Add HeaderMismatchError to schema and reserve -32000..-32099 for MCP - #2642
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Add HeaderMismatchError to schema and reserve -32000..-32099 for MCP#2642pja-ant wants to merge 2 commits into
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Declares the -32001 HeaderMismatch error (previously only in transport prose) as a schema type with constant and example, and adds normative language to the Error Responses section reserving the -32000..-32099 range for MCP-defined errors so implementations don't allocate conflicting codes.
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pja-ant
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April 23, 2026 20:59
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Per WG feedback (Kurtis), revert the bare-header-tolerated design and restore strict byte-equality between _meta and the corresponding HTTP header on both request (Accept-Language) and response (Content-Language) sides, with header absence on requests tolerated for CDN compatibility. Use error code -32005 for HeaderMismatch instead of -32001, which is already in conflicting use across SDKs (REQUEST_TIMEOUT in Python and Kotlin vs HeaderMismatch in Go and C#); the code is provisional pending SEP-2243 / SEP-2678 / PR modelcontextprotocol#2642 schema-level reservation work. Add a Normalization footgun section covering Fastly, Varnish, CloudFront and reverse-proxy header rewriting; consolidate operator-config detail in that section so Security and Backward Compatibility just link to it. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Confirmed with @pja-ant that this is something that is covered in @felixweinberger's earlier PR. |
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Motivation and Context
The -32001 HeaderMismatch error code from SEP-2243 was defined only in transport prose, not in the schema. This adds it as a proper schema type alongside the other error interfaces.
This also adds normative language to the Error Responses section reserving -32000 to -32099 for MCP-defined errors. JSON-RPC designates this range as "implementation-defined server errors"; MCP claims it so that SDKs and applications don't allocate codes that later collide with protocol-defined ones.
How Has This Been Tested?
npm run prep passes (schema validation, example validation, link check, formatting).
Breaking Changes
None to the wire protocol. Implementations that have already allocated custom codes in -32000..-32099 would be non-conforming under the new MUST NOT, but that was always the intent of this range.
Types of changes
Checklist