MCP server not recognizing x-session-id/x-user-id headers via POST /messages/ in FastAPI integration

[thejakia1]

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Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
• Documentation
• Other

Question

We're trying to integrate MCP using the official mcp Python SDK, and we’ve hit a persistent issue where the MCP server (via sse_app()) fails to recognize the x-session-id and x-user-id headers — even when they are present and correct.

Our Setup

Backend: FastAPI (Python 3.10)

MCP Integration:
We're using app = mcp.sse_app() from FastMCP, mounted at /mcp

Frontend: Nextjs + Redux
It calls our backend /mcp/query, which acts as a proxy to /mcp/messages/

Session Management:
We're generating a proper sessionid and userid per user, storing in Redux and headers

Proxy Logic (/mcp/query)
Our backend proxy forwards the request to MCP like this:

headers = {
  "content-type": "application/json",
  "x-session-id": sessionid,
  "x-user-id": userid
}

await httpx.post(
  "http://localhost:8000/mcp/messages/",
  json={ "message": prompt },
  headers=headers
)

Diagnostics:
Inside our MCP prompt (@mcp.prompt()), we added:

def mlp_stage_1(ctx: Context) -> str:
    print("HEADERS RECEIVED BY MCP:")
    for k, v in ctx.request.headers.items():
        print(k, ":", v)

🔍 Result: the headers never show up as x-session-id or x-user-id.

Instead, we keep getting: Received request without session_id

Even though we confirmed those headers are being sent.

Even Direct cURL Fails
We ran:

curl -X POST http://localhost:8000/mcp/messages/ \
  -H "Content-Type: application/json" \
  -H "x-session-id: test-session" \
  -H "x-user-id: test-user" \
  -d '{"message": {"role": "user", "content": "hello"}}'

❌ Still got session_id is required.

Can anybody please clarify:

• Does sse_app() actually support reading session info from headers like x-session-id, x-user-id?

• Is there any middleware/filter stripping headers we should account for?

• Would it be possible to support reading session directly from the JSON body? (some parts of the docs mention it, but SDK doesn't seem to support that)

• Any example of sse_app() working in a FastAPI-based fullstack integration (Nextjs/React + FastAPI)?

[abdushkur]

the With Session Management also does not work, it creates session id when initialize, and it was sent through response header, but after that, client sends initialized event imideiately but without session id.

[Euraxluo]

that is a bug