unslothai/unsloth · error · RuntimeError

Failed to send command to subprocess: {exc}

Error message

Failed to send command to subprocess: {exc}

What it means

Raised when queue.put(cmd) itself fails with OSError or ValueError: the command queue's underlying pipe is broken (subprocess died and the pipe reader is gone) or the queue was closed. This distinguishes 'queue exists but pipe is dead' from 277's 'no queue at all', both meaning the worker is not reachable.

Source

Thrown at studio/backend/core/inference/orchestrator.py:544

                    " This usually means the system killed it under memory pressure. "
                    "Try a smaller model, lower context length, or close other GPU-heavy apps."
                )
            return f"{message}{suffix} Details: pid={pid}, signal={sig_name}, exitcode={exitcode}."

        return f"{message} Details: pid={pid}, exitcode={exitcode}."

    # ------------------------------------------------------------------
    # Queue helpers
    # ------------------------------------------------------------------

    def _send_cmd(self, cmd: dict) -> None:
        """Send a command to the subprocess."""
        if self._cmd_queue is None:
            raise RuntimeError("No inference subprocess running")
        try:
            self._cmd_queue.put(cmd)
        except (OSError, ValueError) as exc:
            raise RuntimeError(f"Failed to send command to subprocess: {exc}")

    def _read_resp(self, timeout: float = 1.0) -> Optional[dict]:
        """Read a response from the subprocess (non-blocking with timeout)."""
        if self._resp_queue is None:
            return None
        try:
            return self._resp_queue.get(timeout = timeout)
        except queue.Empty:
            return None
        except (EOFError, OSError, ValueError):
            return None

    def _wait_response(
        self,
        expected_type: str,
        timeout: float = 300.0,
    ) -> dict:
        """Block until a response of the expected type arrives.

View on GitHub (pinned to 203007d190)

Solutions

1. Treat as worker-down: verify subprocess health, restart it, then resend the command
2. Add a small retry-with-restart wrapper around command sends that converts this into a supervised restart
3. Investigate why the subprocess died (see _subprocess_crash_message output elsewhere) to stop recurrence — often OOM

Example fix

# before
try:
    orchestrator._send_cmd(cmd)
except RuntimeError as e:
    abort_request()  # user-visible failure

# after
try:
    orchestrator._send_cmd(cmd)
except RuntimeError:
    await orchestrator.restart_subprocess()
    orchestrator._send_cmd(cmd)

if not orchestrator._ensure_subprocess_alive():
    await orchestrator.restart_subprocess()  # pipe is dead; recreate before send

try:
    orchestrator._send_cmd(cmd)
except RuntimeError as exc:
    if 'Failed to send command' in str(exc):
        await orchestrator.restart_subprocess()
        orchestrator._send_cmd(cmd)  # fallback: resend after restart
    else:
        raise

When it happens

Trigger: _send_cmd called while/after the inference subprocess crashed, so the multiprocessing queue's feeder pipe raises BrokenPipeError (an OSError); or ValueError from putting on a closed queue.

Common situations: Worker OOM-killed or segfaulted moments before a command was sent; teardown racing a final status poll; long-lived queue whose pipe buffer hit EPIPE after child exit.

Related errors

• No inference subprocess running
• No export subprocess running
• Failed to send command to subprocess: {exc}
• The inference worker stopped unexpectedly while loading the
• Export subprocess crashed during wait

AI-assisted analysis of unslothai/unsloth@203007d190 (2026-08-15). Data as JSON: /api/errors/b492fbef5c7ce93e. Report an issue: GitHub.