SSE transport: "Received request before initialization was complete" error with Claude Code client

[Tech-Fumi]

Environment

• MCP version: 1.25.0
• Transport: SSE (FastMCP)
• Client: Claude Code CLI

Problem

When using SSE transport with Claude Code, tool calls fail with:

RuntimeError: Received request before initialization was complete

The client connects to /sse endpoint and sends tool requests, but the initialization sequence (InitializeRequest → InitializeResponse → InitializedNotification) is not completing before tool calls are made.

Server log shows:

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

Current Workaround

Patching mcp/server/session.py to auto-initialize on first request:

case _:
 if self._initialization_state != InitializationState.Initialized:
 # Auto-initialize on first request
 import logging
 logging.warning("Auto-initializing session on first request")
 self._initialization_state = InitializationState.Initialized

Expected Behavior

SSE transport should either:

1. Wait for proper initialization before accepting tool calls
2. Auto-initialize for SSE clients
3. Provide a configuration option for stateless SSE sessions (similar to stateless_http for Streamable HTTP)

Reproduction

1. Create FastMCP server with SSE transport
2. Connect from Claude Code CLI
3. Call any tool → Error occurs

Server Code Example

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("my-server", host="0.0.0.0", port=8027)

@mcp.tool(description="Test tool")
def test_tool() -> str:
 return "Hello"

if __name__ == "__main__":
 mcp.run(transport="sse")

[maxisbey]

Closing as a duplicate of #423 — same SDK code path (request rejected because it arrives before initialize). Feel free to add your repro to the discussion there. If this turns out to be a distinct issue, feel free to reopen or open a new one.

_{AI Disclaimer}