Invalid method names return error code -32602 instead of -32601

[corv89]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

How to reproduce:

echo '{"jsonrpc":"2.0","method":"invalid/method","id":1,"params":{}}' | your-mcp-server

Expected result:

{"jsonrpc":"2.0","id":1,"error":{"code":-32601,"message":"Method not found"}}

Actual result:

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

The issue appears to occur because request validation in mcp/shared/session.py (lines 360-375) catches the validation exception and returns -32602 before the proper method-not-found handler in mcp/server/lowlevel/server.py (lines 716-718) can be reached.

Example Code

Python & MCP Python SDK

**Environment:**
- Python SDK version: `1.2.0`
- Python version: `3.12`
- Platform: macOS

[giskard09]

Confirmed on MCP Python SDK v1.26.0 with SSE transport.

Specific vector: tools/register (used in security harness testing for tool injection attacks).

Reproduction — SSE transport:

import requests, threading, json, time

base = "http://localhost:PORT"
session_id = None
responses = []

def listen_sse():
    global session_id
    with requests.get(f"{base}/sse", headers={"Accept": "text/event-stream"}, stream=True, timeout=10) as r:
        for line in r.iter_lines():
            if line:
                decoded = line.decode()
                if decoded.startswith("data: "):
                    data = decoded[6:]
                    if data.startswith("/messages/"):
                        session_id = data
                    else:
                        responses.append(json.loads(data))

t = threading.Thread(target=listen_sse, daemon=True)
t.start()
time.sleep(1)

requests.post(f"{base}{session_id}",
    headers={"Content-Type": "application/json"},
    json={"jsonrpc":"2.0","id":1,"method":"initialize",
          "params":{"protocolVersion":"2024-11-05","capabilities":{},
                    "clientInfo":{"name":"test","version":"1.0"}}})
time.sleep(0.5)

requests.post(f"{base}{session_id}",
    headers={"Content-Type": "application/json"},
    json={"jsonrpc":"2.0","id":2,"method":"tools/register",
          "params":{"name":"injected","description":"test","inputSchema":{"type":"object"}}})
time.sleep(1)

for r in responses:
    if r.get("id") == 2:
        print(r)  # → {"error": {"code": -32602, "message": "Invalid request parameters", "data": ""}}

Expected: {"error": {"code": -32601, "message": "Method not found"}}
Actual: {"error": {"code": -32602, "message": "Invalid request parameters", "data": ""}}

Security relevance: Security harnesses testing MCP-002 (Tool Registration via Call Injection) rely on the correct -32601 rejection format to distinguish between "method unknown" and "params malformed". Returning -32602 causes false negatives in injection detection tests.

The root cause matches what the original report identified: Pydantic validation in session.py fires first and swallows the unknown-method path before _handle_request's else branch (which correctly returns -32601) is reached.