fix(bedrock): fall back to toolChoice.auto in structured_output for unsupported models

src/strands/models/bedrock.py

@@ -1064,15 +1064,36 @@ async def structured_output(
         """
         tool_spec = convert_pydantic_to_tool_spec(output_model)
 
-        response = self.stream(
-            messages=prompt,
-            tool_specs=[tool_spec],
-            system_prompt=system_prompt,
-            tool_choice=cast(ToolChoice, {"any": {}}),
-            **kwargs,
-        )
-        async for event in streaming.process_stream(response):
-            yield event
+        tool_choice: ToolChoice = cast(ToolChoice, {"any": {}})
+        try:
+            response = self.stream(
+                messages=prompt,
+                tool_specs=[tool_spec],
+                system_prompt=system_prompt,
+                tool_choice=tool_choice,
+                **kwargs,
+            )
+            async for event in streaming.process_stream(response):
+                yield event
+        except ClientError as e:
+            error_message = str(e)
+            if "toolChoice.any" not in error_message and "toolChoice" not in error_message:
+                raise
+
+            logger.debug(
+                "model_id=<%s> | toolChoice.any not supported, falling back to toolChoice.auto",
+                self.config.get("model_id"),
+            )
+            tool_choice = cast(ToolChoice, {"auto": {}})
+            response = self.stream(
+                messages=prompt,
+                tool_specs=[tool_spec],
+                system_prompt=system_prompt,
+                tool_choice=tool_choice,
+                **kwargs,
+            )
+            async for event in streaming.process_stream(response):
+                yield event
 
         stop_reason, messages, _, _ = event["stop"]
 

tests/strands/models/test_bedrock.py

@@ -1419,7 +1419,45 @@ async def test_structured_output(bedrock_client, model, test_output_model_cls, a
     assert tru_output == exp_output
 
 
-@pytest.mark.skipif(sys.version_info < (3, 11), reason="This test requires Python 3.11 or higher (need add_note)")
+@pytest.mark.asyncio
+async def test_structured_output_fallback_tool_choice_auto(bedrock_client, model, test_output_model_cls, alist):
+    """When toolChoice.any is not supported, structured_output falls back to toolChoice.auto."""
+    messages = [{"role": "user", "content": [{"text": "Generate a person"}]}]
+
+    success_stream = {
+        "stream": [
+            {"messageStart": {"role": "assistant"}},
+            {"contentBlockStart": {"start": {"toolUse": {"toolUseId": "123", "name": "TestOutputModel"}}}},
+            {"contentBlockDelta": {"delta": {"toolUse": {"input": '{"name": "John", "age": 30}'}}}},
+            {"contentBlockStop": {}},
+            {"messageStop": {"stopReason": "tool_use"}},
+        ]
+    }
+
+    error_response = {
+        "Error": {
+            "Code": "ValidationException",
+            "Message": "This model doesn't support the toolConfig.toolChoice.any field. "
+            "Remove toolConfig.toolChoice.any and try again",
+        }
+    }
+
+    bedrock_client.converse_stream.side_effect = [
+        ClientError(error_response, "ConverseStream"),
+        success_stream,
+    ]
+
+    stream = model.structured_output(test_output_model_cls, messages)
+    events = await alist(stream)
+
+    assert events[-1] == {"output": test_output_model_cls(name="John", age=30)}
+    assert bedrock_client.converse_stream.call_count == 2
+
+    first_call = bedrock_client.converse_stream.call_args_list[0]
+    assert first_call.kwargs["toolConfig"]["toolChoice"] == {"any": {}}
+
+    second_call = bedrock_client.converse_stream.call_args_list[1]
+    assert second_call.kwargs["toolConfig"]["toolChoice"] == {"auto": {}}
 @pytest.mark.asyncio
 async def test_add_note_on_client_error(bedrock_client, model, alist, messages):
     """Test that add_note is called on ClientError with region and model ID information."""