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 <[email protected]>
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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
-32001HeaderMismatcherror 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
-32000to-32099for 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 preppasses (schema validation, example validation, link check, formatting).Breaking Changes
None to the wire protocol. Implementations that have already allocated custom codes in
-32000..-32099would be non-conforming under the new MUST NOT, but that was always the intent of this range.Types of changes
Checklist
npm run generate:schema)npm run preppasses