Proposal: i18n via per-request language negotiation#42
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Mirrors SEP-2792 (modelcontextprotocol#2792) into the Transports WG proposals/ directory for review of the transport-touching aspects: Streamable HTTP Accept-Language / Content-Language mirroring, byte-equality enforcement, Vary semantics, and per-request statelessness. Co-authored-by: Copilot <[email protected]>
kurtisvg
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Jun 19, 2026
Mirrors changes from modelcontextprotocol#2792. Addresses @kurtisvg review comments on this PR: - Schema-property descriptions: explicit MAY with a caveat that translation alters what the model sees, with a concrete Codex reference instead of handwaving. - Open-ended "any other displayed field" bullet replaced with an enumerated MUST/MAY/MUST NOT list anchored to the spec's own classification of each field. Co-authored-by: Copilot <[email protected]>
- Resolve duplicate MUST-NOT-translate list (Scope is canonical). - Fix broken Scope anchor. - Resolve notifications open question into the Specification. - Make error.message localization choice explicit. - Update SEP-2243/2575 link targets to merged seps/ files. - Trim Reference Implementation paragraph. Co-authored-by: Copilot <[email protected]>
Drop the parallel localizedMessage field in favor of translating error.message directly. error.code remains the machine-interpreted identifier; error.message is the display string. Co-authored-by: Copilot <[email protected]>
kurtisvg
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Jul 2, 2026
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Overall, supportive of voting on this to bring to the CMs.
Add concrete public-sector and mixed-language examples, make UI localization a SHOULD, adopt HeaderMismatch -32020, and narrow contentLanguage requirements to negotiated responses. Tighten RFC semantics, header matching, and citations. Co-authored-by: Copilot App <[email protected]> Copilot-Session: b3246f61-3161-4e03-88b5-1137fcbd59e9
Co-authored-by: Copilot App <[email protected]> Copilot-Session: b3246f61-3161-4e03-88b5-1137fcbd59e9
Specify comparison over decoded JSON and parsed HTTP field values, including repeated fields, OWS, normalization, and cache behavior. Co-authored-by: Copilot App <[email protected]> Copilot-Session: b3246f61-3161-4e03-88b5-1137fcbd59e9
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Bringing this to the Transports WG for review and comment before bringing to the core maintainers.
Upstream SEP: modelcontextprotocol/modelcontextprotocol#2792 (now draft, pending WG input).
Reference implementation: modelcontextprotocol/typescript-sdk#2158 — full client + server, stdio and Streamable HTTP, byte-equality enforcement, 11-case integration matrix.
What this proposes
A single, transport-agnostic way for clients to express language preference per request, and for servers to indicate the language used per response:
params._meta['io.modelcontextprotocol/acceptLanguage']on requestsresult._meta['io.modelcontextprotocol/contentLanguage'](anderror.data._meta[...]) on responsesAccept-Language/Content-Languagestrings (BCP 47 language-range lists with quality values), so no new grammar.Why this is a transports concern
For Streamable HTTP, the proposal mirrors
_metainto the standardAccept-Language/Content-LanguageHTTP headers:Mcp-Method/Mcp-Name(RPC content surfaced as headers for infra observability and routing)._metawhen both are present, rejected via a newHeaderMismatchJSON-RPC error (provisional-32005, HTTP 400). Header absence is tolerated (CDNs stripAccept-Languageby default);_metais the authoritative carrier.Vary: Accept-Languageis MUST on cacheable responses whose body depends on negotiated language._metais still authoritative.For stdio: same
_metafields, no transport changes.Why per-request, not session
Aligns with the recent stateless-by-default direction: language can change mid-conversation (user retries in another language, agent serves multiple end-users) without renegotiation. There is no
initialize-time language state.