diff --git a/server.py b/server.py
index bdea6a9..5bd30eb 100644
--- a/server.py
+++ b/server.py
@@ -606,6 +606,17 @@ async def ui_activity_detail(activity_index: int, request: Request):
         return HTMLResponse(base_html("Activity Not Found", content, username))
 
     activity = activities[activity_index]
+    return await _render_activity_detail(activity, request)
+
+
+async def ui_activity_detail_by_data(activity: dict, request: Request):
+    """Activity detail page taking activity data directly."""
+    return await _render_activity_detail(activity, request)
+
+
+async def _render_activity_detail(activity: dict, request: Request):
+    """Core activity detail rendering logic."""
+    username = get_user_from_cookie(request)
     activity_type = activity.get("activity_type", "")
     activity_id = activity.get("activity_id", "")
     actor_id = activity.get("actor_id", "")
@@ -2501,16 +2512,36 @@ async def get_activities(request: Request, page: int = 1, limit: int = 20):
     }
 
 
-@app.get("/activities/{activity_index}")
-async def get_activity_detail(activity_index: int, request: Request):
-    """Get single activity. HTML for browsers (default), JSON only if explicitly requested."""
-    activities = await load_activities()
+@app.get("/activities/{activity_ref}")
+async def get_activity_detail(activity_ref: str, request: Request):
+    """Get single activity by index or activity_id. HTML for browsers (default), JSON only if explicitly requested."""
 
     # Check if JSON explicitly requested
     accept = request.headers.get("accept", "")
     wants_json = ("application/json" in accept or "application/activity+json" in accept) and "text/html" not in accept
 
-    if activity_index < 0 or activity_index >= len(activities):
+    activity = None
+    activity_index = None
+
+    # Check if it's a numeric index or an activity_id (hash)
+    if activity_ref.isdigit():
+        # Numeric index (legacy)
+        activity_index = int(activity_ref)
+        activities = await load_activities()
+        if 0 <= activity_index < len(activities):
+            activity = activities[activity_index]
+    else:
+        # Activity ID (hash) - look up directly
+        activity = await db.get_activity(activity_ref)
+        if activity:
+            # Find index for UI rendering
+            activities = await load_activities()
+            for i, a in enumerate(activities):
+                if a.get("activity_id") == activity_ref:
+                    activity_index = i
+                    break
+
+    if not activity:
         if wants_json:
             raise HTTPException(404, "Activity not found")
         content = '''
@@ -2520,13 +2551,15 @@ async def get_activity_detail(activity_index: int, request: Request):
         '''
         return HTMLResponse(base_html("Activity Not Found", content, get_user_from_cookie(request)))
 
-    activity = activities[activity_index]
-
     if wants_json:
         return activity
 
     # Default to HTML for browsers
-    return await ui_activity_detail(activity_index, request)
+    if activity_index is not None:
+        return await ui_activity_detail(activity_index, request)
+    else:
+        # Render activity directly if no index found
+        return await ui_activity_detail_by_data(activity, request)
 
 
 @app.get("/activity/{activity_index}")