fix(server): check tool callback output schemas

[hngpt52]

Closes #2754.

A tool declaring outputSchema: z.object({ data: z.string() }) currently accepts a callback returning structuredContent: "pong". That compiles and only fails when the tool runs. This change makes the mismatch a TypeScript error; returning { data: "pong" } compiles.

The registration overloads now carry the inferred output schema into callback result types. Successful results require matching, defined structured content, including when the schema output is unknown. Error and input-required results remain valid, and registrations without an output schema keep their existing broad result type. The change covers Standard Schema providers and deprecated raw Zod shapes; runtime validation and serialization are unchanged.

Compiler regressions cover valid and invalid output, sync/async callbacks, input inference, exceptional branches, non-object roots, coercion, and ArkType/Valibot. The provider assertions run in the server package's CI typecheck. ESM/CJS consumer checks exercise the built declarations with skipLibCheck: false. Deliberately malformed runtime fixtures remain in place with explicit casts so runtime rejection is still tested.

The tool guide and API comments explain the output contract, including defaults/coercion and the fact that validation does not replace returned values with parsed data. A server patch changeset calls out the stricter compile-time behavior.

Validation

Local environment: macOS, Node 25.6.1, pnpm 10.26.1, clean worktree based on b6542615.

• pnpm install --frozen-lockfile
• pnpm run check:all — passes, including workspace typechecks, lint, TypeDoc, and VitePress.
• pnpm run build:all
• pnpm --filter @modelcontextprotocol/server exec tsc -p tsconfig.json --noEmit
• node scripts/smoke-dist-types.mjs — ESM and CJS pass.
• pnpm -r --filter '!@modelcontextprotocol/test-e2e' test — 4,233 tests pass.
• git diff --check

The full e2e run finished with 2,640 passed, 147 expected failures, one failure in protocol:timeout:max-total [sse 2026-07-28] (ticks.length was 0), and two unhandled request timeouts. Running the unchanged SSE timeout cases on pristine upstream b6542615 also exits nonzero with the same two unhandled timeouts; the assertion failure did not recur in that focused baseline run. No protocol timing code is changed here. The hosted Node 20/22/24 test and e2e matrix subsequently passed; see the CI results below.

CI results

All 14 reported checks passed for commit d5669c47:

• Main workflow: build, tests and e2e on Node 20/22/24, plus Bun and Deno runtime tests.
• Client/server conformance.
• Examples build and e2e.
• Preview package publication and Continuous Releases.

[changeset-bot]

🦋 Changeset detected

Latest commit: d5669c4

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

This PR includes changesets to release 6 packages

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

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commit: d5669c4