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[Bug]: MCP SSE transport rejects concurrent requests on connect with "Received request before initialization was complete" (-32602)

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
66/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
python
Área
api, backend-api-design

Línea de trabajo

Start with /app/mcp_bridge.py, comparing the /mcp/ws initialization gate with the SSE /mcp/sse and /mcp/messages flow through SseServerTransport. Reproduce with parallel tools/call requests during the initialize handshake and inspect the resulting session behavior. Done means concurrent SSE requests no longer fail with the pre-initialization -32602 response.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

⚙️ In-progress 🐞 Bug 📌 Root caused
crawl4ai version

0.9.2 (Docker server, unclecode/crawl4ai:latest)

Expected Behavior

When an MCP client (such as OpenCode, Claude Code, or multi-agent runtimes) connects over SSE (/mcp/sse) and issues tool calls, inbound requests arriving while initialization is completing should either be briefly buffered/queued or await initialization before processing, similar to how /mcp/ws gates incoming traffic.

Current Behavior

When a client establishes an SSE connection and dispatches tool calls concurrently (e.g. parallel fan-out upon connecting), the requests hit /mcp/messages/?session_id=... before the initialize -> notifications/initialized handshake has completed inside mcp.server.session.ServerSession.

This causes mcp/server/session.py to raise:

WARNING - Failed to validate request: Received request before initialization was complete

The client receives:

{"jsonrpc": "2.0", "id": 1, "error": {"code": -32602, "message": "Invalid request parameters"}}

The error code -32602 misleads clients into reporting that tool call parameters are malformed, when in reality the arguments were never validated because the transport rejected the uninitialized session state.

Context & Contrast with WebSocket

In /app/mcp_bridge.py, the WebSocket handler (/mcp/ws) explicitly mitigates this race condition by synchronizing on initialization:

# /app/mcp_bridge.py (WebSocket handler)
first = adapter.validate_python(await ws.receive_json())
await c2s_send.send(first)
await init_done.wait()          # <-- explicitly waits for server readiness
while True:
    data = await ws.receive_json()
    await c2s_send.send(adapter.validate_python(data))

However, the SSE transport (/mcp/sse + /mcp/messages) directly passes through Starlette to SseServerTransport:

sse = SseServerTransport(f"{base}/messages/")

Without any gating on incoming POST requests during the handshake window, any concurrent requests arriving in the first ~100–500ms get rejected.

Upstream References

This is a known race condition in the underlying Python SDK:

Suggested Fix

A minimal fix in mcp_bridge.py for the SSE transport:

  1. Wrap or intercept incoming POST messages to /mcp/messages to await session initialization (with a small timeout, e.g. 2.0s) before feeding them to the read stream writer, OR
  2. Provide an init_done event per SSE session matching the WebSocket pattern.
Steps to Reproduce
  1. Connect an MCP client over SSE (/mcp/sse).
  2. Immediately upon receiving the endpoint SSE event, dispatch 3+ parallel tools/call requests in under 50ms alongside initialize.
  3. Observe Received request before initialization was complete in Docker logs and -32602 returned to the client.
Environment
  • Docker container unclecode/crawl4ai:latest (0.9.2)
  • Python 3.12 (inside container)
  • Client: OpenCode / Claude Code via SSE transport
Lenguaje dominante
Python
Estrellas
84.5k
Forks
8.7k
Merge medio
3 d 9 h
PR fusionados (30 d)
17

Preparar el entorno

Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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