Server's protocol version is not supported: 2025-03-26

[zachdaniel]

Using this with a server that states its protocol version as 2025-03-26 shows the above error. However, if I just "lie" and return the 2024 version of the protocol, it does work (therefore it clearly does support the 2025-03-26 version of the protocol).

[zachdaniel]

I'm using it like so: npx mcp-remote@0.1.2 http://localhost:4000/mcp

[ooojustin]

The version pinned in pnpm-lock is currently 1.10.2. In that version, the official protocol version was 2024-11-05. Version 1.11.0 was released last week, which bumped the protocol version to 2025-03-26. However, the client sending an older version to a newer server shouldn't be an issue. For example, Claude Desktop still sends protocol version 2024-11-05.

I might be missing something here, but it sounds like your server is returning a constant version, which should not be the case. It should be negotiated with the client during init. Meaning if the client sends protocol version 2024-11-05, the server should respond with the same version.

See:
Version Negotiation section from the official spec
Server init code from official typescript sdk
Client init code which is the source of this error

[zachdaniel]

🤔 Well, my server doesn't support that protocol version.

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

My server only supports the 2025-03-26 version of the specification. Which mcp-remote appears to support also, but only if I return 2024-11-05 as my protocol version back.

[ooojustin]

I see. Well I don't think any code changes are required in this project, the SDK version just needs to be bumped to 1.11.0. For the time being you could just build locally and use node proxy.js http://localhost:4000/mcp from the build dir.

[zmaks]

I have the same issue. My remote MCP server supports only the 2025-03-26 version: it uses the HTTP-Streamable transport without SSE (which is allowed by the spec). This means that only MCP clients with 2025-03-26 version can connect to the server, otherwise their text/event-stream requests will be rejected by the server according to the MCP spec.

At the same time, mcp-remote proxy doesn't need all the HTTP-related capabilities from the client side since it uses stdio transport. Therefore, the proxy allows 2024-11-05 clients connecting to 2025-03-26 servers. That's where I face the server version issues: it's proxied as is and the client disconnects due to version incompatibility (f.e. Claude Desktop).
So the only option to use mcp-remote in such case is to adjust the server to return the same version as requested in the client, which will be wrong for the case of direct connection between client and server.

What if mcp-remote supports an --override-version flag which means the version returned by the server will be replaced with the version requested by the client? @geelen WDYT?