Free online CORS checker: enter URL, method, headers and body to predict preflight and get common fixes for CORS errors.
Free online CORS checker by Fireoa Tools: enter an API URL, HTTP method, request headers and body to predict whether the browser will trigger an OPTIONS preflight request, and get actionable fix suggestions for common CORS failures (missing Access-Control-Allow-Origin, methods/headers mismatch, credentials with wildcard origin, and more). Runs fully in-browser; no data is uploaded.
Request
Request headers (one per line: Key: Value)
Request body
Analysis
Origin:-→https://httpbin.org
Will trigger a preflight (OPTIONS)： Yes (the server must respond correctly to OPTIONS)
CORS for cross-origin requests is decided by server response headers; frontend code cannot bypass it.
Handle the OPTIONS preflight by returning 204/200 with Allow-Origin, Allow-Methods, Allow-Headers, and related response headers.
Ensure the preflight Access-Control-Allow-Headers covers: content-type.
Ensure the preflight Access-Control-Allow-Methods includes: POST.
When CORS fails, browsers usually show only “TypeError: Failed to fetch”. Inspect preflight and response headers in DevTools -> Network for the exact cause.
Request result
Duration：-
Status：-
Response
Diagnostics
Copy response headers or preflight response headers from DevTools -> Network, then paste one Key: Value header per line.
Paste response headers you received (optional, for more precise diagnostics)
