diff --git a/apps/ai_agent/tests/__init__.py b/apps/ai_agent/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/apps/ai_agent/tests/test_index.py b/apps/ai_agent/tests/test_index.py
new file mode 100644
index 0000000..c628870
--- /dev/null
+++ b/apps/ai_agent/tests/test_index.py
@@ -0,0 +1,121 @@
+from unittest.mock import MagicMock, patch
+
+import pytest
+from fastapi.testclient import TestClient
+
+from main import app
+
+client = TestClient(app)
+
+VALID_PAYLOAD = {
+    "messageId": "msg-1",
+    "conversationId": "conv-1",
+    "senderId": "user-1",
+    "content": "Hello, world!",
+}
+
+EMBEDDING_DIM = 1536
+
+
+def _mock_embedding(mock_openai):
+    mock_openai.return_value.embeddings.create.return_value = _fake_embedding_response()
+
+
+def _fake_embedding_response():
+    resp = MagicMock()
+    resp.data = [MagicMock()]
+    resp.data[0].embedding = [0.1] * EMBEDDING_DIM
+    return resp
+
+
+class TestIndexMessage:
+    def test_weaviate_connection_failure_returns_503(self):
+        with patch("main.weaviate.connect_to_local", side_effect=Exception("no weaviate")):
+            resp = client.post("/index/message", json=VALID_PAYLOAD)
+
+        assert resp.status_code == 503
+        assert resp.json()["detail"] == "Weaviate connection failed"
+
+    def test_creates_collection_if_missing(self):
+        mock_client = MagicMock()
+        mock_client.collections.exists.return_value = False
+        collection = MagicMock()
+        collection.data.exists.return_value = False
+        mock_client.collections.get.return_value = collection
+
+        with (
+            patch("main.weaviate.connect_to_local", return_value=mock_client),
+            patch("main._openai_client") as mock_openai,
+        ):
+            _mock_embedding(mock_openai)
+            resp = client.post("/index/message", json=VALID_PAYLOAD)
+
+        assert resp.status_code == 200
+        mock_client.collections.create.assert_called_once_with(name="Message")
+
+    def test_inserts_new_message(self):
+        mock_client = MagicMock()
+        mock_client.collections.exists.return_value = True
+        collection = MagicMock()
+        collection.data.exists.return_value = False
+        mock_client.collections.get.return_value = collection
+
+        with (
+            patch("main.weaviate.connect_to_local", return_value=mock_client),
+            patch("main._openai_client") as mock_openai,
+        ):
+            _mock_embedding(mock_openai)
+            resp = client.post("/index/message", json=VALID_PAYLOAD)
+
+        assert resp.status_code == 200
+        collection.data.insert.assert_called_once()
+        collection.data.replace.assert_not_called()
+
+    def test_replaces_existing_message(self):
+        mock_client = MagicMock()
+        mock_client.collections.exists.return_value = True
+        collection = MagicMock()
+        collection.data.exists.return_value = True
+        mock_client.collections.get.return_value = collection
+
+        with (
+            patch("main.weaviate.connect_to_local", return_value=mock_client),
+            patch("main._openai_client") as mock_openai,
+        ):
+            _mock_embedding(mock_openai)
+            resp = client.post("/index/message", json=VALID_PAYLOAD)
+
+        assert resp.status_code == 200
+        collection.data.replace.assert_called_once()
+        collection.data.insert.assert_not_called()
+
+    def test_closes_weaviate_on_success(self):
+        mock_client = MagicMock()
+        mock_client.collections.exists.return_value = True
+        collection = MagicMock()
+        collection.data.exists.return_value = False
+        mock_client.collections.get.return_value = collection
+
+        with (
+            patch("main.weaviate.connect_to_local", return_value=mock_client),
+            patch("main._openai_client") as mock_openai,
+        ):
+            _mock_embedding(mock_openai)
+            resp = client.post("/index/message", json=VALID_PAYLOAD)
+
+        assert resp.status_code == 200
+        mock_client.close.assert_called_once()
+
+    def test_closes_weaviate_on_error(self):
+        mock_client = MagicMock()
+        mock_client.collections.exists.side_effect = Exception("boom")
+
+        with patch("main.weaviate.connect_to_local", return_value=mock_client):
+            resp = client.post("/index/message", json=VALID_PAYLOAD)
+
+        assert resp.status_code == 503
+        mock_client.close.assert_called_once()
+
+    def test_missing_fields_returns_422(self):
+        resp = client.post("/index/message", json={})
+        assert resp.status_code == 422