feat(bedrock): gate forced tool_choice on supports_forced_tool_use in converse

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

litellm/llms/anthropic/common_utils.py

@@ -326,19 +326,13 @@ class AnthropicModelInfo(BaseLLMModelInfo):
             )
 
     @staticmethod
-    def _apply_forced_tool_choice(
-        model: str,
-        tool_choice: AnthropicMessagesToolChoice,
-        drop_params: bool,
-    ) -> AnthropicMessagesToolChoice:
-        """Forward ``tool_choice`` unless the model map flags the model with
+    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``), in which case downgrade to ``auto`` (with
-        drop_params) or raise a clean client-side 400."""
-        if tool_choice["type"] not in ("any", "tool"):
-            return tool_choice
+        ``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:
-            return tool_choice
+            return False
         if not (litellm.drop_params or drop_params):
             raise litellm.utils.UnsupportedParamsError(
                 message=(
@@ -349,6 +343,18 @@ class AnthropicModelInfo(BaseLLMModelInfo):
                 status_code=400,
             )
         litellm.verbose_logger.warning(DROP_FORCED_TOOL_CHOICE_WARNING, model)
+        return True
+
+    @staticmethod
+    def _apply_forced_tool_choice(
+        model: str,
+        tool_choice: AnthropicMessagesToolChoice,
+        drop_params: bool,
+    ) -> AnthropicMessagesToolChoice:
+        if tool_choice["type"] not in ("any", "tool"):
+            return tool_choice
+        if not AnthropicModelInfo.forced_tool_use_downgraded(model, drop_params):
+            return tool_choice
         disable_parallel: Final = tool_choice.get("disable_parallel_tool_use")
         if disable_parallel is None:
             return AnthropicMessagesToolChoice(type="auto")

litellm/llms/bedrock/chat/converse_transformation.py

@@ -588,6 +588,10 @@ class AmazonConverseConfig(BaseConfig):
             supported_params.append("context_management")
         return supported_params
 
+    @staticmethod
+    def _auto_tool_choice() -> ToolChoiceValuesBlock:
+        return ToolChoiceValuesBlock(auto={})
+
     def map_tool_choice_values(
         self, model: str, tool_choice: str | dict, drop_params: bool
     ) -> ToolChoiceValuesBlock | None:
@@ -600,10 +604,14 @@ class AmazonConverseConfig(BaseConfig):
                     status_code=400,
                 )
         elif tool_choice == "required":
+            if AnthropicModelInfo.forced_tool_use_downgraded(model, drop_params):
+                return self._auto_tool_choice()
             return ToolChoiceValuesBlock(any={})
         elif tool_choice == "auto":
-            return ToolChoiceValuesBlock(auto={})
+            return self._auto_tool_choice()
         elif isinstance(tool_choice, dict):
+            if AnthropicModelInfo.forced_tool_use_downgraded(model, drop_params):
+                return self._auto_tool_choice()
             # only supported for anthropic + mistral models - https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_ToolChoice.html
             specific_tool: Final = SpecificToolChoiceBlock(
                 name=make_valid_bedrock_tool_name(tool_choice.get("function", {}).get("name", ""))

tests/test_litellm/llms/bedrock/chat/test_converse_transformation.py

@@ -6449,3 +6449,62 @@ def test_disabled_thinking_omitted_for_always_on_models_converse(
         assert "thinking" not in additional
     else:
         assert additional.get("thinking") == {"type": "disabled"}
+
+@pytest.mark.parametrize(
+    "model",
+    ["anthropic.claude-fable-5-1", "us.anthropic.claude-fable-5-1"],
+)
+@pytest.mark.parametrize(
+    "tool_choice",
+    ["required", {"type": "function", "function": {"name": "get_weather"}}],
+)
+def test_forced_tool_choice_downgraded_to_auto_on_fable_5_1_converse(
+    local_model_cost_map, model, tool_choice
+):
+    config = AmazonConverseConfig()
+
+    result = config.map_tool_choice_values(
+        model=model, tool_choice=tool_choice, drop_params=True
+    )
+
+    assert result == {"auto": {}}
+
+
+@pytest.mark.parametrize(
+    "tool_choice",
+    ["required", {"type": "function", "function": {"name": "get_weather"}}],
+)
+def test_forced_tool_choice_raises_clean_error_on_fable_5_1_converse(
+    local_model_cost_map, tool_choice, monkeypatch
+):
+    monkeypatch.setattr(litellm, "drop_params", False)
+    config = AmazonConverseConfig()
+
+    with pytest.raises(litellm.utils.UnsupportedParamsError, match="forced tool use"):
+        config.map_tool_choice_values(
+            model="anthropic.claude-fable-5-1", tool_choice=tool_choice, drop_params=False
+        )
+
+
+@pytest.mark.parametrize("tool_choice", ["auto", "none"])
+def test_unforced_tool_choice_unaffected_on_fable_5_1_converse(local_model_cost_map, tool_choice):
+    config = AmazonConverseConfig()
+
+    result = config.map_tool_choice_values(
+        model="anthropic.claude-fable-5-1", tool_choice=tool_choice, drop_params=True
+    )
+
+    assert result == ({"auto": {}} if tool_choice == "auto" else None)
+
+
+def test_forced_tool_choice_forwarded_on_converse_models_that_support_it(
+    local_model_cost_map, monkeypatch
+):
+    monkeypatch.setattr(litellm, "drop_params", False)
+    config = AmazonConverseConfig()
+
+    result = config.map_tool_choice_values(
+        model="anthropic.claude-fable-5", tool_choice="required", drop_params=False
+    )
+
+    assert result == {"any": {}}