Incorrect URL in sse.js when providing a custom path

[AbduGh]

Summary
When adding an sse server using a custom path (e.g. http://localhost:3001/custom/path/sse), the server sends messages requests to http://localhost:3001 instead of http://localhost:3001/custom/path.

Steps to reproduce
Steps to reproduce the behavior:

1. Launch an SSE MCP server on a custom path (e.g. http://localhost:3001/custom/path)
2. Connect the inspector to it using the UI (enter http://localhost:3001/custom/path/sse in the URL field)
3. Try to connect. Instead of successful connection, yo get an error because it sends requests to http://localhost:3001

Issue
this._endpoint = new URL(messageEvent.data, this._url); in sse.js ignores custom path, which is needed for the upcoming requests.

Impact
No connection is possible

[jerome3o-anthropic]

Hey I had a quick dig into this issue. We'd originallly suspected a bug in the typescript SDK

I wrote some tests that I would expect to fail if it were an issue with the typescript sdk client implementation (draft pr here).

However, they don't fail, which leads me to suspect that you have an MCP server that doesn't set the endpoint SSE event correctly:

When a client connects, the server MUST send an endpoint event containing a URI for the client to use for sending messages. All subsequent client messages MUST be sent as HTTP POST requests to this endpoint.

@AbdullatifGhajar are you able to provide more information on the MCP server you're using to hit this bug? (i.e. is it in typescript/python, how did you set the custom path, source code would be amazing)

[cliffhall]

Steps to reproduce the behavior:

1. Launch an SSE MCP server on a custom path (e.g. http://localhost:3001/custom/path)
2. Connect the inspector to it using the UI (enter http://localhost:3001/custom/path/sse in the URL field)
3. Try to connect. Instead of successful connection, yo get an error because it sends requests to http://localhost:3001

I made custom paths in an SSE server for the initial connection AND for the subsequent messages, ran it, then connected to it with the latest version of the Inspector. No issues.

Screen.Recording.2025-05-21.at.1.57.23.PM.mov

Please note

The Inspector has a proxy server to which it connects and sends messages via the normal endpoints. So even though my server is receiving connections at http://localhost:3001/my/custom/sse and receiving subsequent messages at http://localhost:3001/my/custom/message the browser shows /message as the endpoint because that's the endpoint on the proxy server itself. The MCP server ends up receiving connections and messages at the custom endpoints.

[olaservo]

@AbdullatifGhajar I'm going to close this since it doesn't seem like its a bug with Inspector, as mentioned in the latest comments. Feel free to re-open if you're still having issues and can share the server code where you're running into this.