fix(stdio): add stdin close/end listeners to prevent zombie/orphan processes

[ElliotDrel]

Summary

Fixes #264.

When an MCP client closes its end of the stdin pipe (restart, crash, or normal shutdown), StdioServerTransport never fires a close event because the underlying @modelcontextprotocol/sdk only registers data and error listeners on process.stdin — not close or end. The FastMCP server process therefore lingers indefinitely as a zombie/orphan.

This PR adds belt-and-suspenders process.stdin listeners immediately after session.connect(transport) for the stdio transport path. When either close or end fires, transport.close() is called and the process exits cleanly.

What changed

src/FastMCP.ts — inside the if (config.transportType === "stdio") block, after await session.connect(transport):

const onStdinClose = () => { transport.close().catch(() => {}); };
process.stdin.on("close", onStdinClose);
process.stdin.on("end", onStdinClose);

Why here and not just rely on the SDK fix?

• The SDK fix (fix(server): exit when MCP client closes stdin pipe modelcontextprotocol/typescript-sdk#2003) is not yet released — users on current npm versions are unprotected.
• Belt-and-suspenders: even after the SDK ships the patch, having FastMCP detect the condition independently protects against future regressions.
• FastMCP is the natural place for this guard since it owns the transport reference and the session lifecycle.

References

• Closes stdio transport: server becomes zombie/orphan process when MCP client closes stdin #264
• SDK upstream fix: fix(server): exit when MCP client closes stdin pipe modelcontextprotocol/typescript-sdk#2003
• Downstream report: Add HTTP/SSE transport support to eliminate per-session process overhead karthikcsq/google-tools-mcp#26

Test plan

• Start a FastMCP stdio server as a child process
• Destroy the child's stdin (child.stdin.destroy()) to simulate a client disconnect
• Confirm the server process exits (previously it would hang)
• Confirm existing tests still pass (npm test)

[Copilot]

Pull request overview

This PR addresses stdio-mode process leaks by ensuring FastMCP reacts when the MCP client closes its end of the stdin pipe, triggering transport shutdown so the server can terminate instead of lingering.

Changes:

• Add process.stdin close and end listeners after session.connect(transport) in the stdio transport path.
• On stdin shutdown, call transport.close() to trigger session/transport teardown.

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