Fix DG keepalive thread permanently killing connection (#5870)

backend/routers/transcribe.py

@@ -1139,6 +1139,8 @@ async def deepgram_socket_send(data):
                     )
 
                 # Stabilization delay before switching to main socket
+                # SafeDeepgramSocket's auto-keepalive thread keeps the main DG socket
+                # alive during this idle window — no manual keepalive needed (#5870)
                 if is_active():
                     logger.info(
                         f"Speech profile sent, waiting {SPEECH_PROFILE_STABILIZE_DELAY}s for stabilization {uid} {session_id}"
@@ -2290,7 +2292,7 @@ async def receive_data(dg_socket, dg_profile_socket, soniox_sock, soniox_profile
         stt_buffer_flush_size = int(sample_rate * 2 * 0.03)  # 30ms at 16-bit mono (e.g., 6400 bytes at 16kHz)
 
         async def flush_stt_buffer(force: bool = False):
-            nonlocal stt_audio_buffer, soniox_profile_socket, deepgram_profile_socket, dg_usage_ms_pending
+            nonlocal stt_audio_buffer, soniox_profile_socket, deepgram_profile_socket, dg_usage_ms_pending, dg_socket
 
             if not stt_audio_buffer:
                 return
@@ -2303,6 +2305,12 @@ async def flush_stt_buffer(force: bool = False):
             # Use event-based routing instead of time-based
             profile_complete = speech_profile_complete.is_set()
 
+            # Check if DG connection died (keepalive or send failure) (#5870)
+            # Separated from routing so profile socket still receives audio if main dies.
+            if dg_socket is not None and dg_socket.is_connection_dead:
+                logger.error('DG connection died mid-session uid=%s session=%s', uid, session_id)
+                dg_socket = None  # Stop sending to dead connection
+
             if dg_socket is not None:
                 if profile_complete or not deepgram_profile_socket:
                     # DG budget gate: skip sending if restricted user's daily budget is exhausted (#5746)
@@ -2336,6 +2344,9 @@ async def close_dg_profile():
                         spawn(close_dg_profile())
                 else:
                     deepgram_profile_socket.send(chunk)
+            elif deepgram_profile_socket and not profile_complete:
+                # Main socket dead but profile socket still alive — keep routing (#5870)
+                deepgram_profile_socket.send(chunk)
 
             if soniox_sock is not None and not fair_use_dg_budget_exhausted:
                 if profile_complete or not soniox_profile_socket: