MCP endpoint answered initialize without an MCP-Protocol-Version response header

What it detects

This site's MCP server completed the initialize handshake successfully but did not return an MCP-Protocol-Version response header.

Why it matters

Clients and intermediaries that rely on the response header to confirm which protocol version was actually negotiated have no way to verify it without parsing the JSON-RPC response body.

How to fix

- Set the `MCP-Protocol-Version` response header on the `initialize` response (and ideally on every response) to the protocol version your server actually negotiated. - If you're behind a reverse proxy or gateway, confirm it isn't stripping the header before it reaches the client. - Verify: re-issue the `initialize` handshake and inspect the raw response headers for `MCP-Protocol-Version`.

References

• Model Context Protocol specification
• WAI-ARIA 1.2 (W3C)