fix(anthropic): use native structured output for claude-fable-5-1 on Vertex AI and Bedrock Invoke

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/llms/anthropic/chat/transformation.py

@@ -1516,7 +1516,7 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                     _tool = self.map_response_format_to_anthropic_tool(value, optional_params, is_thinking_enabled)
                     if _tool is None:
                         continue
-                    if not is_thinking_enabled:
+                    if not is_thinking_enabled and not AnthropicModelInfo.forced_tool_use_unsupported(model):
                         _tool_choice = {
                             "name": RESPONSE_FORMAT_TOOL_NAME,
                             "type": "tool",

litellm/llms/anthropic/common_utils.py

@@ -325,13 +325,17 @@ class AnthropicModelInfo(BaseLLMModelInfo):
                 status_code=400,
             )
 
+    @staticmethod
+    def forced_tool_use_unsupported(model: str) -> bool:
+        return AnthropicModelInfo._get_model_capability(model, "supports_forced_tool_use") is False
+
     @staticmethod
     def forced_tool_use_downgraded(model: str, drop_params: bool) -> bool:
         """True when the model map flags the model with
         ``supports_forced_tool_use: false`` (Fable 5.1 / Mythos 5.1 400 on
         ``any``/``tool``) and ``drop_params`` asks for the ``auto`` downgrade;
         raises a clean client-side 400 for such models without ``drop_params``."""
-        if AnthropicModelInfo._get_model_capability(model, "supports_forced_tool_use") is not False:
+        if not AnthropicModelInfo.forced_tool_use_unsupported(model):
             return False
         if not (litellm.drop_params or drop_params):
             raise litellm.utils.UnsupportedParamsError(

litellm/llms/bedrock/chat/invoke_transformations/anthropic_claude3_transformation.py

@@ -74,10 +74,14 @@ class AmazonAnthropicClaudeConfig(AmazonInvokeConfig, AnthropicConfig):
         drop_params: bool,
     ) -> dict:
         # Force tool-based structured outputs for Bedrock Invoke
-        # (similar to VertexAI fix in #19201)
-        # Bedrock Invoke doesn't support output_format parameter
+        # (similar to VertexAI fix in #19201) unless the model map advertises
+        # native structured output
+        from litellm.utils import supports_native_structured_output
+
         original_model: Final = model
-        if "response_format" in non_default_params:
+        if "response_format" in non_default_params and not supports_native_structured_output(
+            model=model, custom_llm_provider="bedrock"
+        ):
             # Use a model name that forces tool-based approach
             model = "claude-3-sonnet-20240229"
 

litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/transformation.py

@@ -153,14 +153,17 @@ class VertexAIAnthropicConfig(AnthropicConfig):
         drop_params: bool,
     ) -> dict:
         """
-        Override parent method to ensure VertexAI always uses tool-based structured outputs.
-        VertexAI doesn't support the output_format parameter, so we force all models
-        to use the tool-based approach for structured outputs.
+        Override parent method so VertexAI uses tool-based structured outputs
+        unless the vertex map entry advertises native structured output
+        (``output_format``, which Vertex AI Claude forwards for those models).
         """
-        # Temporarily override model name to force tool-based approach
-        # This ensures Claude Sonnet 4.5 uses tools instead of output_format
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
         original_model: Final = model
-        if "response_format" in non_default_params:
+        native_structured_output: Final = AnthropicModelInfo._get_provider_resolved_capability(
+            model, "supports_native_structured_output", "vertex_ai"
+        )
+        if "response_format" in non_default_params and native_structured_output is not True:
             model = "claude-3-sonnet-20240229"  # Use a model that will use tool-based approach
 
         # Call parent method with potentially modified model name

litellm/model_prices_and_context_window_backup.json

@@ -3254,6 +3254,7 @@
         "supports_computer_use": true,
         "supports_forced_tool_use": false,
         "supports_function_calling": true,
+        "supports_native_structured_output": true,
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
@@ -44132,6 +44133,7 @@
         "supports_computer_use": true,
         "supports_forced_tool_use": false,
         "supports_function_calling": true,
+        "supports_native_structured_output": true,
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
@@ -44202,6 +44204,7 @@
         "supports_computer_use": true,
         "supports_forced_tool_use": false,
         "supports_function_calling": true,
+        "supports_native_structured_output": true,
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,

model_prices_and_context_window.json

@@ -3254,6 +3254,7 @@
         "supports_computer_use": true,
         "supports_forced_tool_use": false,
         "supports_function_calling": true,
+        "supports_native_structured_output": true,
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
@@ -44132,6 +44133,7 @@
         "supports_computer_use": true,
         "supports_forced_tool_use": false,
         "supports_function_calling": true,
+        "supports_native_structured_output": true,
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
@@ -44202,6 +44204,7 @@
         "supports_computer_use": true,
         "supports_forced_tool_use": false,
         "supports_function_calling": true,
+        "supports_native_structured_output": true,
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,

tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py

@@ -6354,3 +6354,33 @@ def test_anthropic_drop_params_reduces_mixed_output_config_to_format(monkeypatch
     )
 
     assert result.get("output_config") == {"format": schema_format}
+
+
+def test_response_format_tool_path_skips_forced_tool_choice_when_unsupported(local_model_cost_map, monkeypatch):
+    """Backstop: on the tool-based structured-output path, a model flagged
+    ``supports_forced_tool_use: false`` must not get the forced response-format
+    tool_choice the provider would 400 on."""
+    monkeypatch.setitem(
+        litellm.model_cost,
+        "claude-test-no-forced-tools",
+        {"litellm_provider": "anthropic", "mode": "chat", "supports_forced_tool_use": False},
+    )
+    config = AnthropicConfig()
+
+    result = config.map_openai_params(
+        non_default_params={
+            "response_format": {
+                "type": "json_schema",
+                "json_schema": {
+                    "name": "test_schema",
+                    "schema": {"type": "object", "properties": {"result": {"type": "string"}}},
+                },
+            }
+        },
+        optional_params={},
+        model="claude-test-no-forced-tools",
+        drop_params=False,
+    )
+
+    assert "tools" in result
+    assert "tool_choice" not in result

tests/test_litellm/llms/bedrock/chat/invoke_transformations/test_bedrock_chat_invoke_transformations_anthropic_claude3_transformation.py

@@ -643,3 +643,30 @@ def test_bedrock_chat_invoke_drop_params_still_inlines_for_non_native(local_mode
     assert "output_config" not in result
     last_content = result["messages"][-1]["content"]
     assert json.loads(last_content[-1]["text"]) == schema
+
+
+@pytest.mark.parametrize(
+    "model",
+    ["us.anthropic.claude-fable-5-1", "anthropic.claude-fable-5-1"],
+)
+def test_bedrock_chat_invoke_fable_5_1_response_format_uses_native_path(local_model_cost_map, model):
+    """Regression: Fable 5.1 rejects forced tool use, so invoke must skip the
+    tool-based structured-output stub and emit ``output_format`` instead of a
+    forced ``tool_choice``."""
+    result = AmazonAnthropicClaudeConfig().map_openai_params(
+        non_default_params={
+            "response_format": {
+                "type": "json_schema",
+                "json_schema": {
+                    "name": "test_schema",
+                    "schema": {"type": "object", "properties": {"result": {"type": "string"}}},
+                },
+            }
+        },
+        optional_params={},
+        model=model,
+        drop_params=False,
+    )
+
+    assert "output_format" in result
+    assert "tool_choice" not in result

tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_transformation.py

@@ -727,3 +727,28 @@ def test_sanitize_strips_effort_for_haiku_45():
     data = {"output_config": {"effort": "high"}}
     sanitize_vertex_anthropic_output_params(data, "vertex_ai/claude-opus-4-6")
     assert data["output_config"] == {"effort": "high"}
+
+
+def test_vertex_ai_fable_5_1_response_format_uses_native_output_format(local_model_cost_map):
+    """Regression: Fable 5.1 rejects forced tool use, so the vertex map entry
+    advertises native structured output and ``response_format`` must map to
+    ``output_format`` instead of the tool-based path's forced tool_choice."""
+    config = VertexAIAnthropicConfig()
+    response_format = {
+        "type": "json_schema",
+        "json_schema": {
+            "name": "test_schema",
+            "schema": {"type": "object", "properties": {"result": {"type": "string"}}},
+        },
+    }
+
+    result_params = config.map_openai_params(
+        non_default_params={"response_format": response_format},
+        optional_params={},
+        model="claude-fable-5-1",
+        drop_params=False,
+    )
+
+    assert "output_format" in result_params
+    assert "tool_choice" not in result_params
+    assert "tools" not in result_params