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The MCP client using a tool launched with uvx does not terminate. #320
Description
Activity
Hi @ynaoto thanks for reporting this - just checking whether this issue is still affecting you as it's been some time since your initial report?
fixed in PR #1200 (ace9b62, merged Dec 1 2025). tested on main (b8886e7) and v1.x — both have the fix.
old
close()usedAbortController.abort()which didn't reliably kill uvx-launched processes that don't exit on stdin close. newclose()does:stdin.end()→ 2 s wait → SIGTERM → 2 s wait → SIGKILL.repro script + output
// repro.ts — run: npx tsx repro.ts import { spawn } from "node:child_process"; import { StdioClientTransport } from "@modelcontextprotocol/client"; const TIMEOUT_MS = 10_000; // minimal server that stays alive even after stdin closes (simulates uvx behavior) const hangingServerScript = ` const readline = require("readline"); const rl = readline.createInterface({ input: process.stdin }); rl.on("line", (line) => { try { const msg = JSON.parse(line); if (msg.method === "initialize") { const resp = { jsonrpc: "2.0", id: msg.id, result: { protocolVersion: "2024-11-05", serverInfo: { name: "hang-server", version: "0.0.1" }, capabilities: {} } }; process.stdout.write(JSON.stringify(resp) + "\\n"); } } catch {} }); rl.on("close", () => { setInterval(() => {}, 100_000); }); // stay alive setInterval(() => {}, 100_000); `; async function main() { console.log("Starting repro for issue #320..."); const transport = new StdioClientTransport({ command: "node", args: ["-e", hangingServerScript] }); await transport.start(); const childPid = transport.pid; console.log(`Child process spawned with PID: ${childPid}`); try { process.kill(childPid!, 0); console.log("Child is alive before close() — OK"); } catch { console.error("ERROR: child already dead before close()"); process.exit(1); } const closeStart = Date.now(); console.log("Calling transport.close()..."); await transport.close(); const elapsed = Date.now() - closeStart; console.log(`close() returned after ${elapsed}ms`); let childAlive = false; try { process.kill(childPid!, 0); childAlive = true; } catch { childAlive = false; } if (childAlive) { console.error(`BUG REPRODUCED: child process ${childPid} is still alive after close()`); process.exit(2); } else { console.log(`Child process ${childPid} is dead after close() — PASS`); } } const timer = setTimeout(() => { console.error("TIMEOUT — event loop is hung"); process.exit(3); }, TIMEOUT_MS); timer.unref(); main().catch(err => { console.error(err); process.exit(1); });
$ npx tsx repro.ts Starting repro for issue #320... Child process spawned with PID: 943 Child is alive before close() — OK Calling transport.close()... close() returned after 2011ms Child process 943 is dead after close() — PASS Process should exit cleanly now.The 2011 ms delay is expected: the server ignores stdin close, so
close()waits the 2 s grace period then sends SIGTERM, which kills it.reopen if still seeing this on the current release.
Describe the bug
As shown in the attached sample, a client that uses a tool launched with uvx does not terminate.
While it is possible to forcefully terminate the process using something like process.exit(0), doing so leaves the uvx cache in a state where it appears to still be in use by the MCP client process.
This issue does not seem to occur when using npx instead.
A similar issue also occurs on Windows Claude Desktop as well.
To Reproduce
Run the attached sample.
Expected behavior
Exit the client process.
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