fix(server): respect Accept header q=0 (RFC 7231 "not acceptable")#3158
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fix(server): respect Accept header q=0 (RFC 7231 "not acceptable")#3158SpiliosDimakopoulos wants to merge 1 commit into
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check_accept_headers() discarded every Accept-header parameter after the media type, so \�pplication/json;q=0\ (explicitly rejecting JSON) was parsed the same as plain \�pplication/json\ (accepting it). Parse the q value per RFC 7231 §5.3.1 and treat q<=0 as not-acceptable, with a specific type's rejection taking precedence over a broader wildcard. Fixes modelcontextprotocol#3154
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Fixes #3154
What
check_accept_headers()stripped every Accept-header parameter after the mediatype (
media_type.split(";")[0]), soapplication/json;q=0— which per RFC 7231§5.3.1 explicitly marks JSON as not acceptable — was parsed identically to
plain
application/json. A client that explicitly ruled out a representationcould still receive it.
Details
qparameter (default 1.0 if absent/malformed).q <= 0moves the type from "accepted" to "rejected".would otherwise accept it (e.g.
application/json;q=0, */*still rejectsJSON) — most-specific-wins, consistent with RFC 7231 §5.3.2 precedence.
representations by preference, it only gates on q=0 vs not.
Testing
test_accept_q0_for_the_required_type_is_not_acceptable— the exact examplefrom the issue.
test_accept_q0_for_a_specific_type_overrides_a_present_wildcard— specificrejection beats a present wildcard.
test_accept_nonzero_q_for_the_required_type_is_still_acceptable— regressionguard for nonzero q.
uv run pytest tests/server/,uv run ruff check .,uv run ruff format --check .,uv run pyrightall pass.Disclosure
I used AI (Claude) to help draft this fix and its tests; I've reviewed and
understand the change and I'm happy to answer questions about it.