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