claude-code - 💡(How to fix) Fix [BUG] v2.1.111: ESC during MCP tool call kills all Python stdio MCPs (regression from v2.1.104, 30-line repro) PLEASE FIX NOW! [2 comments, 1 participants]

The following MCP servers have disconnected: esc-test
  The following deferred tools are no longer available (their MCP server
  disconnected)                                                                 
  1 MCP server failed · /mcp
  Nothing on the server-side stderr — no Python traceback, no KeyboardInterrupt,
   no CancelledError, no custom signal-handler log. The process is killed
  externally.

---

pip install "mcp[cli]>=1.0"                                                
                                                                  
  2. Save as ~/esc_test.py:
  """Minimal FastMCP Python server — reproduces ESC-kill regression."""         
  import asyncio, os, signal, sys
  from mcp.server.fastmcp import FastMCP                                        
                                                                  
  mcp = FastMCP("esc-test")                                                     
  print(f"[esc-test] pid={os.getpid()} booting", file=sys.stderr, flush=True)
                                                                                
  @mcp.tool()
  async def sleep_long(seconds: int = 30) -> str:                               
      """Sleep for N seconds. ESC mid-call should cancel but NOT kill the 
  MCP."""                                                                       
      try:
          await asyncio.sleep(seconds)                                          
          return f"slept {seconds}s"                              
      except BaseException as e:
          print(f"[esc-test] cancelled with {type(e).__name__}: {e}",
  file=sys.stderr, flush=True)                                                  
          raise
                                                                                
  @mcp.tool()                                                     
  async def ping() -> str:
      """Health check. If this still works after ESC, the MCP survived."""
      return f"alive pid={os.getpid()}"                                         
  
  def _diag(signum, frame):                                                     
      try: name = signal.Signals(signum).name                     
      except Exception: name = f"sig-{signum}"
      print(f"[esc-test] received {name} ({signum}); ignored", file=sys.stderr, 
  flush=True)
                                                                                
  if __name__ == "__main__":                                                    
      for s in ("SIGINT","SIGTERM","SIGHUP","SIGPIPE"):
          sig = getattr(signal, s, None)                                        
          if sig is not None:                                     
              try: signal.signal(sig, _diag)                                    
              except Exception: pass
      mcp.run()                                                                 
  3. Register:                                                    
  claude mcp add esc-test -- python3 ~/esc_test.py
  4. Start a fresh Claude Code session and run /mcp — confirm esc-test · ✔      
  connected.                                                                    
  5. Prompt Claude: "call the sleep_long tool from the esc-test MCP with        
  seconds=30".                                                                  
  6. As soon as the tool call starts, press ESC once to cancel.   
  7. Run /mcp — esc-test · ✘ failed.                                            
  8. Prompt Claude: "call the ping tool from the esc-test MCP" — fails with "MCP
   server has disconnected". Requires manual /mcp to recover.                   
                                                                  
  Expected (v2.1.104 behavior): esc-test remains connected after ESC. ping      
  returns alive pid=<N> immediately with the same PID as before. No reconnect
  needed.                 

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

2.1.104

### Claude Code Version

2.1.111 (Claude Code)

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information