Example MCP Project does not work with MCP Inspector

[cirego]

I'm trying to write an MCP server that will work with Zed as an extension. I initially tested using the client that was generated as part of the sample application and that seemed to work. But once I tried using my server with other tools, it failed. As part of debugging this, I tried using the MCP Inspector to test the server that I had written and it gave me connection errors. It looks like the example generated by scaffolding project doesn't work properly.

Steps to reproduce:

1. Create a new project using the MCPR CLI: mcpr generate-project --name simple-mcp-server
2. Build the server: cd simple-mcp-server/server && cargo build
3. Run the MCP Inspector: npx @modelcontextprotocol/inspector ./target/debug/simple-mcp-server-server
4. Open the Connector in your browser and hit the Connect button.

Expected result:

The inspector shows a "connected" status and shows the hello tool

Actual result:

The inspector shows a "Connection error" message. Looking in the console logs, I see these two error messages:

index-BK1TrpAN.js:13731 Failed to connect to MCP server: ZodError: [
  {
    "code": "invalid_type",
    "expected": "string",
    "received": "undefined",
    "path": [
      "protocolVersion"
    ],
    "message": "Required"
  },
  {
    "code": "invalid_type",
    "expected": "object",
    "received": "undefined",
    "path": [
      "capabilities"
    ],
    "message": "Required"
  },
  {
    "code": "invalid_type",
    "expected": "object",
    "received": "undefined",
    "path": [
      "serverInfo"
    ],
    "message": "Required"
  }
]
    at get error (index-BK1TrpAN.js:7922:23)
    at ZodObject.parse (index-BK1TrpAN.js:7998:18)
    at index-BK1TrpAN.js:12430:39
    at Client._onresponse (index-BK1TrpAN.js:12361:7)
    at _transport.onmessage (index-BK1TrpAN.js:12251:14)
    at _eventSource.onmessage (index-BK1TrpAN.js:13429:71)
    at index-BK1TrpAN.js:12884:119
    at dispatchEvent (index-BK1TrpAN.js:12754:24)
    at parseLine (index-BK1TrpAN.js:12707:7)
    at Object.feed (index-BK1TrpAN.js:12702:7)
connect @ index-BK1TrpAN.js:13731

index-BK1TrpAN.js:13759 ZodError: [
  {
    "code": "invalid_type",
    "expected": "string",
    "received": "undefined",
    "path": [
      "protocolVersion"
    ],
    "message": "Required"
  },
  {
    "code": "invalid_type",
    "expected": "object",
    "received": "undefined",
    "path": [
      "capabilities"
    ],
    "message": "Required"
  },
  {
    "code": "invalid_type",
    "expected": "object",
    "received": "undefined",
    "path": [
      "serverInfo"
    ],
    "message": "Required"
  }
]
    at get error (index-BK1TrpAN.js:7922:23)
    at ZodObject.parse (index-BK1TrpAN.js:7998:18)
    at index-BK1TrpAN.js:12430:39
    at Client._onresponse (index-BK1TrpAN.js:12361:7)
    at _transport.onmessage (index-BK1TrpAN.js:12251:14)
    at _eventSource.onmessage (index-BK1TrpAN.js:13429:71)
    at index-BK1TrpAN.js:12884:119
    at dispatchEvent (index-BK1TrpAN.js:12754:24)
    at parseLine (index-BK1TrpAN.js:12707:7)
    at Object.feed (index-BK1TrpAN.js:12702:7)
connect @ index-BK1TrpAN.js:13759

Fixes:

I modified handle_initialize to return the following and now MCP inspector will connect (although tools listing is broken):

        let response = mcpr::schema::json_rpc::JSONRPCResponse::new(
            id,
            serde_json::json!({
                "protocolVersion": mcpr::constants::LATEST_PROTOCOL_VERSION,
                "serverInfo": {
                    "name": self.config.name,
                    "version": self.config.version
                },
                "capabilities": {
                    "tools": {},
                },
            }),
        );

[conikeec]

Hey @cirego - I am working on a more cleaner and fluid interface to build and distribute clients and servers .. Will respond here when I wrap it up

[cirego]

Hi @conikeec, great! FYI, with some additional edits, I was able to get the example stuff all working with both MCP inspector and Cursor. Thank you for sharing this code! It really helped me learn how these things work!

[gabrieligbastos]

hey @cirego can u share in this meantime any snippet or something else i can try to keep on going here too?

[flyingzl]

@cirego same problem ,could you share your additional edits? thanks !

[conikeec]

Hey @cirego feel free to issue a PR with doc updates for others to use.
Hey @flyingzl and @gabrieligbastos - the new version will be a whole lot simpler .. I am excited by how its shaping up

[cirego]

@gabrieligbastos, @flyingzl, I have opened PR #16 that includes the fixes I made to get the example stdio server working with MCP inspector.

@conikeec, feel free to do what you want with it. The client doesn't work and I didn't change the SSE examples. It's also based on an older commit because the current master branch disabled project generation.

[JoshJarabek7]

I believe that Cursor uses MCP Inspector to connect as a client, so SSE with Cursor is not working.

[gabrieligbastos]

Awesome @cirego
With those, and some little stuffs to my specific need, I also could set it all up and running just to test and learn things! Thanks very much.

Cant wait for new version that will be released by @conikeec now :)

[flyingzl]

I also found two rust mcp library: mcp-attr and poem-mcpserver, which are also very useful