[gentil93]

Summary

Fixes #3171.

In @modelcontextprotocol/server 2.4.0, RequestMetaEnvelope is published as {} (see dist/mcp-DbUPMDd5.d.cts and dist/mcp-BCJHyo7l.d.mts). The source type is keyed by the public meta-key constants ([CLIENT_CAPABILITIES_META_KEY], …). The tsdown declaration bundler dropped those computed keys, so a handler cannot read the envelope without a cast:

error TS7053: Element implicitly has an 'any' type because expression of type '"io.modelcontextprotocol/clientCapabilities"' can't be used to index type 'Partial<RequestMetaEnvelope>'.

The v2.4.0 source of this type is the same as current main. A local tsdown 0.18.4 build happens to keep the computed key names, but that still depends on the bundler not dropping them again.

This change writes the envelope keys as the wire string literals (the type stays a type alias, not an interface, so it remains assignable to _meta's string-indexed slot). After pnpm build:all, the published .d.ts contains:

type RequestMetaEnvelope = {
  'io.modelcontextprotocol/protocolVersion': string;
  'io.modelcontextprotocol/clientInfo'?: Implementation;
  'io.modelcontextprotocol/clientCapabilities': ClientCapabilities;
  'io.modelcontextprotocol/logLevel'?: LoggingLevel;
};

So ctx.mcpReq.envelope?.[CLIENT_CAPABILITIES_META_KEY] is typed as ClientCapabilities | undefined.

Tests

• packages/core-internal/test/types/requestMetaEnvelope.test.ts pins the literal keys to the public meta-key constants, so the two cannot drift.
• scripts/smoke-dist-types.mjs (run in CI after build:all) now compiles the issue's consumer pattern against the built declarations, for both the ESM and CommonJS consumers. The same check fails with TS7053 against the published 2.4.0 typings.

How I verified

• pnpm build:all
• Inspected packages/server/dist/mcp-*.d.mts and *.d.cts for the keyed RequestMetaEnvelope (not {})
• node scripts/smoke-dist-types.mjs
• pnpm typecheck:all
• pnpm lint:all
• pnpm test:all

[changeset-bot]

🦋 Changeset detected

Latest commit: 613276e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/core-internal	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/client	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server-legacy	Patch

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commit: 613276e