Custom scripts

Trace includes a JavaScript-based scripting engine to modify requests and responses on the fly.

What scripts can do

• Rewrite URLs or headers
• Transform request or response bodies
• Mock dynamic responses
• Apply logic that is hard to express with simple rules

Script types

• Request scripts: run before the request is sent
• Response scripts: run after the response is received
• Both: run on both phases

Execution model

• Scripts run inside Trace on-device.
• A script can inspect or modify headers, bodies, and metadata.
• If a script fails, Trace logs the error and continues with the original request.

When to use scripts

• You need conditional logic (if/else) instead of static rules
• You need to compute a response body dynamically
• You want to prototype backend changes without deploying

Tips

• Keep scripts small and focused for easier debugging.
• Prefer rewrite rules for simple header or URL changes.
• Export scripts to share them with teammates.

Debugging scripts

• Start with a narrow filter so you see only the requests your script touches.
• Add temporary logging in the script to validate inputs and outputs.
• Disable scripts one by one if you see unexpected behavior.