Scripting

Scripting lets you write small Rhai rules that change live traffic as it passes through WePROXA. Use it when a breakpoint would be too manual, or when you want the same request or response edit applied every time a URL matches.

How It Works

A script rule combines a URL pattern, an optional HTTP method filter, and one or both script phases:

• Request phase - runs after request breakpoints and before Map Local matching or upstream forwarding.
• Response phase - runs on proxied upstream responses before they are sent back to the client.

If multiple enabled script rules match the same request, WePROXA runs them in creation order. Each rule receives the current request or response state, including changes made by earlier rules.

Note

Free accounts are limited to 2 Scripting rules. Upgrade to Pro for unlimited script rules.

Creating a Script Rule

1. Open the Scripting tool from the toolbar, Tools menu, or tray menu.
2. Click Add Rule.
3. Enter a rule name and URL pattern, such as https://api.example.com/v1/*.
4. Optionally choose an HTTP method.
5. Select Run on request, Run on response, or both.
6. Write the script and save the rule.

Each rule can be enabled or disabled individually, duplicated, edited, deleted, and sorted by name or date. The global Scripting toggle turns the whole rule engine on or off without deleting rules.

Entry Points

Define on_request to modify requests and on_response to modify responses. The recommended style is to mutate this:

fn on_request() {
    this.headers["x-debug-client"] = "weproxa";
}

fn on_response() {
    if this.status == 200 {
        this.headers["x-served-through"] = "weproxa";
    }
}

You can also accept the request or response map as an argument, but you must return it:

fn on_request(req) {
    req.headers["x-debug-client"] = "weproxa";
    req
}

Request Fields

Request scripts can read and update these fields:

Field	Type	Notes
url	string	Full request URL. Updating this is the most direct way to rewrite a destination.
method	string	HTTP method, such as GET or POST.
scheme	string	URL scheme, usually http or https.
host	string	Request host.
port	number	Resolved URL port.
path	string	Path portion of the URL.
query	map	Query parameters. Duplicate query keys are represented by the first value.
headers	map	Lowercase header name to string value.
body	string	Request body decoded as text.

If you change body, WePROXA updates Content-Length automatically.

URL Rewrites and Default Ports

Changing url directly is the most explicit way to route a request somewhere else. You can also update scheme, host, port, path, or query independently.

When a script changes only the scheme, WePROXA uses the new scheme’s default port instead of carrying over the old one. For example, changing http to https routes to 443 unless you set port yourself.

fn on_request() {
    this.scheme = "https";
}

Set this.port explicitly whenever you need a non-default destination port.

Response Fields

Response scripts can read and update these fields:

Field	Type	Notes
status	number	HTTP status code.
headers	map	Lowercase header name to string value.
body	string	Response body decoded as text.

If an upstream response is compressed, scripts see the plaintext body. When a script changes the body, WePROXA removes stale content-encoding headers and sends the updated plaintext body.

Response scripts run only when the response body can be safely buffered for scripting. Very large or long-lived streaming responses continue through the proxy without forcing unbounded buffering.

Examples

Add a Header to Matching Requests

fn on_request() {
    this.headers["x-debug-session"] = "local-dev";
}

Rewrite an API Path

fn on_request() {
    if this.path == "/v1/profile" {
        this.path = "/v1/profile-preview";
    }
}

Simulate an Error Response

fn on_response() {
    this.status = 503;
    this.headers["content-type"] = "application/json";
    this.body = "{\"error\":\"temporarily unavailable\"}";
}

Tag Responses

fn on_response() {
    this.headers["x-weproxa-script"] = "tagged";
}

Working With Other Tools

• Breakpoints - request scripts run after request breakpoints, so scripts can automate edits after a manual review step.
• Map Local - request scripts can rewrite the URL before Map Local checks for a matching file rule.
• Diff Requests - add scripted and unscripted captures to Diff to verify exactly what changed.
• Network Conditioning - delays still apply to matching traffic after request-phase scripts complete.

Safety Limits

Scripting is designed for fast request and response edits. WePROXA limits script operations, call depth, string size, collection size, and scriptable response body size so accidental infinite loops or very large generated data cannot stall capture indefinitely.

If a script has a syntax error, WePROXA rejects the rule before saving it. If a saved script throws at runtime, that invocation is skipped and traffic continues with the last valid state.