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406

Not Acceptable

4xx Client Error

The server can't produce a response matching the Accept headers.

406 Not Acceptable is content negotiation falling through: the request's Accept headers (Accept, Accept-Language, Accept-Encoding) ruled out every format the server can produce. Demand XML from an endpoint that only speaks JSON and a strict server answers 406. Most servers just ignore the mismatch and send what they have, so a real 406 usually means the API takes negotiation seriously.

What 406 looks like on the wire

GET /api/report HTTP/1.1
Host: api.example.com
Accept: application/xml

HTTP/1.1 406 Not Acceptable
Content-Type: application/json

{"supported": ["application/json", "text/csv"]}

About 4xx Client Error

The request has a problem the client needs to fix, like a bad URL, missing auth, or invalid input.

How to fix 406 Not Acceptable

  • Loosen or drop the Accept header so the server can pick a format it supports.
  • Check the API docs for which content types the endpoint can return.
  • Watch for an SDK or client setting a default Accept header you didn't choose.

Tools that help with 406

MIME Type Lookup
Look up the exact MIME types your format maps to before loosening the Accept header.

Where it's defined

406 Not Acceptable is defined in RFC 9110 §15.5.7. MDN's 406 reference covers browser behavior and examples.

Looking for a different one? See the full HTTP status code reference.