CoplayDev/unity-mcp · error · Exception

Connection closed before receiving data

Error message

Connection closed before receiving data

What it means

Raised in the legacy (non-framing) receive loop (unity_connection.py:220): the very first sock.recv() returned empty (TCP EOF) and no chunks had been collected yet, so the peer closed before sending any data.

Source

Thrown at Server/src/transport/legacy/unity_connection.py:220

                        f"Received framed response ({len(payload)} bytes)")
                    return payload
            except socket.timeout as exc:
                logger.warning("Socket timeout during framed receive")
                raise TimeoutError("Timeout receiving Unity response") from exc
            except TimeoutError:
                raise
            except Exception as exc:
                logger.error(f"Error during framed receive: {exc}")
                raise

        chunks = []
        # Respect the socket's currently configured timeout
        try:
            while True:
                chunk = sock.recv(buffer_size)
                if not chunk:
                    if not chunks:
                        raise Exception(
                            "Connection closed before receiving data")
                    break
                chunks.append(chunk)

                # Process the data received so far
                data = b''.join(chunks)
                decoded_data = data.decode('utf-8')

                # Check if we've received a complete response
                try:
                    # Special case for ping-pong
                    if decoded_data.strip().startswith('{"status":"success","result":{"message":"pong"'):
                        logger.debug("Received ping response")
                        return data

                    # Handle escaped quotes in the content
                    if '"content":' in decoded_data:
                        # Find the content field and its value

View on GitHub (pinned to c21bf496bc)

Solutions

1. Ensure Unity and the MCPForUnity bridge are still running and responsive.
2. Prefer framing mode (default) where possible — it is more robust to mid-stream drops.
3. Let send_command retry after reconnecting.

# Check the bridge is registered before relying on a legacy connection
from pathlib import Path

def bridge_registered() -> bool:
    return bool(list(Path.home().joinpath('.unity-mcp').glob('unity-mcp-status-*.json')))

def is_legacy_eof(e: BaseException) -> bool:
    return (isinstance(e, Exception)
            and 'Connection closed before receiving data' in str(e))

try:
    resp = conn.send_command(cmd, params)
except Exception as e:
    if 'Connection closed before receiving data' in str(e):
        conn.disconnect()
        resp = conn.send_command(cmd, params)
    else:
        raise

When it happens

Trigger: receive_full_response in legacy mode (config.require_framing=False) when Unity closes the socket before writing a response — domain reload, editor quit, or connecting to a port whose listener immediately drops the connection.

Common situations: Same as the framed EOF (error 182) but in legacy interop mode; also seen when the port is held by a process that accepts then closes.

Understand the failure class

• Connection failures: ECONNREFUSED, ECONNRESET, and friends — why connections get refused, reset, or dropped.

Related errors

• Connection closed before reading expected bytes
• Cannot connect to Unity MCP server at {cfg.host}:{cfg.port}.
• Timeout receiving Unity response
• Could not connect to Unity
• Failed to connect to Unity instance '{target.id}' on port {t

AI-assisted analysis of CoplayDev/unity-mcp@c21bf496bc (2026-08-13). Data as JSON: /api/errors/3a9d5c62bc38f4a9. Report an issue: GitHub.