sse timeout set dont work & always close after 5mins

Describe the bug
always res.on('close') tigger when 5mins after.

To Reproduce

app.get('/sse', async (req, res) => {
  // Create SSE transport for legacy clients
  const transport = new SSEServerTransport('/messages', res);
  transports.sse[transport.sessionId] = transport;
  
  res.on("close", () => {
    delete transports.sse[transport.sessionId];
  });
  
  await server.connect(transport);
});

Expected behavior
to set a bigger timeout

callTool(xx,undefined, {timeout: 20mins})

but dont work

Hey there! The problem is that os kills the network connection after 5min of inactivity. The simplest way to prevent it is send some data regularly, "notifications/progress" MCP message works perfectly for it.

Please find below working example with 20min task (here I send a notification progress message every 30sec):

minimal-sse-fix.ts:

import express from 'express';
import { Client } from '@modelcontextprotocol/sdk/client/index.js';
import { SSEClientTransport } from '@modelcontextprotocol/sdk/client/sse.js';
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { SSEServerTransport } from '@modelcontextprotocol/sdk/server/sse.js';
import { z } from 'zod';

// Test: 20-minute task with 30-minute timeout and progress notifications
const TASK_SECONDS = 1200; // 20 minutes
const CLIENT_TIMEOUT = 1800000; // 30 minutes

// Server
function createServer() {
  const app = express();
  app.use(express.json());
  const transports: Record<string, SSEServerTransport> = {};
  
  const server = new McpServer({ name: 'test', version: '1.0.0' }, { capabilities: {} });
  
  server.tool('task', 'Long task with progress', { seconds: z.number() },
    async ({ seconds }, { sendNotification }) => {
      const start = Date.now();
      
      for (let elapsed = 0; elapsed < seconds; elapsed += 30) {
        await new Promise(r => setTimeout(r, Math.min(30, seconds - elapsed) * 1000));
        
        const actualElapsed = Math.round((Date.now() - start) / 1000);
        const remaining = Math.max(0, seconds - actualElapsed);
        
        await sendNotification({
          method: "notifications/progress",
          params: { 
            progressToken: "task-progress",
            progress: (actualElapsed / seconds) * 100,
            message: `${actualElapsed}s/${seconds}s (${remaining}s remaining)`
          }
        });
        
        if (actualElapsed >= seconds) break;
      }
      
      return { content: [{ type: 'text', text: `Completed ${seconds}s task` }] };
    }
  );

  app.get('/sse', async (req, res) => {
    const transport = new SSEServerTransport('/messages', res);
    transports[transport.sessionId] = transport;
    transport.onclose = () => delete transports[transport.sessionId];
    await server.connect(transport);
  });

  app.post('/messages', async (req, res) => {
    const sessionId = req.query.sessionId as string;
    const transport = transports[sessionId];
    if (!transport) return res.status(404).send('Not found');
    await transport.handlePostMessage(req, res, req.body);
  });

  return app.listen(3004);
}

// Client
async function runTest(): Promise<boolean> {
  const transport = new SSEClientTransport(new URL('http://localhost:3004/sse'));
  const client = new Client({ name: 'test', version: '1.0.0' }, { capabilities: {} });

  const start = Date.now();
  try {
    await client.connect(transport);
    
    console.log(`[${ts()}] Starting ${TASK_SECONDS}s task...`);
    
    await client.callTool({ 
      name: 'task', 
      arguments: { seconds: TASK_SECONDS }
    }, undefined, { timeout: CLIENT_TIMEOUT });
    
    const duration = (Date.now() - start) / 1000;
    console.log(`[${ts()}] ✅ SUCCESS: Completed after ${duration.toFixed(1)}s`);
    return true;
    
  } catch (error: any) {
    const duration = (Date.now() - start) / 1000;
    console.log(`[${ts()}] ❌ FAILED: ${error.message} (after ${duration.toFixed(1)}s)`);
    return false;
  } finally {
    await client.close();
    await transport.close();
  }
}

// Main
function ts() { return new Date().toISOString().slice(11, 19); }

async function main() {
  console.log(`[${ts()}] Testing: 20-minute task with progress notifications (no timeout config)`);
  
  const server = createServer();
  await new Promise(r => setTimeout(r, 1000));
  
  const result = await runTest();
  server.close();
  
  console.log(`\n[${ts()}] ` + (result ? 
    '🎯 SOLUTION: Progress notifications keep SSE alive past 20 minutes!' : 
    '❌ Still fails - more investigation needed'
  ));
}

if (import.meta.url === `file://${process.argv[1]}`) {
  main().catch(console.error);
}

package.json:

{
  "name": "issue_876",
  "version": "1.0.0",
  "type": "module",
  "main": "index.js",
  "scripts": {
    "minimal-fix": "npx tsx minimal-sse-fix.ts"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "description": "",
  "dependencies": {
    "@modelcontextprotocol/sdk": "^1.17.3",
    "express": "^5.1.0",
    "zod": "^3.25.76"
  },
  "devDependencies": {
    "@types/express": "^5.0.3",
    "@types/node": "^24.3.0",
    "ts-node": "^10.9.2",
    "tsx": "^4.20.4",
    "typescript": "^5.9.2"
  }
}

output:

issue_876 % npm run minimal-fix

> issue_876@1.0.0 minimal-fix
> npx tsx minimal-sse-fix.ts

[10:46:48] Testing: 20-minute task with progress notifications (no timeout config)
[10:46:49] Starting 1200s task...

[11:06:49] ✅ SUCCESS: Completed after 1200.1s

[11:06:49] 🎯 SOLUTION: Progress notifications keep SSE alive past 20 minutes!

Hope that helps! MCP team, should we add this tip to README.md?

@antonpk1

MCP team, should we add this tip to README.md?

@antonpk1 good idea, may want to point to references of os-specific default timeouts if applicable

The spec actually says that it should be sending pings anyway, which would keep the connection alive.

https://modelcontextprotocol.io/specification/2025-06-18/basic/utilities/ping

Implementations SHOULD periodically issue pings to detect connection health

I don't think implementations do that now, so it's a bug. I don't think clients should have to do this manually.

Hey! I ran into a similar pattern in our bug knowledge base and thought this might help.

The fix mentioned above looks right — we've seen the same pattern work in other projects. ✅

Added this to our shared KB so other folks hitting the same issue can find it faster.

_{🦞 Confucius Debug — community knowledge base for AI agent bugs. Free to search via MCP.}

feat(server): add keepAliveInterval to standalone GET SSE stream

The reproduction uses the old SSE server transport (SSEServerTransport), which writes nothing while a tool runs, so a client or proxy that gives up on a silent stream after five minutes (Node's fetch does) drops it; the request timeout option does not change that. That transport is deprecated: Streamable HTTP sends a keep-alive comment every 15 seconds since 1.30.0 (#2538) and 2.0.0 (#2541), and on SSE the progress notifications shown above keep the stream open. Closing.

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sse timeout set dont work & always close after 5mins #876

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sse timeout set dont work & always close after 5mins #876

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antonpk1 commented on Aug 18, 2025

ochafik commented on Sep 17, 2025

pja-ant commented on Oct 13, 2025

sstklen commented on Feb 24, 2026

felixweinberger commented on Oct 5, 2026

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