diff --git a/gateway/platforms/signal.py b/gateway/platforms/signal.py
index 39c3814fb..cbe12a87c 100644
--- a/gateway/platforms/signal.py
+++ b/gateway/platforms/signal.py
@@ -279,6 +279,12 @@ class SignalAdapter(BasePlatformAdapter):
                             line = line.strip()
                             if not line:
                                 continue
+                            # SSE keepalive comments (":") prove the connection
+                            # is alive — update activity so the health monitor
+                            # doesn't report false idle warnings.
+                            if line.startswith(":"):
+                                self._last_sse_activity = time.time()
+                                continue
                             # Parse SSE data lines
                             if line.startswith("data:"):
                                 data_str = line[5:].strip()