diff --git a/bincio/serve/server.py b/bincio/serve/server.py
index 3646648..1b07965 100644
--- a/bincio/serve/server.py
+++ b/bincio/serve/server.py
@@ -286,11 +286,18 @@ async def register(request: Request) -> JSONResponse:
 
     # Create per-user directories
     dd = _get_data_dir()
-    (dd / handle / "activities").mkdir(parents=True, exist_ok=True)
-    (dd / handle / "edits").mkdir(parents=True, exist_ok=True)
+    user_dir = dd / handle
+    (user_dir / "activities").mkdir(parents=True, exist_ok=True)
+    (user_dir / "edits").mkdir(parents=True, exist_ok=True)
+
+    # Write an empty index.json so the shard URL resolves immediately,
+    # even before the user uploads any activities.
+    from bincio.extract.writer import write_index
+    index_path = user_dir / "index.json"
+    if not index_path.exists():
+        write_index([], user_dir, {"handle": handle, "display_name": display or handle})
 
     # Update root manifest so the new user's shard is discoverable immediately
-    # (Astro dev re-evaluates getStaticPaths() on each request from this file)
     from bincio.render.cli import _write_root_manifest
     _write_root_manifest(dd)