Feedback: Throws error on `Unsupported protocol version`

[jpmcb]

Inspector Version

0.17.5 latest

Describe the bug

The inspector seems to fail with some older servers I have:

McpError: MCP error -32602: Unsupported protocol version: 2025-11-25 - supported versions: 2025-06-18,2025-03-26,2024-11-05,2024-10-07
    McpError http://localhost:6274/assets/index-CneFQkmB.js:16182
    fromError http://localhost:6274/assets/index-CneFQkmB.js:16197
    _onresponse http://localhost:6274/assets/index-CneFQkmB.js:27833
    onmessage http://localhost:6274/assets/index-CneFQkmB.js:27626
    processStream http://localhost:6274/assets/index-CneFQkmB.js:42007
    _handleSseStream http://localhost:6274/assets/index-CneFQkmB.js:42039
    send http://localhost:6274/assets/index-CneFQkmB.js:42166
    request http://localhost:6274/assets/index-CneFQkmB.js:28055
    request http://localhost:6274/assets/index-CneFQkmB.js:27941
    connect http://localhost:6274/assets/index-CneFQkmB.js:40972
    connect http://localhost:6274/assets/index-CneFQkmB.js:42829
    Nb http://localhost:6274/assets/index-CneFQkmB.js:1277
    Tb http://localhost:6274/assets/index-CneFQkmB.js:1289
    Ub http://localhost:6274/assets/index-CneFQkmB.js:1292
    nf http://localhost:6274/assets/index-CneFQkmB.js:2229
    se http://localhost:6274/assets/index-CneFQkmB.js:2252
    hd http://localhost:6274/assets/index-CneFQkmB.js:2524
    Qk http://localhost:6274/assets/index-CneFQkmB.js:5828
    Jb http://localhost:6274/assets/index-CneFQkmB.js:1230
    hd http://localhost:6274/assets/index-CneFQkmB.js:2325
    fd http://localhost:6274/assets/index-CneFQkmB.js:1723
    ed http://localhost:6274/assets/index-CneFQkmB.js:1706

It seems that the inspector is sending the 11-25 protocol and my servers aren't negotiating correctly (something I think I'll need to go and fix). This wasn't immediately intuitive to me since the inspector just showed "Connection Error - Did you add the proxy session token in Configuration?" error.

To Reproduce

Steps to reproduce the behavior:

1. Use a server that doesn't support the newer spec
2. This server sends a -32602 MCP error indicating it doesn't support a protocol version
3. Notice no helpful error in MCP inspector

Expected behavior

It'd be very helpful, as a server builder to see these errors surfaced more easily.

Inspector V2 is under development to address architectural and UX improvements. See CONTRIBUTING.md for information about V2 development.

[olaservo]

Hi @jpmcb , thanks for the feedback! I created an issue for improving the UX/DX here, since I agree that the generic error isn't very helpful: #962

For others reading this issue:

The MCP spec for version negotiation says that:

1. Client sends initialize with its latest supported protocolVersion (e.g., 2025-11-25)
2. Server MUST respond (not error) with:

• The same version if it supports it, OR
• A different version it supports (should be its latest)

3. Client checks if it supports the server's returned version

• If yes: proceed with that version
• If no: disconnect

How the Inspector (via SDK) handles it:

• Sends LATEST_PROTOCOL_VERSION (2025-11-25 in SDK v1.24.3)
• Validates server's response is in SUPPORTED_PROTOCOL_VERSIONS
• Would accept downgrades to 2025-06-18, 2025-03-26, etc.

So a server returning an error instead of proposing a version it supports, is non-compliant with the spec - the server should respond with a supported version and not an error.

The SDK's Server._oninitialize() does this:

const protocolVersion = SUPPORTED_PROTOCOL_VERSIONS.includes(requestedVersion)
    ? requestedVersion
    : LATEST_PROTOCOL_VERSION;  // Falls back to server's latest, doesn't error

[jpmcb]

So a server returning an error instead of proposing a version it supports, is non-compliant with the spec

Yes, exactly, great callout and thanks for clearing up how clients should work with this: this is something I need to fix in @zuplo/mcp for our downstream servers. I'll close this since you opened a separate issue re: the DX for version negotiation. Thanks!