Unhandled 'EPIPE' in 'StdioServerTransport' causes fatal process crash on client disconnect #1564

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1. manthanghasadiya

   added

   bugSomething isn't workingSomething isn't working

   on Feb 21, 2026
2. stakeswky

   added a commit that references this issue on Feb 22, 2026

   fix: handle EPIPE errors in StdioServerTransport

   ...

   This issue will close once commit af8ccdc is merged into the 'main' branch.

   af8ccdc
3. sstklen commented on Feb 25, 2026

   

   sstklen

   on Feb 25, 2026

   More actions

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

   What's happening: StdioServerTransport.send() calls stdout.write() without any error handling. When a client disconnects abruptly (closes stdin/stdout pipes), the subsequent write to the now-broken stdout pipe emits an unhandled 'EPIPE' error. Node.js treats unhandled stream errors as fatal exceptions, crashing the entire server process. The core issue is that stream.write() can fail asynchronously via the 'error' event or synchronously throw, and neither path is caught in the current implementation.

   What worked for us:

   Add error handling around stdout.write() in StdioServerTransport: (1) attach an 'error' event listener on the stdout stream to catch async EPIPE/ERR_STREAM_DESTROYED errors gracefully, and (2) wrap the write() call in try/catch for synchronous throws. On EPIPE, log a debug message and trigger graceful shutdown instead of crashing.

   Steps:

   1. In StdioServerTransport constructor or start(), add stdout.on('error', handler) that catches EPIPE/ERR_STREAM_DESTROYED and calls close() gracefully instead of letting it propagate as uncaught
   2. Wrap the stdout.write() call in send() with try/catch to handle synchronous write errors on an already-destroyed stream
   3. In the error handler, check for err.code === 'EPIPE' || err.code === 'ERR_STREAM_DESTROYED' and treat these as client-disconnect signals rather than fatal errors
   4. Optionally add process.on('SIGPIPE', () => {}) as a defense-in-depth measure to ignore SIGPIPE at the process level (Node.js default already ignores it, but Bun may not)

   // In StdioServerTransport class:
   
   start() {
     // ... existing setup ...
     
     // Add error listener to prevent unhandled EPIPE crash
     this._stdout.on('error', (err) => {
       if (err.code === 'EPIPE' || err.code === 'ERR_STREAM_DESTROYED') {
         // Client disconnected — graceful shutdown, not a crash
         this.close();
       } else {
         this.onerror?.(err);
       }
     });
   }
   
   async send(message: JSONRPCMessage): Promise<void> {
     try {
       const json = JSON.stringify(message);
       if (this._stdout.writable) {
         this._stdout.write(json + '\n');
       }
     } catch (err: any) {
       if (err?.code === 'EPIPE' || err?.code === 'ERR_STREAM_DESTROYED') {
         await this.close();
       } else {
         throw err;
       }
     }
   }

   📊 We found 3 similar cases in our knowledge base with the same pattern — this gives us high confidence in this analysis.

   Hope this helps! Let me know if it doesn't match your case — happy to dig deeper. 🦞

   Disclosure: This analysis is from Confucius Debug, an AI-powered community KB for agent bugs. Please verify before applying.

   ---

   _{🦞 Confucius Debug — community knowledge base for AI agent bugs. Free to search via MCP.}
4. manthanghasadiya commented on Feb 25, 2026

   

   manthanghasadiya

   on Feb 25, 2026

   Author

   More actions

   @sstklen Thanks for the analysis! This matches what I found through fuzzing with mcpsec. PR #1566 is already addressing this - hopefully it gets merged soon.

   The core issue is that any MCP server using StdioServerTransport will crash on client disconnect, which is a DoS vector.
5. xXMrNidaXx commented on Feb 25, 2026

   

   xXMrNidaXx

   on Feb 25, 2026

   More actions

   This is a critical issue for production deployments. EPIPE handling should definitely be defensive in any transport layer.

   Immediate workaround:

   You can patch this at the process level until the SDK is fixed:

   // Add at the top of your MCP server entry point
   process.stdout.on('error', (err) => {
     if (err.code === 'EPIPE') {
       // Client disconnected - graceful exit
       console.error('[MCP] Client disconnected (EPIPE)');
       process.exit(0);
     }
     throw err; // Re-throw other errors
   });
   
   // Also catch uncaughtException as a fallback
   process.on('uncaughtException', (err) => {
     if (err.code === 'EPIPE') {
       process.exit(0);
     }
     console.error('Uncaught exception:', err);
     process.exit(1);
   });

   More robust wrapper for the transport:

   // Wrap the send method with EPIPE handling
   const originalSend = transport.send.bind(transport);
   transport.send = async (message: JsonRpcMessage) => {
     try {
       await originalSend(message);
     } catch (err: any) {
       if (err.code === 'EPIPE' || err.code === 'ERR_STREAM_DESTROYED') {
         // Emit close event and clean up
         transport.emit('close');
         return;
       }
       throw err;
     }
   };

   Suggested SDK fix:

   In StdioServerTransport.send(), the stdout.write() call at line 66 should be wrapped in try/catch, and the stdout stream should have an 'error' listener registered in the constructor:

   constructor() {
     // ... existing code ...
     process.stdout.on('error', (err) => {
       if (err.code === 'EPIPE') {
         this.close();
       }
     });
   }

   Happy to submit a PR if helpful.
6. stakeswky

   added a commit that references this issue on Feb 26, 2026

   fix: handle EPIPE errors in StdioServerTransport

   ...

   This issue will close once commit 5bfe860 is merged into the 'main' branch.

   5bfe860
7. MaxwellCalkin

   added a commit that references this issue on Mar 8, 2026

   fix: handle EPIPE errors in StdioServerTransport gracefully

   ...

   This issue will close once commit 75c8800 is merged into the 'main' branch.

   75c8800
8. felixweinberger

   added a commit that references this issue on Mar 12, 2026

   fix: handle EPIPE errors in StdioServerTransport

   ...

   This issue will close once commit 546d56f is merged into the 'main' branch.

   546d56f
9. felixweinberger

   closed this as completedin #1568on Mar 25, 2026