Method not found

[liu-collab]

import { FastMCP } from "fastmcp";
import { z } from "zod";

const server = new FastMCP({
  name: "My Server",
  version: "1.0.0",
});

server.addTool({
  name: "add",
  description: "Add two numbers",
  parameters: z.object({
    a: z.number(),
    b: z.number(),
  }),
  execute: async (args) => {
    return String(args.a + args.b);
  },
});

server.start({
  transportType: "sse",
  sse: {
    endpoint: "/sse",
    port: 8080,
  },
});

$ node fastmcp-demo.js
server is running on SSE at http://localhost:8080/sse
[warning] FastMCP could not infer client capabilities
[warning] FastMCP could not infer client capabilities
Error connecting to server: McpError: MCP error -32601: Method not found
at Server._onresponse (file:///C:/workspace/MCP/calculate/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:156:27)
at _transport.onmessage (file:///C:/workspace/MCP/calculate/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:45:22)
at SSEServerTransport.handleMessage (file:///C:/workspace/MCP/calculate/node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:93:71)
at SSEServerTransport.handlePostMessage (file:///C:/workspace/MCP/calculate/node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:72:24)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
at async Server. (file:///C:/workspace/MCP/calculate/node_modules/mcp-proxy/dist/chunk-A5VUTWBC.js:194:7) {
code: -32601,
data: undefined
}
node:_http_server:344
throw new ERR_HTTP_HEADERS_SENT('write');
^

Error [ERR_HTTP_HEADERS_SENT]: Cannot write headers after they are sent to the client
at ServerResponse.writeHead (node:_http_server:344:11)
at Server. (file:///C:/workspace/MCP/calculate/node_modules/mcp-proxy/dist/chunk-A5VUTWBC.js:175:15)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5) {
code: 'ERR_HTTP_HEADERS_SENT'
}

Node.js v22.11.0

[LanfordCai]

Same here

[punkpeye]

Please use dev or inspect to start the server.

# Test the addition server example using CLI:
npx fastmcp dev src/examples/addition.ts
# Test the addition server example using MCP Inspector:
npx fastmcp inspect src/examples/addition.ts

If server cannot detect client capabilities, it cannot fully boot.

[liu-collab]

$ npx fastmcp inspect  fastmcp-demo.js 
Starting MCP inspector...
Proxy server listening on port 3000

🔍 MCP Inspector is up and running at http://localhost:5173 🚀
New SSE connection
Query parameters: { transportType: 'sse', url: 'http://localhost:3000/sse' }
sse transportType
SSE transport: url=http://localhost:3000/sse, headers=
New SSE connection
Invalid transport type: undefined
Query parameters: {}
undefined transportType
undefined transportType
Error in /sse route: Error: Invalid transport type specified
    at createTransport (file:///C:/Users/Administrator/AppData/Local/npm-cache/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/inspector/server/build/index.js:74:15)
    at file:///C:/Users/Administrator/AppData/Local/npm-cache/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/inspector/server/build/index.js:82:44
    at Layer.handle [as handle_request] (C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\express\lib\router\layer.js:95:5)
    at next (C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\express\lib\router\route.js:149:13)
    at Route.dispatch (C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\express\lib\router\route.js:119:3)
    at Layer.handle [as handle_request] (C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\express\lib\router\layer.js:95:5)
    at C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\express\lib\router\index.js:284:15  
    at Function.process_params (C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\express\lib\router\index.js:346:12)
    at next (C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\express\lib\router\index.js:280:10)
    at cors (C:\Users\Administrator\AppData\Local\npm-cache\_npx\5a9d879542beca3a\node_modules\cors\lib\index.js:188:7)      
Error in /sse route: SseError: SSE error: Non-200 status code (500)
    at _eventSource.onerror (file:///C:/Users/Administrator/AppData/Local/npm-cache/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/sdk/dist/esm/client/sse.js:69:31)
    at EventSource.failConnection_fn (file:///C:/Users/Administrator/AppData/Local/npm-cache/_npx/5a9d879542beca3a/node_modules/eventsource/dist/index.js:234:53)
    at file:///C:/Users/Administrator/AppData/Local/npm-cache/_npx/5a9d879542beca3a/node_modules/eventsource/dist/index.js:76:74
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5) {
  code: 500,
  event: {
    type: 'error',
    message: 'Non-200 status code (500)',
    code: 500,
    defaultPrevented: false,
    cancelable: false,
    timeStamp: 4538.1117
  }
}

It looks like the second connection parameter is gone