fix(gemini): Gemini UnboundLocal Exception raised during stream (#1420)

Author: emattiza

src/strands/models/gemini.py

@@ -426,6 +426,8 @@ async def stream(
             yield self._format_chunk({"chunk_type": "content_start", "data_type": "text"})
 
             tool_used = False
+            candidate = None
+            event = None
             async for event in response:
                 candidates = event.candidates
                 candidate = candidates[0] if candidates else None
@@ -455,7 +457,8 @@ async def stream(
                     "data": "TOOL_USE" if tool_used else (candidate.finish_reason if candidate else "STOP"),
                 }
             )
-            yield self._format_chunk({"chunk_type": "metadata", "data": event.usage_metadata})
+            if event:
+                yield self._format_chunk({"chunk_type": "metadata", "data": event.usage_metadata})
 
         except genai.errors.ClientError as error:
             if not error.message:

tests/strands/models/test_gemini.py

@@ -566,6 +566,25 @@ async def test_stream_response_none_candidates(gemini_client, model, messages, a
     assert tru_chunks == exp_chunks
 
 
+@pytest.mark.asyncio
+async def test_stream_response_empty_stream(gemini_client, model, messages, agenerator, alist):
+    """Test that empty stream doesn't raise UnboundLocalError.
+
+    When the stream yields no events, the candidate variable must be initialized
+    to None to avoid UnboundLocalError when referenced in message_stop chunk.
+    """
+    gemini_client.aio.models.generate_content_stream.return_value = agenerator([])
+
+    tru_chunks = await alist(model.stream(messages))
+    exp_chunks = [
+        {"messageStart": {"role": "assistant"}},
+        {"contentBlockStart": {"start": {}}},
+        {"contentBlockStop": {}},
+        {"messageStop": {"stopReason": "end_turn"}},
+    ]
+    assert tru_chunks == exp_chunks
+
+
 @pytest.mark.asyncio
 async def test_stream_response_throttled_exception(gemini_client, model, messages):
     gemini_client.aio.models.generate_content_stream.side_effect = genai.errors.ClientError(