diff --git a/tests/test_sync_utils.py b/tests/test_sync_utils.py
new file mode 100644
index 0000000..d8bd585
--- /dev/null
+++ b/tests/test_sync_utils.py
@@ -0,0 +1,21 @@
+import json
+from pathlib import Path
+
+from agentic_index_api import sync_utils
+
+
+def test_sync_filters_and_writes(tmp_path, monkeypatch):
+    repos = [
+        {"name": "a", "maintainer": "openai", "topics": "ai,agents"},
+        {"name": "b", "maintainer": "openai", "topics": "ml"},
+        {"name": "c", "maintainer": "other", "topics": "agents"},
+    ]
+    monkeypatch.setattr(sync_utils, "search_and_harvest", lambda min_stars=0, max_pages=1: repos)
+    monkeypatch.setattr(sync_utils, "STATE_PATH", tmp_path / "sync.json")
+
+    result = sync_utils.sync(org="openai", topics=["agents"])
+    assert {r["name"] for r in result} == {"a"}
+    assert (tmp_path / "sync.json").exists()
+    saved = json.loads(Path(tmp_path / "sync.json").read_text())
+    assert len(saved) == len(result)
+