fix: fix stop reason for bedrock model when stop_reason (strands-agents#767)

* fix: fix stop reason for bedrock model when stop_reason is end_turn in tool use response.

* change logger info to warning, optimize if condition

* fix: add unit tests

---------

Co-authored-by: Jack Yuan <[email protected]>

Author: JackYPCOnline

src/strands/models/bedrock.py

@@ -435,6 +435,8 @@ def _stream(
             logger.debug("got response from model")
             if streaming:
                 response = self.client.converse_stream(**request)
+                # Track tool use events to fix stopReason for streaming responses
+                has_tool_use = False
                 for chunk in response["stream"]:
                     if (
                         "metadata" in chunk
@@ -446,7 +448,24 @@ def _stream(
                             for event in self._generate_redaction_events():
                                 callback(event)
 
-                    callback(chunk)
+                    # Track if we see tool use events
+                    if "contentBlockStart" in chunk and chunk["contentBlockStart"].get("start", {}).get("toolUse"):
+                        has_tool_use = True
+
+                    # Fix stopReason for streaming responses that contain tool use
+                    if (
+                        has_tool_use
+                        and "messageStop" in chunk
+                        and (message_stop := chunk["messageStop"]).get("stopReason") == "end_turn"
+                    ):
+                        # Create corrected chunk with tool_use stopReason
+                        modified_chunk = chunk.copy()
+                        modified_chunk["messageStop"] = message_stop.copy()
+                        modified_chunk["messageStop"]["stopReason"] = "tool_use"
+                        logger.warning("Override stop reason from end_turn to tool_use")
+                        callback(modified_chunk)
+                    else:
+                        callback(chunk)
 
             else:
                 response = self.client.converse(**request)
@@ -582,9 +601,17 @@ def _convert_non_streaming_to_streaming(self, response: dict[str, Any]) -> Itera
             yield {"contentBlockStop": {}}
 
         # Yield messageStop event
+        # Fix stopReason for models that return end_turn when they should return tool_use on non-streaming side
+        current_stop_reason = response["stopReason"]
+        if current_stop_reason == "end_turn":
+            message_content = response["output"]["message"]["content"]
+            if any("toolUse" in content for content in message_content):
+                current_stop_reason = "tool_use"
+                logger.warning("Override stop reason from end_turn to tool_use")
+
         yield {
             "messageStop": {
-                "stopReason": response["stopReason"],
+                "stopReason": current_stop_reason,
                 "additionalModelResponseFields": response.get("additionalModelResponseFields"),
             }
         }

tests/strands/models/test_bedrock.py

@@ -1227,6 +1227,53 @@ async def test_stream_logging(bedrock_client, model, messages, caplog, alist):
     assert "finished streaming response from model" in log_text
 
 
+@pytest.mark.asyncio
+async def test_stream_stop_reason_override_streaming(bedrock_client, model, messages, alist):
+    """Test that stopReason is overridden from end_turn to tool_use in streaming mode when tool use is detected."""
+    bedrock_client.converse_stream.return_value = {
+        "stream": [
+            {"messageStart": {"role": "assistant"}},
+            {"contentBlockStart": {"start": {"toolUse": {"toolUseId": "123", "name": "test_tool"}}}},
+            {"contentBlockDelta": {"delta": {"test": {"input": '{"param": "value"}'}}}},
+            {"contentBlockStop": {}},
+            {"messageStop": {"stopReason": "end_turn"}},
+        ]
+    }
+
+    response = model.stream(messages)
+    events = await alist(response)
+
+    # Find the messageStop event
+    message_stop_event = next(event for event in events if "messageStop" in event)
+
+    # Verify stopReason was overridden to tool_use
+    assert message_stop_event["messageStop"]["stopReason"] == "tool_use"
+
+
+@pytest.mark.asyncio
+async def test_stream_stop_reason_override_non_streaming(bedrock_client, alist, messages):
+    """Test that stopReason is overridden from end_turn to tool_use in non-streaming mode when tool use is detected."""
+    bedrock_client.converse.return_value = {
+        "output": {
+            "message": {
+                "role": "assistant",
+                "content": [{"toolUse": {"toolUseId": "123", "name": "test_tool", "input": {"param": "value"}}}],
+            }
+        },
+        "stopReason": "end_turn",
+    }
+
+    model = BedrockModel(model_id="test-model", streaming=False)
+    response = model.stream(messages)
+    events = await alist(response)
+
+    # Find the messageStop event
+    message_stop_event = next(event for event in events if "messageStop" in event)
+
+    # Verify stopReason was overridden to tool_use
+    assert message_stop_event["messageStop"]["stopReason"] == "tool_use"
+
+
 def test_format_request_cleans_tool_result_content_blocks(model, model_id):
     """Test that format_request cleans toolResult blocks by removing extra fields."""
     messages = [