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CORS Diagnoser

Find out why a cross-origin request gets blocked

chromeedgebraveoperafirefox🔒 Zero telemetryVersion 1.0.0
Download the extension for ChromeDownload the extension for Firefox

Description

Replays the fetch from the active tab's real origin and explains, header by header, why the browser allows or blocks a cross-origin request: missing Access-Control-Allow-Origin, wildcard incompatible with credentials, failed OPTIONS preflight, disallowed method. Every finding ships with a server-side fix hint. Diagnostic and read-only: it never bypasses CORS. 100% offline, zero telemetry.

Features

  • Replays the fetch from the active tab's real origin (activeTab + scripting)
  • Structured diagnosis: missing ACAO, wildcard + credentials, preflight, disallowed method
  • Server-side fix hints for every finding
  • Clear verdict: allowed / blocked by CORS / network error / undetermined
  • Read-only: never disables the Same-Origin Policy nor uses a bypass proxy
  • 100% offline, no data leaves the browser

Privacy

100% local processing in your browser. Zero telemetry: no data is sent to external servers.

Required permissions

  • activeTab
  • scripting

Screenshots

CORS Diagnoser

Supported browsers

chromeedgebraveoperafirefox