diff --git a/logger.py b/logger.py
new file mode 100644
index 0000000..f435729
--- /dev/null
+++ b/logger.py
@@ -0,0 +1,105 @@
+"""Activity Logger - Logs all events to JSON Lines file."""
+
+import json
+import uuid
+import threading
+from datetime import datetime, timezone
+from typing import List, Dict, Any, Optional
+from config import LOG_FILE
+
+
+class ActivityLogger:
+    """Thread-safe activity logger that writes to JSON Lines file."""
+
+    def __init__(self, log_file: str = None):
+        self.log_file = log_file or LOG_FILE
+        self._lock = threading.Lock()
+
+    def _append_line(self, entry: Dict[str, Any]):
+        """Append a single JSON line to the file."""
+        with open(self.log_file, 'a') as f:
+            f.write(json.dumps(entry) + '\n')
+
+    def log_activity(self, event_data: Dict[str, Any]) -> Dict[str, Any]:
+        """
+        Log an activity entry to the JSON Lines file.
+        
+        Args:
+            event_data: Dictionary containing event information
+                - timestamp: ISO format timestamp (optional, auto-generated)
+                - event_type: Type of event (e.g., pull_request, issue)
+                - action: Action performed (e.g., opened, closed)
+                - repository: Repository full name
+                - sender: Actor who triggered the event
+                - payload: Full webhook payload
+                - routed_to: List of agents/targets triggered
+        
+        Returns:
+            The logged entry with added id and timestamp
+        """
+        with self._lock:
+            entry = {
+                "id": str(uuid.uuid4()),
+                "timestamp": datetime.now(timezone.utc).isoformat(),
+                "event_type": event_data.get("event_type", ""),
+                "action": event_data.get("action", ""),
+                "repository": event_data.get("repository", ""),
+                "sender": event_data.get("sender", ""),
+                "payload": event_data.get("payload", {}),
+                "routed_to": event_data.get("routed_to", [])
+            }
+            
+            self._append_line(entry)
+            return entry
+
+    def get_activities(self, limit: int = 50) -> List[Dict[str, Any]]:
+        """
+        Get recent activity entries from the log file.
+        
+        Args:
+            limit: Maximum number of entries to return
+            
+        Returns:
+            List of activity entries (most recent last)
+        """
+        with self._lock:
+            if not self.log_file:
+                return []
+                
+            activities = []
+            try:
+                with open(self.log_file, 'r') as f:
+                    for line in f:
+                        line = line.strip()
+                        if line:
+                            try:
+                                activities.append(json.loads(line))
+                            except json.JSONDecodeError:
+                                continue
+            except FileNotFoundError:
+                return []
+            
+            # Return most recent entries
+            return activities[-limit:] if len(activities) > limit else activities
+
+    def clear(self):
+        """Clear all activity logs."""
+        with self._lock:
+            try:
+                open(self.log_file, 'w').close()
+            except IOError:
+                pass
+
+
+# Global logger instance
+logger = ActivityLogger()
+
+
+def log_activity(event_data: Dict[str, Any]) -> Dict[str, Any]:
+    """Convenience function to log activity."""
+    return logger.log_activity(event_data)
+
+
+def get_activities(limit: int = 50) -> List[Dict[str, Any]]:
+    """Convenience function to get activities."""
+    return logger.get_activities(limit)
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