Always negotiates the oldest version

version-downgrade · Protocol versioning

Endpoint (Streamable HTTP)

https://chaos-mcp.thomascdnns.com/s/version-downgrade/mcp

On the wire — recorded from a real request

Symptom

what you'll observe

You request a modern protocol version (e.g. 2025-06-18) but initialize always answers 2024-11-05, so newer features (the version header, structured tool output, elicitation) are silently off.

Cause

what this reproduces

protocolVersion is pinned to a stale hard-coded constant instead of negotiating against the client's requested version.

Fix

how you'd resolve it

Negotiate: if you support the client's requested version echo it back; otherwise return the newest version you actually support.

Try it with curl

bash

curl -sS -X POST \
  https://chaos-mcp.thomascdnns.com/s/version-downgrade/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json' \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "2024-11-05",
      "capabilities": {},
      "clientInfo": {"name": "curl", "version": "1"}
    }
  }'