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fix: accept wildcard media types in Accept header per RFC 7231 - #2152

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maxisbey merged 2 commits into
modelcontextprotocol:mainfrom
shivama205:fix/accept-header-wildcards
Mar 9, 2026
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fix: accept wildcard media types in Accept header per RFC 7231#2152
maxisbey merged 2 commits into
modelcontextprotocol:mainfrom
shivama205:fix/accept-header-wildcards

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@shivama205

@shivama205 shivama205 commented Feb 27, 2026

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Summary

  • Fix _check_accept_headers to accept wildcard media types (*/*, application/*, text/*) in the Accept header, per RFC 7231, section 5.3.2
  • Strip quality value parameters (e.g. ;q=0.8) before matching media types
  • Add comprehensive tests for wildcard acceptance and rejection scenarios

Problem

The server rejected requests with wildcard Accept headers like */* or application/*, returning 406 Not Acceptable. This violated RFC 7231 and broke interoperability with HTTP clients that send Accept: */* by default (e.g. httpx, requests).

Closes #1641

Test plan

  • Wildcard */* accepted for both SSE and JSON-only modes
  • application/* + text/* accepted for SSE mode
  • application/* alone correctly rejected for SSE mode (doesn't cover text/event-stream)
  • Quality parameters (*/*;q=0.8) handled correctly
  • Truly missing Accept header still rejected
  • Incompatible types (text/html) still rejected
  • All 1134 existing tests pass
  • Linting, formatting, and type checks clean

@shivama205

shivama205 commented Feb 27, 2026

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Hi @Kludex @maxisbey — could you please review this when you get a chance? This fixes #1641 (wildcard Accept header support per RFC 7231). All CI checks are passing. Thanks!

@shivama205

shivama205 commented Feb 28, 2026

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@felixweinberger @maxisbey could you please review this when you get a chance? Thanks!

@shivama205
shivama205 force-pushed the fix/accept-header-wildcards branch from b5d0136 to 6c49c5f Compare March 4, 2026 18:36
@shivama205

shivama205 commented Mar 4, 2026

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Bumping this — just rebased onto latest main (clean, no conflicts). This fixes a real interoperability bug: clients like httpx and requests send Accept: */* by default and currently get a 406 Not Acceptable. All CI is passing.

If anyone's on the MCP Discord, would really appreciate a ping to get eyes on this. Happy to make any changes to get it merged. 🙏

@maxisbey maxisbey added bug Something isn't working P2 Moderate issues affecting some users, edge cases, potentially valuable feature improves spec compliance When a change improves ability of SDK users to comply with spec definition labels Mar 5, 2026
maxisbey
maxisbey requested changes Mar 6, 2026
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Thanks for this — the approach is correct and the test coverage is great. Two small asks before merge (suggestions inline, should be one-click applies).

AI Disclaimer

Comment thread src/mcp/server/streamable_http.py Outdated
Comment thread tests/shared/test_streamable_http.py Outdated
Comment thread tests/shared/test_streamable_http.py Outdated
Comment thread tests/shared/test_streamable_http.py Outdated
Shivam and others added 2 commits March 7, 2026 03:12
Shivam Shivam
fix: accept wildcard media types in Accept header per RFC 7231
6af5bfe 
The server rejected requests with wildcard Accept headers like `*/*`,
`application/*`, or `text/*`, returning 406 Not Acceptable. This violated
RFC 7231, section 5.3.2, which defines wildcard media ranges as valid.

Update `_check_accept_headers` to recognize `*/*` as matching all types,
`application/*` as matching `application/json`, and `text/*` as matching
`text/event-stream`. Also strip quality parameters before matching.

Github-Issue:modelcontextprotocol#1641
Reported-by:rh-fr
@shivama205 Shivam
fix: address review feedback for accept header parsing
8e2ec6f 
- Normalize media types to lowercase for case-insensitive matching per RFC 7231
- Use exact match instead of startswith to prevent over-matching
- Replace headers.update with headers.pop for cleaner Accept removal in tests
@shivama205
shivama205 force-pushed the fix/accept-header-wildcards branch from d8bd3e4 to 8e2ec6f Compare March 6, 2026 21:44
@shivama205

shivama205 commented Mar 6, 2026

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Thanks for the thorough review @maxisbey! Applied all your suggestions:

  • Added .lower() for case-insensitive media type matching per RFC 7231
  • Switched from startswith to exact match to prevent over-matching (e.g. application/json-patch+json) — good eye on that one
  • Replaced headers.update({"Accept": None}) with headers.pop("Accept") at all three locations, much cleaner

@shivama205
shivama205 requested a review from maxisbey March 6, 2026 21:57
@shivama205 shivama205 mentioned this pull request Mar 8, 2026
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@maxisbey
maxisbey merged commit 51c53f2 into modelcontextprotocol:main Mar 9, 2026
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@maxisbey maxisbey mentioned this pull request Jun 11, 2026
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bug Something isn't working improves spec compliance When a change improves ability of SDK users to comply with spec definition P2 Moderate issues affecting some users, edge cases, potentially valuable feature

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MCP Server won't work with wildcard in "Accept" header and therefore is non‑compliant with HTTP spec

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