diff --git a/engine/app/main.py b/engine/app/main.py
index 593d534..259204c 100644
--- a/engine/app/main.py
+++ b/engine/app/main.py
@@ -190,6 +190,12 @@ async def websocket_endpoint(websocket: WebSocket):
         # Receive loop
         while True:
             message = await websocket.receive()
+            message_type = message.get("type")
+
+            if message_type == "websocket.disconnect":
+                logger.info(f"WebSocket disconnected: {session_id}")
+                break
+
             last_received_at[0] = time.monotonic()
 
             # Handle binary audio data
diff --git a/engine/core/session.py b/engine/core/session.py
index 460cd6d..6c59bd7 100644
--- a/engine/core/session.py
+++ b/engine/core/session.py
@@ -1,5 +1,6 @@
 """Session management for active calls."""
 
+import asyncio
 import uuid
 import json
 import time
@@ -78,6 +79,8 @@ class Session:
         self._history_turn_index: int = 0
         self._history_call_started_mono: Optional[float] = None
         self._history_finalized: bool = False
+        self._cleanup_lock = asyncio.Lock()
+        self._cleaned_up = False
 
         self.pipeline.conversation.on_turn_complete(self._on_turn_complete)
 
@@ -288,10 +291,15 @@ class Session:
 
     async def cleanup(self) -> None:
         """Cleanup session resources."""
-        logger.info(f"Session {self.id} cleaning up")
-        await self._finalize_history(status="connected")
-        await self.pipeline.cleanup()
-        await self.transport.close()
+        async with self._cleanup_lock:
+            if self._cleaned_up:
+                return
+
+            self._cleaned_up = True
+            logger.info(f"Session {self.id} cleaning up")
+            await self._finalize_history(status="connected")
+            await self.pipeline.cleanup()
+            await self.transport.close()
 
     async def _start_history_bridge(self, metadata: Dict[str, Any]) -> None:
         """Initialize backend history call record for this session."""
diff --git a/engine/core/transports.py b/engine/core/transports.py
index 6945225..3df04ba 100644
--- a/engine/core/transports.py
+++ b/engine/core/transports.py
@@ -5,6 +5,7 @@ import json
 from abc import ABC, abstractmethod
 from typing import Optional
 from fastapi import WebSocket
+from starlette.websockets import WebSocketState
 from loguru import logger
 
 # Try to import aiortc (optional for WebRTC functionality)
@@ -107,9 +108,24 @@ class SocketTransport(BaseTransport):
 
     async def close(self) -> None:
         """Close the WebSocket connection."""
+        if self._closed:
+            return
+
         self._closed = True
+        if (
+            self.ws.client_state == WebSocketState.DISCONNECTED
+            or self.ws.application_state == WebSocketState.DISCONNECTED
+        ):
+            return
+
         try:
             await self.ws.close()
+        except RuntimeError as e:
+            # Already closed by another task/path; safe to ignore.
+            if "close message has been sent" in str(e):
+                logger.debug(f"WebSocket already closed: {e}")
+                return
+            logger.error(f"Error closing WebSocket: {e}")
         except Exception as e:
             logger.error(f"Error closing WebSocket: {e}")