Response Mocker Doctor Ultimate

Tracking since May 18, 2026.

Changelog

• May 31, 2026
  permissions

  debugger, storage, contextMenus, offscreen

  debugger, storage, contextMenus, offscreen, alarms

• May 24, 2026
  description

  Response Mocker Doctor Ultimate is the most powerful HTTP & WebSocket request mocker for Chrome. Match by anything, mock everything, with surgical precision.
  
    WHY THIS EXISTS
  
    Ever wanted to see how your frontend behaves when an API returns 500, when a WebSocket sends an unusual message, when a CORS preflight fails, or when an endpoint takes forever? Existing tools make this
    painful. This extension makes it trivial.
  
    Built on Chrome's DevTools Protocol, it intercepts requests before they leave the browser. No proxy. No certs. No backend changes.
  
    FEATURES
  
    HTTP mocking
    • Intercept any request via Chrome's DevTools Protocol
    • Match by URL, method, body, headers, or query parameters
    • Three matching modes per field: substring, glob, or regex
    • Return any status code, headers, and body — JSON, HTML, text, anything
    • One-click Add CORS to make cross-origin mocks just work
  
    WebSocket mocking (actually built in — most "mockers" don't support this)
    • Match by URL, method, body, headers, query
    • Intercept ws.send() and reply with mocked server messages
    • MAIN-world content script — works on any site, no proxy required
  
    JavaScript predicates
    • Write (request, query, headers, body, url, method, state) => boolean
    • Decide whether a rule matches based on dynamic conditions
    • Sandboxed evaluation in an offscreen iframe — no CSP holes
  
    Dynamic responses
    • Write a JS function that returns the response body
    • Generate responses that depend on the actual request
    • Mutate `state` — it persists per-rule across reloads (chrome.storage)
  
    Convenience
    • Master toggle: one click to enable/disable all mocking
    • Per-rule toggle and ordering
    • CodeMirror editors for predicates and bodies
    • Import / export rule sets as JSON
    • Right-click toolbar icon for fast actions
    • Tab badge: ON / OFF / attaching state
  
    PERMISSIONS — and why each one is needed
  
    • debugger — Required to intercept HTTP via Chrome's DevTools Protocol. This is the only API that lets an extension truly mock responses (instead of redirecting requests like most "mockers" do).
    • storage — Saves your rules, master toggle, and per-rule state.
    • contextMenus — Right-click toolbar icon menu items.
    • offscreen — Hosts a sandboxed iframe for safely evaluating user JavaScript.
    • <all_urls> — chrome.debugger.attach needs host permission for any mocked tab; the WebSocket content script must run on any origin.
  
    PRIVACY
  
    Runs entirely locally. No telemetry. No analytics. No external servers. Your rules and request data never leave your machine.

  Response Mocker Doctor Ultimate is the most powerful HTTP & WebSocket request mocker for Chrome. Match by anything, mock everything, with surgical precision.
  
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  WHY THIS EXISTS
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  
  You're building a UI. The API returns 200. You need it to return 500 — right now, in this tab, without touching the backend, without spinning up Postman, without editing a staging config. You just want to say "this endpoint returns this" and see what your frontend does.
  
  That's it. That's the whole extension.
  
  Built on Chrome's DevTools Protocol, it intercepts requests before they leave the browser. No proxy. No certs. No backend changes. No switching tools.
  
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  WHO IT'S FOR
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  
  - Frontend developers building UI while the backend API is still in progress or unstable
  - QA engineers who need to reliably reproduce edge cases — timeouts, 500s, malformed JSON, empty arrays
  - Full-stack teams testing production-like failure scenarios without deploying broken backends
  - Anyone tired of the DevTools → Postman → staging → back-to-DevTools loop
  
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  FEATURES
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  
  HTTP mocking
  - Intercept any request via Chrome's DevTools Protocol
  - Match by URL, method, body, headers, or query parameters
  - Three matching modes per field: substring, glob, or regex
  - Return any status code, headers, and body — JSON, HTML, text, anything
  - One-click Add CORS to make cross-origin mocks just work
  
  WebSocket mocking (actually built in — most "mockers" don't support this)
  - Match by URL, method, body, headers, query
  - Intercept ws.send() and reply with mocked server messages
  - MAIN-world content script — works on any site, no proxy required
  
  JavaScript predicates
  - Write (request, query, headers, body, url, method, state) => boolean
  - Decide whether a rule matches based on dynamic conditions
  - Sandboxed evaluation in an offscreen iframe — no CSP holes
  
  Dynamic responses
  - Write a JS function that returns the response body
  - Generate responses that depend on the actual request
  - Mutate state — it persists per-rule across reloads (chrome.storage)
  
  Convenience
  - Master toggle: one click to enable/disable all mocking
  - Per-rule toggle and ordering
  - CodeMirror editors for predicates and bodies
  - Import / export rule sets as JSON
  - Right-click toolbar icon for fast actions
  - Tab badge: ON / OFF / attaching state
  
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  PERMISSIONS — and why each one is needed
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  
  - debugger — Required to intercept HTTP via Chrome's DevTools Protocol. This is the only API that lets an extension truly mock responses (instead of redirecting requests like most "mockers" do).
  - storage — Saves your rules, master toggle, and per-rule state.
  - contextMenus — Right-click toolbar icon menu items.
  - offscreen — Hosts a sandboxed iframe for safely evaluating user JavaScript.
  - <all_urls> — chrome.debugger.attach needs host permission for any mocked tab; the WebSocket content script must run on any origin.
  
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  PRIVACY
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  
  No accounts. No tracking. No analytics. No remote servers. Your rules and request data never leave your machine.

Permissions & access

Permissions

debuggerstoragecontextMenusoffscreenalarms

Host access

<all_urls>

Screenshots