406 Not Acceptable

406 Not Acceptable is a content-negotiation failure: the server cannot produce a response that satisfies the constraints the client stated in its Accept, Accept-Language, or Accept-Charset headers, and unlike most negotiation mismatches, it has no acceptable representation to fall back to. RFC 9110 treats this as a genuine end state rather than something the server should silently work around, though in practice most servers prefer to relax the negotiation rather than return a hard failure.

The typical trigger is an API that only serves JSON receiving a request with Accept: application/xml, or a strict server-driven negotiation layer rejecting any media type it wasn't explicitly configured to serve. Accept-Language mismatches are less common in APIs but appear in content platforms that only have translations for a subset of locales and choose to fail rather than serve a default language silently.

406 is genuinely rare in production because most APIs simply ignore an unsupported Accept header and return their default representation instead of enforcing strict negotiation, treating a mismatched Accept as advisory rather than mandatory. When a 406 does appear, it usually indicates a deliberately strict server or an intentionally versioned API using media-type negotiation such as Accept: application/vnd.example.v2+json, where an unrecognized version string is a meaningful, actionable error rather than a nuisance to be silently smoothed over.