Unpredictable timeout error

[luiyongsheng]

The error message:

server is running on SSE at http://localhost:8080/sse
node:events:507
    const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
                ^

Error [ERR_UNHANDLED_ERROR]: Unhandled error. ({
  error: Error: Not connected
      at file:///Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:221:24
      at new Promise (<anonymous>)
      at Server.request (file:///Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:218:16)
      at Server.ping (file:///Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/server/index.js:167:21)
      at Timeout._onTimeout (file:///Users/<project-dir>/node_modules/fastmcp/dist/FastMCP.js:262:28)
      at listOnTimeout (node:internal/timers:573:17)
      at process.processTimers (node:internal/timers:514:7)
})
    at FastMCPSession.emit (node:events:507:17)
    at Timeout._onTimeout (file:///Users/<project-dir>/node_modules/fastmcp/dist/FastMCP.js:264:14) {
  code: 'ERR_UNHANDLED_ERROR',
  context: {
    error: Error: Not connected
        at file:///Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:221:24
        at new Promise (<anonymous>)
        at Server.request (file:///Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:218:16)
        at Server.ping (file:///Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/server/index.js:167:21)
        at Timeout._onTimeout (file:///Users/<project-dir>/node_modules/fastmcp/dist/FastMCP.js:262:28)
        at listOnTimeout (node:internal/timers:573:17)
        at process.processTimers (node:internal/timers:514:7)
  }
}

error message when run using bun:

server is running on SSE at http://localhost:8080/sse
[warning] FastMCP could not infer client capabilities
259 |     }
260 |     this.#pingInterval = setInterval(async () => {
261 |       try {
262 |         await this.#server.ping();
263 |       } catch (error) {
264 |         this.emit("error", {
                   ^
error: Unhandled error. ({
  error: McpError: MCP error -32001: Request timed out
      at new McpError (/Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/types.js:998:9)
      at timeoutHandler (/Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:277:53) {
    code: -32001,
    data: { timeout: 60000 }
  }
})
 context: {
  error: 993 |     CallToolResultSchema,
994 |     ListToolsResultSchema,
995 | ]);
996 | export class McpError extends Error {
997 |     constructor(code, message, data) {
998 |         super(`MCP error ${code}: ${message}`);
              ^
McpError: MCP error -32001: Request timed out
 code: -32001,
 data: [Object ...],

      at new McpError (/Users/<project-dir>/node_modules/fastmcp/node_modules/@modelcontextprotocol/sdk/dist/esm/types.js:998:9)
,
},
    code: "ERR_UNHANDLED_ERROR"

      at emitError (node:events:52:31)
      at <anonymous> (/Users/<project-dir>/node_modules/fastmcp/dist/FastMCP.js:264:14)

Bun v1.2.4 (macOS arm64)

I'm serving the FastMCP server like this:

#!/usr/bin/env node

import { FastMCP } from "fastmcp"

const server = new FastMCP({
  name: "Rokuyo",
  version: "1.0.0",
})

server.addTool({
  // ...
})

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

main().catch((error) => {
  console.error("Server error:", error)
  process.exit(1)
})

The way I write my MCP client and call the MCP server:

import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse"
import { Client } from "@modelcontextprotocol/sdk/client/index.js"
import { NextResponse } from "next/server"

export const runtime = "nodejs"

export async function GET(req) {
  const transport = new SSEClientTransport(new URL("http://localhost:8080/sse"))
  const client = new Client({
    name: "6ren",
    version: "1.0.0",
  })
  await client.connect(transport)
  const tools = await client.listTools()
  console.log("Tools:", tools)
  return NextResponse.json({ id: 1, ...tools })
}

[punkpeye]

My best guess is that these timeouts are coming from SDK itself.

https://github.com/search?q=repo%3Amodelcontextprotocol%2Ftypescript-sdk%20fetch&type=code

fetch has some unfortunate timeout defaults.

CC @jspahrsummers

I was just dealing with a related issue in another context.

nodejs/undici#1373

Ended up replacing fetch with undici's fetch.