fix: handle cancellation for request id zero

[he-yufeng]

Summary

• preserve requestId: 0 when handling notifications/cancelled
• add a regression test that cancels an inbound request whose JSON-RPC id is 0

Fixes #2115.

Validation

• pnpm --filter @modelcontextprotocol/core exec vitest run test/shared/protocol.test.ts -t "notifications/cancelled behavior"
• pnpm --filter @modelcontextprotocol/core exec vitest run test/shared/protocol.test.ts
• pnpm --filter @modelcontextprotocol/core run typecheck
• pnpm --filter @modelcontextprotocol/core run lint
• pre-push hook: pnpm -r typecheck, pnpm -r build, pnpm sync:snippets --check, pnpm -r lint
• git diff --check

[changeset-bot]

🦋 Changeset detected

Latest commit: c640656

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

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/core	Patch

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[pkg-pr-new]

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@modelcontextprotocol/client

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@modelcontextprotocol/express

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@modelcontextprotocol/hono

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@modelcontextprotocol/node

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commit: c640656

[he-yufeng]

Rebased this on the latest main and re-ran the focused/core checks locally:

• pnpm --filter @modelcontextprotocol/core exec vitest run test/shared/protocol.test.ts -t "requestId 0"
• pnpm --filter @modelcontextprotocol/core exec vitest run test/shared/protocol.test.ts
• pnpm --filter @modelcontextprotocol/core run typecheck
• pnpm --filter @modelcontextprotocol/core run lint
• git diff --check upstream/main..HEAD
• pre-push hook: pnpm -r typecheck, pnpm -r build, pnpm sync:snippets --check, pnpm -r lint

[he-yufeng]

Added the @modelcontextprotocol/core patch changeset and force-pushed 553b6b6. Local validation: changeset status shows @modelcontextprotocol/core patch; focused requestId 0 test passed; full core protocol.test.ts passed with 151 tests; core typecheck and lint passed; git diff --check passed. Pre-push typecheck, build, and lint also passed.

[he-yufeng]

Rebased on current main, resolved the protocol test conflict caused by the task-era test removal, and force-pushed the branch.

The resolved diff is still only the intended request-id-zero cancellation fix: _oncancel now only ignores an absent requestId, and the regression test covers requestId: 0.

Validated locally:

• pnpm changeset status --since origin/main (@modelcontextprotocol/core patch)
• pnpm --filter @modelcontextprotocol/core exec vitest run test/shared/protocol.test.ts -t "requestId 0" (1 passed)
• pnpm --filter @modelcontextprotocol/core exec vitest run test/shared/protocol.test.ts (31 passed)
• pnpm --filter @modelcontextprotocol/core typecheck
• pnpm --filter @modelcontextprotocol/core lint
• git diff --check origin/main..HEAD

The pre-push hook also completed the repo-wide build, typecheck, and lint steps successfully.

[he-yufeng]

Closing this during backlog cleanup. This queue has many review-required MCP TypeScript SDK PRs, and this item is lower priority than the remaining transport/protocol fixes with clearer current value. I can reopen or refresh it if maintainers want this exact change.