fix: async onclose, stdin EOF detection, SIGTERM in examples

[MayCXC]

Summary

Three related improvements to server lifecycle handling.

1. Allow async onclose callbacks

MCP servers that hold external resources (browser sessions, database connections) need to await cleanup before the process exits. onclose is the only transport/protocol callback called from an awaitable context (transport.close() is async, awaited by server.close()). The other callbacks (onmessage, onerror) fire from event emitters that cannot await.

The onclose signature changes from () => void to () => void | Promise<void>, matching the existing pattern used by onsessionclosed in StreamableHTTPServerTransport. All transports and Protocol._onclose now await the callback.

Changed files: Transport interface, Protocol, StdioServerTransport, StreamableHTTPServerTransport, StdioClientTransport, WebSocketClientTransport, StreamableHTTPClientTransport, SSEClientTransport, InMemoryTransport, and mock transports in tests.

2. Close StdioServerTransport when stdin ends

The transport listened for data and error on stdin but not EOF. When the MCP client disconnects (closing stdin), the transport stays open and onclose never fires. This prevents servers from cleaning up resources.

This is especially visible with containerized MCP servers using docker run --rm: without onclose, the server process never exits, the container never stops, and containers accumulate on each client reconnect.

3. Add SIGTERM handlers in examples

All 10 examples only handle SIGINT (Ctrl+C). MCP servers run as background processes spawned by clients, not interactively. SIGTERM is what container runtimes and process managers send to stop a process. Added SIGTERM handlers alongside SIGINT in all examples.

Test plan

• New test: should close when stdin ends (push null to stdin, verify onclose fires)
• New test: should await async onclose callback (async cleanup completes before close() resolves)
• Existing debounce test passes (state cleared synchronously before async callbacks)
• All server tests pass (39/39)
• All core tests pass (440/440)
• Client test failure is pre-existing on main (jose/RSA base64 error message mismatch)

[changeset-bot]

🦋 Changeset detected

Latest commit: 3f70c00

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	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/client	Patch
@modelcontextprotocol/node	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/hono	Patch

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commit: 3f70c00