Try Placebo API

This page asks /api/demo.json for data. Make a rule for it and watch the answer change.

1. Open Placebo API (its icon in Chrome's toolbar) and click Add Rule.
2. Pattern: /api/demo.json. Action: Override response. Response body, for example:

   { "message": "Hello from Placebo API!" }

3. Save, then click a button below. No reload needed.

Click a button to send a request.

With no rule, GET returns the real file and POST fails (this static site doesn't accept POST). A rule with Serve without network call answers both. DevTools → Network still shows the real response; the page console logs every override as [Placebo API].