lsp mux smoke failed: no ping response

Error message

lsp mux smoke failed: no ping response (${proc.peekStderr().slice(-500) || "no stderr"})

What it means

The LSP mux smoke test spawns a mux process, sends a ping over the framed link, and waits for a pong within a polling window. If no ping response arrives, the daemon can't be considered healthy; the error includes the last 500 bytes of the child's stderr (or "no stderr") to expose why the process didn't answer.

Solutions

Inspect the stderr excerpt in the message for the child's actual failure (bind error, missing binary, exception)
Remove stale socket/pipe files at the endpoint and retry
Verify the mux binary/entry point runs standalone
Check permissions on the socket directory and that no firewall/AV blocks local IPC

Defensive patterns

Strategy: try-catch

Validate before calling

// Pre-check the endpoint before spawning the mux daemon
import * as fs from 'node:fs';
if (process.platform !== 'win32' && fs.existsSync(sockPath)) {
  fs.unlinkSync(sockPath); // clear stale socket
}

Try / catch

try {
  await smokeTestLspMux(endpoint);
} catch (err) {
  if (err.message.startsWith('lsp mux smoke failed: no ping response')) {
    logger.error('mux daemon failed smoke test', { stderr: err.message });
    // retry once after clearing endpoint, then surface stderr to the user
  }
  throw err;
}

Prevention

Read the stderr excerpt embedded in the error — it names the child's real failure
Clear stale Unix socket files at the endpoint before startup
Verify the mux binary/entrypoint works standalone
Ensure the socket directory exists and is writable

When it happens

Trigger: runSmokeTest → smokeTestLspMux: the spawned mux process fails to bind its socket/pipe, crashes on startup, or hangs, so the ping loop exhausts its retries without a response.

Common situations: Port/socket path conflicts or stale socket files; mux binary incompatible or missing; insufficient permissions on the Unix socket path; platform-specific named-pipe issues on Windows; resource exhaustion preventing process startup.

Related errors

AI-assisted analysis of can1357/oh-my-pi@9690622007 (2026-08-31). Data as JSON: /api/errors/39136acf97fa9d4b. Report an issue: GitHub.

Appendix: source

Thrown at packages/coding-agent/src/lsp/mux/daemon.ts:342

		cwd: spawn.cwd,
		env: workerEnvFromParent({
			[LSP_MUX_SOCKET_ENV]: endpoint,
			[LSP_MUX_PROJECT_DIR_ENV]: process.cwd(),
		}),
	});
	try {
		const deadline = Date.now() + SMOKE_TEST_TIMEOUT_MS;
		let alive = false;
		while (Date.now() < deadline) {
			if (proc.exitCode !== null) break;
			if (await probeMux(endpoint)) {
				alive = true;
				break;
			}
			await Bun.sleep(200);
		}
		if (!alive) {
			throw new Error(`lsp mux smoke failed: no ping response (${proc.peekStderr().slice(-500) || "no stderr"})`);
		}
	} finally {
		proc.kill();
		await proc.exited.catch(() => {});
	}
}

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