if err != nil {
+			t.Close() // Clean up already created engines
+			return nil, types.NewScriptChainError("js", i, err)
+		}
+		t.jsEngines = append(t.jsEngines, engine)
+	}
+
+	return t, nil
+}
+
+// Transform applies all scripts to the request data.
+// Lua scripts run first, then JavaScript scripts.
+// It can return the following errors:
+//   - types.ScriptChainError
+func (t *Transformer) Transform(req *RequestData) error {
+	// Run Lua scripts
+	for i, engine := range t.luaEngines {
+		if err := engine.Transform(req); err != nil {
+			return types.NewScriptChainError("lua", i, err)
+		}
+	}
+
+	// Run JS scripts
+	for i, engine := range t.jsEngines {
+		if err := engine.Transform(req); err != nil {
+			return types.NewScriptChainError("js", i, err)
+		}
+	}
+
+	return nil
+}
+
+// Close releases all engine resources.
+func (t *Transformer) Close() {
+	for _, engine := range t.luaEngines {
+		engine.Close()
+	}
+	for _, engine := range t.jsEngines {
+		engine.Close()
+	}
+}
+
+// IsEmpty returns true if there are no engines.
+func (t *Transformer) IsEmpty() bool {
+	return len(t.luaEngines) == 0 && len(t.jsEngines) == 0
+}

internal/script/js.go

@@ -0,0 +1,198 @@
+package script
+
+import (
+	"errors"
+
+	"github.com/dop251/goja"
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// JsEngine implements the Engine interface using goja (JavaScript).
+type JsEngine struct {
+	runtime   *goja.Runtime
+	transform goja.Callable
+}
+
+// NewJsEngine creates a new JavaScript script engine with the given script content.
+// The script must define a global `transform` function that takes a request object
+// and returns the modified request object.
+//
+// Example JavaScript script:
+//
+//	function transform(req) {
+//	    req.headers["X-Custom"] = ["value"];
+//	    return req;
+//	}
+//
+// It can return the following errors:
+//   - types.ErrScriptTransformMissing
+//   - types.ScriptExecutionError
+func NewJsEngine(scriptContent string) (*JsEngine, error) {
+	vm := goja.New()
+
+	// Execute the script to define the transform function
+	_, err := vm.RunString(scriptContent)
+	if err != nil {
+		return nil, types.NewScriptExecutionError("JavaScript", err)
+	}
+
+	// Get the transform function
+	transformVal := vm.Get("transform")
+	if transformVal == nil || goja.IsUndefined(transformVal) || goja.IsNull(transformVal) {
+		return nil, types.ErrScriptTransformMissing
+	}
+
+	transform, ok := goja.AssertFunction(transformVal)
+	if !ok {
+		return nil, types.NewScriptExecutionError("JavaScript", errors.New("'transform' must be a function"))
+	}
+
+	return &JsEngine{
+		runtime:   vm,
+		transform: transform,
+	}, nil
+}
+
+// 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 {
+	// 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)
+	}
+
+	// Update RequestData from the returned object
+	if err := e.objectToRequestData(result, req); err != nil {
+		return types.NewScriptExecutionError("JavaScript", err)
+	}
+
+	return nil
+}
+
+// Close releases the JavaScript runtime resources.
+func (e *JsEngine) Close() {
+	// goja doesn't have an explicit close method, but we can help GC
+	e.runtime = nil
+	e.transform = nil
+}
+
+// requestDataToObject converts RequestData to a goja Value (JavaScript object).
+func (e *JsEngine) requestDataToObject(req *RequestData) goja.Value {
+	obj := e.runtime.NewObject()
+
+	_ = obj.Set("method", req.Method)
+	_ = obj.Set("path", req.Path)
+	_ = obj.Set("body", req.Body)
+
+	// Headers (map[string][]string -> object of arrays)
+	headers := e.runtime.NewObject()
+	for k, values := range req.Headers {
+		_ = headers.Set(k, e.stringSliceToArray(values))
+	}
+	_ = obj.Set("headers", headers)
+
+	// Params (map[string][]string -> object of arrays)
+	params := e.runtime.NewObject()
+	for k, values := range req.Params {
+		_ = params.Set(k, e.stringSliceToArray(values))
+	}
+	_ = obj.Set("params", params)
+
+	// Cookies (map[string][]string -> object of arrays)
+	cookies := e.runtime.NewObject()
+	for k, values := range req.Cookies {
+		_ = cookies.Set(k, e.stringSliceToArray(values))
+	}
+	_ = obj.Set("cookies", cookies)
+
+	return obj
+}
+
+// objectToRequestData updates RequestData from a JavaScript object.
+func (e *JsEngine) objectToRequestData(val goja.Value, req *RequestData) error {
+	if val == nil || goja.IsUndefined(val) || goja.IsNull(val) {
+		return types.ErrScriptTransformReturnObject
+	}
+
+	obj := val.ToObject(e.runtime)
+	if obj == nil {
+		return types.ErrScriptTransformReturnObject
+	}
+
+	// Method
+	if v := obj.Get("method"); v != nil && !goja.IsUndefined(v) {
+		req.Method = v.String()
+	}
+
+	// Path
+	if v := obj.Get("path"); v != nil && !goja.IsUndefined(v) {
+		req.Path = v.String()
+	}
+
+	// Body
+	if v := obj.Get("body"); v != nil && !goja.IsUndefined(v) {
+		req.Body = v.String()
+	}
+
+	// Headers
+	if v := obj.Get("headers"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+		req.Headers = e.objectToStringSliceMap(v.ToObject(e.runtime))
+	}
+
+	// Params
+	if v := obj.Get("params"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+		req.Params = e.objectToStringSliceMap(v.ToObject(e.runtime))
+	}
+
+	// Cookies
+	if v := obj.Get("cookies"); v != nil && !goja.IsUndefined(v) && !goja.IsNull(v) {
+		req.Cookies = e.objectToStringSliceMap(v.ToObject(e.runtime))
+	}
+
+	return nil
+}
+
+// stringSliceToArray converts a Go []string to a JavaScript array.
+func (e *JsEngine) stringSliceToArray(values []string) *goja.Object {
+	ifaces := make([]any, len(values))
+	for i, v := range values {
+		ifaces[i] = v
+	}
+	return e.runtime.NewArray(ifaces...)
+}
+
+// objectToStringSliceMap converts a JavaScript object to a Go map[string][]string.
+// Supports both single string values and array values.
+func (e *JsEngine) objectToStringSliceMap(obj *goja.Object) map[string][]string {
+	if obj == nil {
+		return make(map[string][]string)
+	}
+
+	result := make(map[string][]string)
+	for _, key := range obj.Keys() {
+		v := obj.Get(key)
+		if v == nil || goja.IsUndefined(v) || goja.IsNull(v) {
+			continue
+		}
+
+		// Check if it's an array
+		if arr, ok := v.Export().([]any); ok {
+			var values []string
+			for _, item := range arr {
+				if s, ok := item.(string); ok {
+					values = append(values, s)
+				}
+			}
+			result[key] = values
+		} else {
+			// Single value - wrap in slice
+			result[key] = []string{v.String()}
+		}
+	}
+	return result
+}

internal/script/lua.go

@@ -0,0 +1,191 @@
+package script
+
+import (
+	"fmt"
+
+	lua "github.com/yuin/gopher-lua"
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// LuaEngine implements the Engine interface using gopher-lua.
+type LuaEngine struct {
+	state     *lua.LState
+	transform *lua.LFunction
+}
+
+// NewLuaEngine creates a new Lua script engine with the given script content.
+// The script must define a global `transform` function that takes a request table
+// and returns the modified request table.
+//
+// Example Lua script:
+//
+//	function transform(req)
+//	    req.headers["X-Custom"] = {"value"}
+//	    return req
+//	end
+//
+// It can return the following errors:
+//   - types.ErrScriptTransformMissing
+//   - types.ScriptExecutionError
+func NewLuaEngine(scriptContent string) (*LuaEngine, error) {
+	L := lua.NewState()
+
+	// Execute the script to define the transform function
+	if err := L.DoString(scriptContent); err != nil {
+		L.Close()
+		return nil, types.NewScriptExecutionError("Lua", err)
+	}
+
+	// Get the transform function
+	transform := L.GetGlobal("transform")
+	if transform.Type() != lua.LTFunction {
+		L.Close()
+		return nil, types.ErrScriptTransformMissing
+	}
+
+	return &LuaEngine{
+		state:     L,
+		transform: transform.(*lua.LFunction),
+	}, nil
+}
+
+// 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 {
+	// Convert RequestData to Lua table
+	reqTable := e.requestDataToTable(req)
+
+	// Call transform(req)
+	e.state.Push(e.transform)
+	e.state.Push(reqTable)
+	if err := e.state.PCall(1, 1, nil); err != nil {
+		return types.NewScriptExecutionError("Lua", err)
+	}
+
+	// Get the result
+	result := e.state.Get(-1)
+	e.state.Pop(1)
+
+	if result.Type() != lua.LTTable {
+		return types.NewScriptExecutionError("Lua", fmt.Errorf("transform function must return a table, got %s", result.Type()))
+	}
+
+	// Update RequestData from the returned table
+	e.tableToRequestData(result.(*lua.LTable), req)
+
+	return nil
+}
+
+// Close releases the Lua state resources.
+func (e *LuaEngine) Close() {
+	if e.state != nil {
+		e.state.Close()
+	}
+}
+
+// requestDataToTable converts RequestData to a Lua table.
+func (e *LuaEngine) requestDataToTable(req *RequestData) *lua.LTable {
+	L := e.state
+	t := L.NewTable()
+
+	t.RawSetString("method", lua.LString(req.Method))
+	t.RawSetString("path", lua.LString(req.Path))
+	t.RawSetString("body", lua.LString(req.Body))
+
+	// Headers (map[string][]string -> table of arrays)
+	headers := L.NewTable()
+	for k, values := range req.Headers {
+		arr := L.NewTable()
+		for _, v := range values {
+			arr.Append(lua.LString(v))
+		}
+		headers.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
+}
+
+// tableToRequestData updates RequestData from a Lua table.
+func (e *LuaEngine) tableToRequestData(t *lua.LTable, req *RequestData) {
+	// Method
+	if v := t.RawGetString("method"); v.Type() == lua.LTString {
+		req.Method = string(v.(lua.LString))
+	}
+
+	// Path
+	if v := t.RawGetString("path"); v.Type() == lua.LTString {
+		req.Path = string(v.(lua.LString))
+	}
+
+	// Body
+	if v := t.RawGetString("body"); v.Type() == lua.LTString {
+		req.Body = string(v.(lua.LString))
+	}
+
+	// Headers
+	if v := t.RawGetString("headers"); v.Type() == lua.LTTable {
+		req.Headers = e.tableToStringSliceMap(v.(*lua.LTable))
+	}
+
+	// Params
+	if v := t.RawGetString("params"); v.Type() == lua.LTTable {
+		req.Params = e.tableToStringSliceMap(v.(*lua.LTable))
+	}
+
+	// Cookies
+	if v := t.RawGetString("cookies"); v.Type() == lua.LTTable {
+		req.Cookies = e.tableToStringSliceMap(v.(*lua.LTable))
+	}
+}
+
+// tableToStringSliceMap converts a Lua table to a Go map[string][]string.
+// Supports both single string values and array values.
+func (e *LuaEngine) tableToStringSliceMap(t *lua.LTable) map[string][]string {
+	result := make(map[string][]string)
+	t.ForEach(func(k, v lua.LValue) {
+		if k.Type() != lua.LTString {
+			return
+		}
+		key := string(k.(lua.LString))
+
+		switch v.Type() {
+		case lua.LTString:
+			// Single string value
+			result[key] = []string{string(v.(lua.LString))}
+		case lua.LTTable:
+			// Array of strings
+			var values []string
+			v.(*lua.LTable).ForEach(func(_, item lua.LValue) {
+				if item.Type() == lua.LTString {
+					values = append(values, string(item.(lua.LString)))
+				}
+			})
+			result[key] = values
+		}
+	})
+	return result
+}

internal/script/script.go

@@ -0,0 +1,197 @@
+package script
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"go.aykhans.me/sarin/internal/types"
+)
+
+// RequestData represents the request data passed to scripts for transformation.
+// Scripts can modify any field and the changes will be applied to the actual request.
+// Headers, Params, and Cookies use []string values to support multiple values per key.
+type RequestData struct {
+	Method  string              `json:"method"`
+	Path    string              `json:"path"`
+	Headers map[string][]string `json:"headers"`
+	Params  map[string][]string `json:"params"`
+	Cookies map[string][]string `json:"cookies"`
+	Body    string              `json:"body"`
+}
+
+// Engine defines the interface for script engines (Lua, JavaScript).
+// Each engine must be able to transform request data using a user-provided script.
+type Engine interface {
+	// Transform executes the script's transform function with the given request data.
+	// The script should modify the RequestData and return it.
+	Transform(req *RequestData) error
+
+	// Close releases any resources held by the engine.
+	Close()
+}
+
+// EngineType represents the type of script engine.
+type EngineType string
+
+const (
+	EngineTypeLua        EngineType = "lua"
+	EngineTypeJavaScript EngineType = "js"
+)
+
+// Source represents a loaded script source.
+type Source struct {
+	Content    string
+	EngineType EngineType
+}
+
+// LoadSource loads a script from the given source string.
+// The source can be:
+//   - Inline script: any string not starting with "@"
+//   - Escaped "@": strings starting with "@@" (literal "@" at start, returns string without first @)
+//   - File reference: "@/path/to/file" or "@./relative/path"
+//   - URL reference: "@http://..." or "@https://..."
+//
+// It can return the following errors:
+//   - types.ErrScriptEmpty
+//   - types.ScriptLoadError
+func LoadSource(ctx context.Context, source string, engineType EngineType) (*Source, error) {
+	if source == "" {
+		return nil, types.ErrScriptEmpty
+	}
+
+	var content string
+	var err error
+
+	switch {
+	case strings.HasPrefix(source, "@@"):
+		// Escaped @ - it's an inline script starting with literal @
+		content = source[1:] // Remove first @, keep the rest
+	case strings.HasPrefix(source, "@"):
+		// File or URL reference
+		ref := source[1:]
+		if strings.HasPrefix(ref, "http://") || strings.HasPrefix(ref, "https://") {
+			content, err = fetchURL(ctx, ref)
+		} else {
+			content, err = readFile(ref)
+		}
+		if err != nil {
+			return nil, types.NewScriptLoadError(ref, err)
+		}
+	default:
+		// Inline script
+		content = source
+	}
+
+	return &Source{
+		Content:    content,
+		EngineType: engineType,
+	}, nil
+}
+
+// LoadSources loads multiple script sources.
+// It can return the following errors:
+//   - types.ErrScriptEmpty
+//   - types.ScriptLoadError
+func LoadSources(ctx context.Context, sources []string, engineType EngineType) ([]*Source, error) {
+	loaded := make([]*Source, 0, len(sources))
+	for _, src := range sources {
+		source, err := LoadSource(ctx, src, engineType)
+		if err != nil {
+			return nil, err
+		}
+		loaded = append(loaded, source)
+	}
+	return loaded, nil
+}
+
+// ValidateScript validates a script source by loading it and checking syntax.
+// It loads the script (from file/URL/inline), parses it, and verifies
+// that a 'transform' function is defined.
+// It can return the following errors:
+//   - types.ErrScriptEmpty
+//   - types.ErrScriptTransformMissing
+//   - types.ScriptLoadError
+//   - types.ScriptExecutionError
+//   - types.ScriptUnknownEngineError
+func ValidateScript(ctx context.Context, source string, engineType EngineType) error {
+	// Load the script source
+	src, err := LoadSource(ctx, source, engineType)
+	if err != nil {
+		return err
+	}
+
+	// Try to create an engine - this validates syntax and transform function
+	var engine Engine
+	switch engineType {
+	case EngineTypeLua:
+		engine, err = NewLuaEngine(src.Content)
+	case EngineTypeJavaScript:
+		engine, err = NewJsEngine(src.Content)
+	default:
+		return types.NewScriptUnknownEngineError(string(engineType))
+	}
+
+	if err != nil {
+		return err
+	}
+
+	// Clean up the engine - we only needed it for validation
+	engine.Close()
+	return nil
+}
+
+// fetchURL downloads content from an HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
+func fetchURL(ctx context.Context, url string) (string, error) {
+	ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return "", types.NewHTTPFetchError(url, err)
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", types.NewHTTPFetchError(url, err)
+	}
+	defer resp.Body.Close() //nolint:errcheck
+
+	if resp.StatusCode != http.StatusOK {
+		return "", types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
+	}
+
+	data, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", types.NewHTTPFetchError(url, err)
+	}
+
+	return string(data), nil
+}
+
+// readFile reads content from a local file.
+// It can return the following errors:
+//   - types.FileReadError
+func readFile(path string) (string, error) {
+	if !filepath.IsAbs(path) {
+		pwd, err := os.Getwd()
+		if err != nil {
+			return "", types.NewFileReadError(path, err)
+		}
+		path = filepath.Join(pwd, path)
+	}
+
+	data, err := os.ReadFile(path) //nolint:gosec
+	if err != nil {
+		return "", types.NewFileReadError(path, err)
+	}
+
+	return string(data), nil
+}

internal/types/errors.go

@@ -6,16 +6,12 @@ import (
 	"strings"
 )
 
-var (
-	// General
-	ErrNoError = errors.New("no error (internal)")
-
-	// CLI
-	ErrCLINoArgs = errors.New("CLI expects arguments but received none")
-)
-
 // ======================================== General ========================================
 
+var (
+	errNoError = errors.New("no error (internal)")
+)
+
 type FieldParseError struct {
 	Field string
 	Value string
@@ -24,7 +20,7 @@ type FieldParseError struct {
 
 func NewFieldParseError(field string, value string, err error) FieldParseError {
 	if err == nil {
-		err = ErrNoError
+		err = errNoError
 	}
 	return FieldParseError{field, value, err}
 }
@@ -72,7 +68,7 @@ type FieldValidationError struct {
 
 func NewFieldValidationError(field string, value string, err error) FieldValidationError {
 	if err == nil {
-		err = ErrNoError
+		err = errNoError
 	}
 	return FieldValidationError{field, value, err}
 }
@@ -118,7 +114,7 @@ type UnmarshalError struct {
 
 func NewUnmarshalError(err error) UnmarshalError {
 	if err == nil {
-		err = ErrNoError
+		err = errNoError
 	}
 	return UnmarshalError{err}
 }
@@ -131,8 +127,133 @@ func (e UnmarshalError) Unwrap() error {
 	return e.error
 }
 
+// ======================================== General I/O ========================================
+
+type FileReadError struct {
+	Path string
+	Err  error
+}
+
+func NewFileReadError(path string, err error) FileReadError {
+	if err == nil {
+		err = errNoError
+	}
+	return FileReadError{path, err}
+}
+
+func (e FileReadError) Error() string {
+	return fmt.Sprintf("failed to read file %s: %v", e.Path, e.Err)
+}
+
+func (e FileReadError) Unwrap() error {
+	return e.Err
+}
+
+type HTTPFetchError struct {
+	URL string
+	Err error
+}
+
+func NewHTTPFetchError(url string, err error) HTTPFetchError {
+	if err == nil {
+		err = errNoError
+	}
+	return HTTPFetchError{url, err}
+}
+
+func (e HTTPFetchError) Error() string {
+	return fmt.Sprintf("failed to fetch %s: %v", e.URL, e.Err)
+}
+
+func (e HTTPFetchError) Unwrap() error {
+	return e.Err
+}
+
+type HTTPStatusError struct {
+	URL        string
+	StatusCode int
+	Status     string
+}
+
+func NewHTTPStatusError(url string, statusCode int, status string) HTTPStatusError {
+	return HTTPStatusError{url, statusCode, status}
+}
+
+func (e HTTPStatusError) Error() string {
+	return fmt.Sprintf("HTTP %d %s (url: %s)", e.StatusCode, e.Status, e.URL)
+}
+
+type URLParseError struct {
+	URL string
+	Err error
+}
+
+func NewURLParseError(url string, err error) URLParseError {
+	if err == nil {
+		err = errNoError
+	}
+	return URLParseError{url, err}
+}
+
+func (e URLParseError) Error() string {
+	return fmt.Sprintf("invalid URL %q: %v", e.URL, e.Err)
+}
+
+func (e URLParseError) Unwrap() error {
+	return e.Err
+}
+
+// ======================================== Template ========================================
+
+var (
+	ErrFileCacheNotInitialized = errors.New("file cache is not initialized")
+	ErrFormDataOddArgs         = errors.New("body_FormData requires an even number of arguments (key-value pairs)")
+)
+
+type TemplateParseError struct {
+	Err error
+}
+
+func NewTemplateParseError(err error) TemplateParseError {
+	if err == nil {
+		err = errNoError
+	}
+	return TemplateParseError{err}
+}
+
+func (e TemplateParseError) Error() string {
+	return "template parse error: " + e.Err.Error()
+}
+
+func (e TemplateParseError) Unwrap() error {
+	return e.Err
+}
+
+type TemplateRenderError struct {
+	Err error
+}
+
+func NewTemplateRenderError(err error) TemplateRenderError {
+	if err == nil {
+		err = errNoError
+	}
+	return TemplateRenderError{err}
+}
+
+func (e TemplateRenderError) Error() string {
+	return "template rendering: " + e.Err.Error()
+}
+
+func (e TemplateRenderError) Unwrap() error {
+	return e.Err
+}
+
 // ======================================== CLI ========================================
 
+var (
+	ErrCLINoArgs = errors.New("CLI expects arguments but received none")
+)
+
 type CLIUnexpectedArgsError struct {
 	Args []string
 }
@@ -153,7 +274,7 @@ type ConfigFileReadError struct {
 
 func NewConfigFileReadError(err error) ConfigFileReadError {
 	if err == nil {
-		err = ErrNoError
+		err = errNoError
 	}
 	return ConfigFileReadError{err}
 }
@@ -168,6 +289,61 @@ func (e ConfigFileReadError) Unwrap() error {
 
 // ======================================== Proxy ========================================
 
+type ProxyUnsupportedSchemeError struct {
+	Scheme string
+}
+
+func NewProxyUnsupportedSchemeError(scheme string) ProxyUnsupportedSchemeError {
+	return ProxyUnsupportedSchemeError{scheme}
+}
+
+func (e ProxyUnsupportedSchemeError) Error() string {
+	return "unsupported proxy scheme: " + e.Scheme
+}
+
+type ProxyParseError struct {
+	Err error
+}
+
+func NewProxyParseError(err error) ProxyParseError {
+	if err == nil {
+		err = errNoError
+	}
+	return ProxyParseError{err}
+}
+
+func (e ProxyParseError) Error() string {
+	return "failed to parse proxy URL: " + e.Err.Error()
+}
+
+func (e ProxyParseError) Unwrap() error {
+	return e.Err
+}
+
+type ProxyConnectError struct {
+	Status string
+}
+
+func NewProxyConnectError(status string) ProxyConnectError {
+	return ProxyConnectError{status}
+}
+
+func (e ProxyConnectError) Error() string {
+	return "proxy CONNECT failed: " + e.Status
+}
+
+type ProxyResolveError struct {
+	Host string
+}
+
+func NewProxyResolveError(host string) ProxyResolveError {
+	return ProxyResolveError{host}
+}
+
+func (e ProxyResolveError) Error() string {
+	return "no IP addresses found for host: " + e.Host
+}
+
 type ProxyDialError struct {
 	Proxy string
 	Err   error
@@ -175,7 +351,7 @@ type ProxyDialError struct {
 
 func NewProxyDialError(proxy string, err error) ProxyDialError {
 	if err == nil {
-		err = ErrNoError
+		err = errNoError
 	}
 	return ProxyDialError{proxy, err}
 }
@@ -187,3 +363,86 @@ func (e ProxyDialError) Error() string {
 func (e ProxyDialError) Unwrap() error {
 	return e.Err
 }
+
+// ======================================== Script ========================================
+
+var (
+	ErrScriptEmpty                 = errors.New("script cannot be empty")
+	ErrScriptSourceEmpty           = errors.New("script source cannot be empty after @")
+	ErrScriptTransformMissing      = errors.New("script must define a global 'transform' function")
+	ErrScriptTransformReturnObject = errors.New("transform function must return an object")
+	ErrScriptURLNoHost             = errors.New("script URL must have a host")
+)
+
+type ScriptLoadError struct {
+	Source string
+	Err    error
+}
+
+func NewScriptLoadError(source string, err error) ScriptLoadError {
+	if err == nil {
+		err = errNoError
+	}
+	return ScriptLoadError{source, err}
+}
+
+func (e ScriptLoadError) Error() string {
+	return fmt.Sprintf("failed to load script from %q: %v", e.Source, e.Err)
+}
+
+func (e ScriptLoadError) Unwrap() error {
+	return e.Err
+}
+
+type ScriptExecutionError struct {
+	EngineType string
+	Err        error
+}
+
+func NewScriptExecutionError(engineType string, err error) ScriptExecutionError {
+	if err == nil {
+		err = errNoError
+	}
+	return ScriptExecutionError{engineType, err}
+}
+
+func (e ScriptExecutionError) Error() string {
+	return fmt.Sprintf("%s script error: %v", e.EngineType, e.Err)
+}
+
+func (e ScriptExecutionError) Unwrap() error {
+	return e.Err
+}
+
+type ScriptChainError struct {
+	EngineType string
+	Index      int
+	Err        error
+}
+
+func NewScriptChainError(engineType string, index int, err error) ScriptChainError {
+	if err == nil {
+		err = errNoError
+	}
+	return ScriptChainError{engineType, index, err}
+}
+
+func (e ScriptChainError) Error() string {
+	return fmt.Sprintf("%s script[%d]: %v", e.EngineType, e.Index, e.Err)
+}
+
+func (e ScriptChainError) Unwrap() error {
+	return e.Err
+}
+
+type ScriptUnknownEngineError struct {
+	EngineType string
+}
+
+func NewScriptUnknownEngineError(engineType string) ScriptUnknownEngineError {
+	return ScriptUnknownEngineError{engineType}
+}
+
+func (e ScriptUnknownEngineError) Error() string {
+	return "unknown engine type: " + e.EngineType
+}

internal/types/proxy.go

@@ -1,7 +1,6 @@
 package types
 
 import (
-	"fmt"
 	"net/url"
 )
 
@@ -17,6 +16,9 @@ func (proxies *Proxies) Append(proxy ...Proxy) {
 	*proxies = append(*proxies, proxy...)
 }
 
+// Parse parses a raw proxy string and appends it to the list.
+// It can return the following errors:
+//   - ProxyParseError
 func (proxies *Proxies) Parse(rawValue string) error {
 	parsedProxy, err := ParseProxy(rawValue)
 	if err != nil {
@@ -27,10 +29,13 @@ func (proxies *Proxies) Parse(rawValue string) error {
 	return nil
 }
 
+// ParseProxy parses a raw proxy URL string into a Proxy.
+// It can return the following errors:
+//   - ProxyParseError
 func ParseProxy(rawValue string) (*Proxy, error) {
 	urlParsed, err := url.Parse(rawValue)
 	if err != nil {
-		return nil, fmt.Errorf("failed to parse proxy URL: %w", err)
+		return nil, NewProxyParseError(err)
 	}
 
 	proxyParsed := Proxy(*urlParsed)