[v2][Web][Streamable HTTP] Remote proxy drops the negotiated Mcp-Protocol-Version header #1935

Which version line?

v2 — current (@modelcontextprotocol/inspector@latest)

Which client?

Web

Inspector version

2.1.0

Node version

v24.15.0

Operating system (and browser, for the web client)

Windows 11 10.0.26200, Chrome

Transport

Streamable HTTP

MCP server under inspection

Server: A stateful local Streamable HTTP MCP server
URL: http://localhost:65535/sdk/v1/mcp
Protocol era: legacy / MCP 2025-11-25
OAuth: no

The server returns both Mcp-Session-Id and Mcp-Protocol-Version from
initialize. Both headers are also exposed through CORS.

Steps to reproduce

1. Run npx @modelcontextprotocol/inspector@2.1.0.
2. Add http://localhost:65535/sdk/v1/mcp as a Streamable HTTP server.
3. Use the legacy protocol era and connect.
4. Observe that initialize requests protocol version 2025-11-25.
5. Observe that the initialize response returns:
   • Mcp-Session-Id
   • Mcp-Protocol-Version: 2025-11-25
   • result.protocolVersion: 2025-11-25
6. Inspect the immediately following notifications/initialized request.

The request contains Mcp-Session-Id, but does not contain
Mcp-Protocol-Version.

Expected behavior

After initialization, every Streamable HTTP request must include the negotiated
protocol version:

Mcp-Protocol-Version: 2025-11-25

This includes notifications/initialized, subsequent JSON-RPC requests, SSE
GET requests, and session DELETE requests.

Actual behavior

The Inspector Web client sends Mcp-Session-Id but drops
Mcp-Protocol-Version in the remote proxy layer.

The MCP server therefore rejects notifications/initialized with HTTP 400:

{
  "jsonrpc": "2.0",
  "id": null,
  "error": {
    "code": -32001,
    "message": "Mcp-Protocol-Version is required for a session",
    "data": {}
  }
}

Logs, errors, or screenshots

The same endpoint works when used directly through
@modelcontextprotocol/client@2.0.0-beta.5; the direct SDK client correctly
sends both headers.
This resembles #679, but reproduces in the new v2.1.0 remote transport
architecture.

Already prototyped a fix?

No response

Before you submit

• I searched existing issues and this is not a duplicate.
• This is not a security vulnerability report (those go through the private advisory process).