when adding MCP server, a invalid token should tell user, but return invalid json(500) on UI

[int0x03]

Describe the bug
on UI, when I add a MCP server, and given the wrong token, it should tell user the token is invalid, but on current UI, it just says: Unexpected token 'I', "Internal S"... is not valid JSON

To Reproduce

1. add an MCP server which need a token, but give the wrong token
2. you can see on the top right corner: Unexpected token 'I', "Internal S"... is not valid JSON
3. in the chainlit UI log, you can see

INFO:httpx:HTTP Request: POST https://example.com/mcp/server "HTTP/1.1 401 Unauthorized"
INFO:     127.0.0.1:62307 - "POST /chat/mcp HTTP/1.1" 500 Internal Server Error

                 ***********************   
    |     raise RuntimeError("No response returned.")
    | RuntimeError: No response returned.

Expected behavior
it should tell users the clearly msg what's wrong for the MCP server

Screenshots

Desktop (please complete the following information):

• OS: [iOS]
• Browser [chrome]
• Version [all]

[dosubot]

Hi @int0x03! I'm Dosu and I’m helping the chainlit team.

This happens because when the MCP server returns a 401 Unauthorized (invalid token), the backend doesn't handle it gracefully and instead throws an unhandled exception, resulting in a 500 error and a non-JSON response. The frontend tries to parse this as JSON, which leads to the confusing "not valid JSON" error you see, instead of a clear message about the invalid token.

To fix this, the backend should catch authentication errors from the MCP server and return a structured JSON error with a clear message (like "Invalid token" or "Authentication failed"). The frontend should then display this message to the user. Improving error handling in the backend MCP integration will prevent the 500 error and ensure users get actionable feedback when their token is invalid.

There are related discussions about MCP server authentication and error handling, but none directly address this specific bug yet (example, another). This is a backend error handling gap that needs to be addressed for better UX.

To reply, just mention @dosu.

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