Fix HTTP 405 Method Not Allowed API Errors

HTTP 405 means the server recognized the target resource but does not permit the method used. Compare the final request with the deployed route and its advertised methods.

Capture the final method and URL

Inspect the request after redirects, client middleware, and form submission behavior. Confirm method, scheme, host, port, path, version, query, and trailing slash. A redirect can change a POST into a GET in some client flows, while a frontend form may default to GET. Record the response body, request ID, and Allow header without logging credentials or private payload data.

Read the Allow header

A 405 response should advertise the methods supported by the target resource in Allow. Compare that list with the API contract. If the required method is absent, the client may be calling the wrong path or the deployed server may not include the expected route. If Allow is missing or clearly unrelated, identify whether a gateway, static server, or framework fallback produced the response.

Compare source, specification, and deployment

Find the operation in the current OpenAPI document and verify that the running environment matches it. A PATCH route may exist on a development branch but not in the production image. Generated clients can retain an old method after an API change. Check route registration, feature flags, deployment version, and whether a framework mounted the router under a different prefix.

Inspect OPTIONS and HEAD handling

Browsers use OPTIONS for CORS preflight. If a proxy or application rejects OPTIONS with 405, the real cross-origin request is never sent. Handle preflight before endpoint authentication and return the required CORS policy. HEAD can often share GET semantics without a body, but support is implementation-specific; do not assume a custom router adds it automatically.

Trace gateways and static hosts

A CDN, web server, ingress, or object-storage host may allow only a restricted method set. Confirm that the public route forwards the method unchanged and does not send API traffic to a static frontend. Review method allowlists, Web Application Firewall rules, proxy rewrites, and host-based routing. Correlate the request ID across hops to find where 405 originated.

Fix the contract owner

If the client method is wrong, update it to the documented operation. If the method should exist, register and test the server route, then update the specification when necessary. Do not bypass a method restriction by tunneling writes through GET. Inspect operations with the OpenAPI Viewer and lint the contract with the OpenAPI Linter.

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