feat(models): add Claude Fable 5.1 across Anthropic, Bedrock, Vertex AI, and Azure AI

litellm/constants.py

@@ -1251,6 +1251,7 @@
     "openai.gpt-oss-120b-1:0",
     "anthropic.claude-haiku-4-5-20251001-v1:0",
     "anthropic.claude-sonnet-4-5-20250929-v1:0",
+    "anthropic.claude-fable-5-1",
     "anthropic.claude-fable-5",
     "anthropic.claude-sonnet-5",
     "anthropic.claude-opus-5",

litellm/llms/anthropic/chat/transformation.py

@@ -1466,7 +1466,9 @@ def map_openai_params(
                 )
 
                 if _tool_choice is not None:
-                    optional_params["tool_choice"] = _tool_choice
+                    optional_params["tool_choice"] = AnthropicConfig._apply_forced_tool_choice(
+                        model=model, tool_choice=_tool_choice, drop_params=drop_params
+                    )
             elif param == "stream" and value is True:
                 optional_params["stream"] = value
             elif param == "stop" and (isinstance(value, str) or isinstance(value, list)):

litellm/llms/anthropic/common_utils.py

@@ -28,10 +28,15 @@
     ANTHROPIC_OAUTH_TOKEN_PREFIX,
     AllAnthropicToolsValues,
     AnthropicMcpServerTool,
+    AnthropicMessagesToolChoice,
 )
 from litellm.types.llms.openai import AllMessageValues
 from litellm.types.proxy.model_listing import ModelInfoResponse
 
+DROP_FORCED_TOOL_CHOICE_WARNING: Final = (
+    "Downgrading forced tool_choice to 'auto' for model=%s (drop_params=True): this model rejects tool_choice type "
+    "'any'/'tool' with a 400 because thinking is always on and a forced call would skip it."
+)
 DROP_DISABLED_THINKING_WARNING: Final = (
     "Dropping `thinking={'type': 'disabled'}` for model=%s: thinking is always on for this model and cannot be "
     "disabled (the alternative is a provider 400). The model will still think adaptively, its response can contain "
@@ -320,6 +325,41 @@ def _apply_sampling_param(
                 status_code=400,
             )
 
+    @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:
+            return False
+        if not (litellm.drop_params or drop_params):
+            raise litellm.utils.UnsupportedParamsError(
+                message=(
+                    f"{model} does not support forced tool use (tool_choice='required' or a named tool). "
+                    "Use tool_choice='auto' and tell the model in the prompt when to call the tool, or set "
+                    "`litellm.drop_params = True` to downgrade to 'auto' automatically."
+                ),
+                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")
+        return AnthropicMessagesToolChoice(type="auto", disable_parallel_tool_use=disable_parallel)
+
     @staticmethod
     def _strip_version_suffix(model: str) -> str:
         at: Final = model.rfind("@")

litellm/llms/bedrock/chat/converse_transformation.py

@@ -588,6 +588,10 @@ def get_supported_openai_params(self, model: str) -> list[str]:
             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 @@ def map_tool_choice_values(
                     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", ""))