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OpenAI API breaking change: request-media-type-removed on POST /responses/input_tokens application/x-www-form-urlencoded

openai · breaking · 2026-07-31 · source: OpenAPI spec diff

What changed

According to the OpenAPI spec diff (openai/openai-oas-5162af98d314-to-117ce5680e42-breaking), the following surface changed:

request-media-type-removed: POST /responses/input_tokens application/x-www-form-urlencoded

openai OAS corridor pair 5162af98d314 -> 117ce5680e42 (breaking subset ingested 2026-08-01)

Who is affected

Switch the request body encoding at every call site that POSTs /responses/input_tokens as a form: replace application/x-www-form-urlencoded submission (URLSearchParams / form-data key-value bodies with the form content-type header) with a JSON body — set the Content-Type header to application/json and JSON.stringify the same parameters.

To find out whether your repo is hit — file and line number, no code leaving your machine:

npx mendapi scan --repo .

How to fix it

Migration path:

Switch the request body encoding at every call site that POSTs /responses/input_tokens as a form: replace application/x-www-form-urlencoded submission (URLSearchParams / form-data key-value bodies with the form content-type header) with a JSON body — set the Content-Type header to application/json and JSON.stringify the same parameters. Official openai SDK clients already send JSON and are unaffected; only raw HTTP callers using form encoding break.

Before:

await fetch('https://api.openai.com/v1/responses/input_tokens', { method: 'POST', headers: { 'Content-Type': 'application/x-www-form-urlencoded' }, body: new URLSearchParams(params) });

After:

await fetch('https://api.openai.com/v1/responses/input_tokens', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify(params) });

Replacement data source: the same endpoint accepts application/json request bodies in the NEW spec

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Related

Change data is recorded from upstream provider releases, changelogs and OpenAPI spec diffs; every article names its source.