schema: use JSONObject for extensions capability values#2508
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felixweinberger merged 1 commit intomainfrom Apr 9, 2026
Merged
schema: use JSONObject for extensions capability values#2508felixweinberger merged 1 commit intomainfrom
felixweinberger merged 1 commit intomainfrom
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The extensions field in ClientCapabilities/ServerCapabilities used bare `object` as the value type, while the sibling experimental field (same semantic concept — open-ended vendor-prefixed config) uses JSONObject. This was the only use of bare `object` in the entire schema file. Over the wire the two validate identical JSON — both mean 'any object'. The difference is TypeScript ergonomics: JSONObject catches accidental Date/Map/function values at compile time. Normal extension config interfaces (including optional fields under exactOptionalPropertyTypes) remain assignable. Ref: SEP-2133
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pja-ant
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Apr 9, 2026
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Change the
extensionscapability value type fromobjecttoJSONObject, matchingexperimentaland every other open-ended record in the schema.Motivation and Context
SEP-2133 added
extensionsusing bareobject:This is the only use of bare
objectin the entire schema file. Over the wire the two validate identical JSON documents (JSON parsing already guarantees JSON-serializable values), but at the TypeScript levelJSONObjectcatches accidentalDate/Map/function values at compile time.The practical impact: the typescript-sdk's nightly spec-sync workflow has a type-parity test that asserts bidirectional assignability between spec types and Zod-inferred types. The SDK's Zod schemas for
extensions(correctly) useJSONObjectsemantics, so the parity test fails whenspec.types.tsreverts toobject. See modelcontextprotocol/typescript-sdk#1782 for context.How Has This Been Tested?
npm run check:schema:tspassesnpm run check:schema:jsonpassesnpm run check:schema:examplespasses (134/134, including both extensions examples)JSONObjectunder both--strictand--strict --exactOptionalPropertyTypesBreaking Changes
None on the wire. TypeScript consumers who were passing non-JSON-serializable values (class instances, Dates, functions) as extension configs will get a compile error, which is the intended effect.
Types of changes
Checklist
Additional context
No type discussion in the SEP-2133 review — the
objectchoice appears to have been unintentional.schema.jsonandschema.mdxregenerated.