What causes "Failed to write ncl CLI response" in nanocoai/nanoclaw?

Retry the ncl command — transient disconnects are the usual cause

Failed to write ncl CLI response

Error message

Failed to write ncl CLI response

What it means

The socket server failed to serialize or write a response frame back to the client and the connection could not be closed cleanly. The client will see a hang or EOF instead of a response; the server itself stays healthy.

Source

Thrown at src/cli/socket-server.ts:103

      },
    });
    return;
  }

  // Host caller — connecting to data/ncl.sock requires file-system access
  // to a 0600 socket owned by the host user, so we treat the socket path
  // itself as the auth boundary.
  const ctx: CallerContext = { caller: 'host' };
  const res = await dispatch(req, ctx);
  write(conn, res);
}

function write(conn: net.Socket, frame: ResponseFrame): void {
  try {
    conn.write(JSON.stringify(frame) + '\n');
    conn.end();
  } catch (err) {
    log.warn('Failed to write ncl CLI response', { err });
  }
}

function isRequestFrame(x: unknown): x is RequestFrame {
  if (!x || typeof x !== 'object') return false;
  const o = x as Record<string, unknown>;
  return typeof o.id === 'string' && typeof o.command === 'string' && typeof o.args === 'object' && o.args !== null;
}

View on GitHub (pinned to 294ef2aee8)

Solutions

Retry the ncl command — transient disconnects are the usual cause
Narrow large queries with `--json` plus explicit ids or list filters
Check that the client isn't timing out (raise timeout or run on-host)

Defensive patterns

Strategy: retry

Try / catch

try { conn.write(JSON.stringify(frame)+'\n'); conn.end(); } catch { /* client gone; nothing to do */ }

Prevention

Retry ncl commands that hang — usually a dropped connection
Keep responses small with filters/--json
Set sane client timeouts so dead connections are reaped

When it happens

Trigger: `conn.write`/`conn.end` throws — client already disconnected (EPIPE), socket destroyed, or the response frame contains a value JSON.stringify chokes on (circular).

Common situations: Client timeout killed the connection before the response arrived; huge response frames on a slow socket.

Related errors

AI-assisted analysis of nanocoai/nanoclaw@294ef2aee8 (2026-08-28). Data as JSON: /api/errors/7dd418f8cc23b7dc. Report an issue: GitHub.