Version negotiation is wrongly implemented

[marcod85]

Version: 3.1.0

References:

• https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle#version-negotiation

Issue Description:
Using the MCP server with latest Claude Desktop (Claude for Mac 0.10.38 e55f4e) fails.
Clause Desktop is sending initialize message with"protocolVersion": "2024-11-05", but the server responds with "protocolVersion": "2025-03-26".

Could not attach to MCP server: Error: Server's protocol version is not supported: 2025-03-26

According to the specs:

...
If the server supports the requested protocol version, it MUST respond with the same version.
...

Checking the implementation, it seems it is always forced to the latest version currently:

server/src/Dispatcher.php

Lines 120 to 128 in 2b4f1d4

	public function handleInitialize(InitializeRequest $request, SessionInterface $session): InitializeResult
	{
	if (! in_array($request->protocolVersion, Protocol::SUPPORTED_PROTOCOL_VERSIONS)) {
	$this->logger->warning("Unsupported protocol version: {$request->protocolVersion}", [
	'supportedVersions' => Protocol::SUPPORTED_PROTOCOL_VERSIONS,
	]);
	}
	$protocolVersion = Protocol::LATEST_PROTOCOL_VERSION;

[CodeWithKyrian]

Thanks for reporting this, and you're right. That’s an oversight on my part. The server should be echoing the client’s version when it’s supported, not forcing the latest. I’ll get that fixed.