all scenarios

Rejects the MCP-Protocol-Version header

rejects-version-header · Protocol versioning

Endpoint (Streamable HTTP)
https://chaos-mcp.thomascdnns.com/s/rejects-version-header/mcp
On the wire — recorded from a real request
your clientchaos-mcp01initializePOST /s/rejects-version-header/mcp200 OK · application/json · 2 ms"jsonrpc""2.0""id"1"result""protocolVersion""2024-11-05""capabilities""tools""serverInfo""name""chaos-mcp/rejects-version-header""version""1.0.0""instructions"02tools/listPOST /s/rejects-version-header/mcpMCP-Protocol-Version: 2025-06-18400 Bad Request · application/json · 2 ms"jsonrpc""2.0""id"null"error""code"-32600"message""unexpected header MCP-Protocol-Version: 2025-06-18"

Symptom

what you'll observe

Modern clients fail immediately: any request carrying an MCP-Protocol-Version header is rejected with HTTP 400. Older clients that omit it work fine.

Cause

what this reproduces

An older/over-strict server validates request headers against an allow-list and treats the (newer) MCP-Protocol-Version header as unexpected.

Fix

how you'd resolve it

Ignore unknown headers; never hard-fail on a protocol header you simply don't recognise.

Try it with curl
bash
curl -sS -X POST \
  https://chaos-mcp.thomascdnns.com/s/rejects-version-header/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"

'