fix(bedrock): disable thinking mode when forcing tool_choice (#1495)

---------

Co-authored-by: Dean Schmigelski <dbschmigelski+github@gmail.com>

Author: strands-agent

src/strands/models/bedrock.py

@@ -255,11 +255,7 @@ def _format_request(
                 if tool_specs
                 else {}
             ),
-            **(
-                {"additionalModelRequestFields": self.config["additional_request_fields"]}
-                if self.config.get("additional_request_fields")
-                else {}
-            ),
+            **(self._get_additional_request_fields(tool_choice)),
             **(
                 {"additionalModelResponseFieldPaths": self.config["additional_response_field_paths"]}
                 if self.config.get("additional_response_field_paths")
@@ -298,6 +294,34 @@ def _format_request(
             ),
         }
 
+    def _get_additional_request_fields(self, tool_choice: ToolChoice | None) -> dict[str, Any]:
+        """Get additional request fields, removing thinking if tool_choice forces tool use.
+
+        Bedrock's API does not allow thinking mode when tool_choice forces tool use.
+        When forcing a tool (e.g., for structured_output retry), we temporarily disable thinking.
+
+        Args:
+            tool_choice: The tool choice configuration.
+
+        Returns:
+            A dict containing additionalModelRequestFields if configured, or empty dict.
+        """
+        additional_fields = self.config.get("additional_request_fields")
+        if not additional_fields:
+            return {}
+
+        # Check if tool_choice is forcing tool use ("any" or specific "tool")
+        is_forcing_tool = tool_choice is not None and ("any" in tool_choice or "tool" in tool_choice)
+
+        if is_forcing_tool and "thinking" in additional_fields:
+            # Create a copy without the thinking key
+            fields_without_thinking = {k: v for k, v in additional_fields.items() if k != "thinking"}
+            if fields_without_thinking:
+                return {"additionalModelRequestFields": fields_without_thinking}
+            return {}
+
+        return {"additionalModelRequestFields": additional_fields}
+
     def _format_bedrock_messages(self, messages: Messages) -> list[dict[str, Any]]:
         """Format messages for Bedrock API compatibility.
 

tests/strands/models/test_bedrock_thinking.py

@@ -0,0 +1,84 @@
+"""Tests for thinking mode behavior in BedrockModel."""
+
+import pytest
+
+from strands.models.bedrock import BedrockModel
+
+
+@pytest.fixture
+def model_with_thinking():
+    """Create a BedrockModel with thinking enabled."""
+    return BedrockModel(
+        model_id="anthropic.claude-sonnet-4-20250514-v1:0",
+        additional_request_fields={"thinking": {"type": "enabled", "budget_tokens": 5000}},
+    )
+
+
+@pytest.fixture
+def model_without_thinking():
+    """Create a BedrockModel without thinking."""
+    return BedrockModel(model_id="anthropic.claude-sonnet-4-20250514-v1:0")
+
+
+@pytest.fixture
+def model_with_thinking_and_other_fields():
+    """Create a BedrockModel with thinking and other additional fields."""
+    return BedrockModel(
+        model_id="anthropic.claude-sonnet-4-20250514-v1:0",
+        additional_request_fields={
+            "thinking": {"type": "enabled", "budget_tokens": 5000},
+            "some_other_field": "value",
+        },
+    )
+
+
+def test_thinking_removed_when_forcing_tool_any(model_with_thinking):
+    """Thinking should be removed when tool_choice forces tool use with 'any'."""
+    tool_choice = {"any": {}}
+    result = model_with_thinking._get_additional_request_fields(tool_choice)
+    assert result == {}  # thinking removed, no other fields
+
+
+def test_thinking_removed_when_forcing_specific_tool(model_with_thinking):
+    """Thinking should be removed when tool_choice forces a specific tool."""
+    tool_choice = {"tool": {"name": "structured_output_tool"}}
+    result = model_with_thinking._get_additional_request_fields(tool_choice)
+    assert result == {}  # thinking removed, no other fields
+
+
+def test_thinking_preserved_with_auto_tool_choice(model_with_thinking):
+    """Thinking should be preserved when tool_choice is 'auto'."""
+    tool_choice = {"auto": {}}
+    result = model_with_thinking._get_additional_request_fields(tool_choice)
+    assert result == {"additionalModelRequestFields": {"thinking": {"type": "enabled", "budget_tokens": 5000}}}
+
+
+def test_thinking_preserved_with_none_tool_choice(model_with_thinking):
+    """Thinking should be preserved when tool_choice is None."""
+    result = model_with_thinking._get_additional_request_fields(None)
+    assert result == {"additionalModelRequestFields": {"thinking": {"type": "enabled", "budget_tokens": 5000}}}
+
+
+def test_other_fields_preserved_when_thinking_removed(model_with_thinking_and_other_fields):
+    """Other additional fields should be preserved when thinking is removed."""
+    tool_choice = {"any": {}}
+    result = model_with_thinking_and_other_fields._get_additional_request_fields(tool_choice)
+    assert result == {"additionalModelRequestFields": {"some_other_field": "value"}}
+
+
+def test_no_fields_when_model_has_no_additional_fields(model_without_thinking):
+    """Should return empty dict when model has no additional_request_fields."""
+    tool_choice = {"any": {}}
+    result = model_without_thinking._get_additional_request_fields(tool_choice)
+    assert result == {}
+
+
+def test_fields_preserved_when_no_thinking_and_forcing_tool():
+    """Additional fields without thinking should be preserved when forcing tool."""
+    model = BedrockModel(
+        model_id="anthropic.claude-sonnet-4-20250514-v1:0",
+        additional_request_fields={"some_field": "value"},
+    )
+    tool_choice = {"any": {}}
+    result = model._get_additional_request_fields(tool_choice)
+    assert result == {"additionalModelRequestFields": {"some_field": "value"}}

tests_integ/models/test_model_bedrock.py

@@ -275,6 +275,43 @@ def test_redacted_content_handling():
     assert isinstance(result.message["content"][0]["reasoningContent"]["redactedContent"], bytes)
 
 
+def test_reasoning_content_in_messages_with_thinking_disabled():
+    """Test that messages with reasoningContent are accepted when thinking is explicitly disabled."""
+    # First, get a real reasoning response with thinking enabled
+    thinking_model = BedrockModel(
+        model_id="us.anthropic.claude-sonnet-4-20250514-v1:0",
+        additional_request_fields={
+            "thinking": {
+                "type": "enabled",
+                "budget_tokens": 1024,
+            }
+        },
+    )
+    agent_with_thinking = Agent(model=thinking_model)
+    result_with_thinking = agent_with_thinking("What is 2+2?")
+
+    # Verify we got reasoning content
+    assert "reasoningContent" in result_with_thinking.message["content"][0]
+
+    # Now create a model with thinking disabled and use the messages from the thinking session
+    disabled_model = BedrockModel(
+        model_id="us.anthropic.claude-sonnet-4-20250514-v1:0",
+        additional_request_fields={
+            "thinking": {
+                "type": "disabled",
+            }
+        },
+    )
+
+    # Use the conversation history that includes reasoning content
+    messages = agent_with_thinking.messages
+
+    agent_disabled = Agent(model=disabled_model, messages=messages)
+    result = agent_disabled("What about 3+3?")
+
+    assert result.stop_reason == "end_turn"
+
+
 def test_multi_prompt_system_content():
     """Test multi-prompt system content blocks."""
     system_prompt_content = [