Placebo API: Mock API Responses, Override & Block Requests

Mock API responses and error status codes (500, 401, 429), override fetch/XHR, block or redirect requests. No backend changes.

Mock any API response in your browser. Placebo API lets frontend developers and testers change what the browser receives, without touching the backend, a proxy or certificates. Works without an account: install it, add the demo rule from the welcome screen and see a mocked response in 30 seconds. Your rules stay in your browser. WHAT YOU CAN DO • Override responses of fetch and XMLHttpRequest calls: any status code (200–599) and any body, for GET, POST, PUT, PATCH, DELETE and more. • Mock without a server: answer locally and never send the request, or let the real request go and only replace what comes back. • Block requests, or redirect them to another URL. Block and redirect also work for scripts, stylesheets, images, frames and workers. • Modify request and response headers: set or remove any header, for every request type including page loads. • Dynamic bodies: write a small JavaScript function that builds the body from the real one (needs "Allow User Scripts", one switch in Chrome's extension settings). • Match URLs by a simple "contains" text or a regular expression, for every method or only the ones you choose. • Organise rules in folders, switch a whole folder on or off, and drag to reorder. The first matching rule wins. • Import and export rules as JSON, and import Requestly exports: bodies, headers, redirects, cancel rules and JavaScript (groups become folders, priority is kept). • Changes apply to open tabs immediately. Every override is logged in the page console. • Sync your rules between browsers (paid plans, opt-in), and share Team Rules with everyone on a Team plan: each person switches team rules on or off for themselves, and their own rules come first. TYPICAL USES • Test error states: 500, 401, 429, empty lists, broken payloads, failed saves. • Keep building while the backend endpoint doesn't exist yet. • Reproduce a bug with the exact payload from a report. • Load your local build of a production script, or block analytics while you develop. PRIVATE BY DESIGN Your rules and response bodies are stored in your browser. They're sent to our server only if you turn on sync (paid plans) or share them as Team Rules, and are then stored encrypted, for your account or team only. The extension doesn't collect your browsing history or the content of pages. An optional free account holds your email address and your plan. PLANS • Without an account: 3 rules, 1 active at a time. • Free account (email and a 6-digit code, no password): 6 rules, 3 active. • Optional paid plans with more rules and sync between browsers (Basic and Pro), and Team with shared Team Rules, are sold on placeboapi.com through our reseller Paddle.com, not by Google. Try it on https://placeboapi.com/demo. Privacy policy: https://placeboapi.com/privacy. Support: support@placeboapi.com.