Free Online CORS Checker - ATools

Free online CORS checker: enter URL, method, headers and body to predict preflight and get common fixes for CORS errors.

View source on GitHub

Request

MethodEndpoint URL

Credentials (Cookies)Timeout (ms)

Request Headers (One per line: Key: Value)

Body TypeOrigin for analysis (Optional)

Only affects suggested response headers; actual request Origin is determined by the current page.

Request Body

Analysis

Origin: - → https://httpbin.org

Will trigger preflight (OPTIONS)?： Yes (server must respond properly to OPTIONS)

Reason

Contains non-safelisted request headers: content-type.

Recommended Allow-Methods

POST

Recommended Allow-Headers

content-type

Example Configuration / Headers

# Preflight (OPTIONS) Response Suggestion
Access-Control-Allow-Origin: https://example.com
Access-Control-Allow-Methods: OPTIONS, POST
Access-Control-Allow-Headers: content-type
Access-Control-Max-Age: 600
Vary: Origin
# Actual Response Suggestion (non-OPTIONS)
Access-Control-Allow-Origin: https://example.com
Vary: Origin
Access-Control-Expose-Headers: X-Request-Id

Fix Suggestions

CORS for cross-origin requests is entirely determined by server response headers; frontend cannot bypass it.
Handle OPTIONS preflight: return 204/200 for OPTIONS with Allow-Origin/Allow-Methods/Allow-Headers.
Ensure preflight response Access-Control-Allow-Headers covers: content-type.
Ensure preflight response Access-Control-Allow-Methods includes: POST.
Browsers typically only show "TypeError: Failed to fetch" on CORS failure. Inspect DevTools -> Network for preflight/response headers.

Actual Request Result

Duration：-

Status：-

Response

Diagnostics

Copy response headers or preflight response headers from DevTools -> Network and paste here (Key: Value per line).

Paste observed response headers (Optional, for detailed diagnostics)

Free Online CORS Checker - ATools

Free online CORS checker: enter URL, method, headers and body to predict preflight and get common fixes for CORS errors.