feat(anthropic): add Claude Fable 5.1

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 +325,35 @@ def _apply_sampling_param(
                 status_code=400,
             )
 
+    @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
+        ``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
+        if AnthropicModelInfo._get_model_capability(model, "supports_forced_tool_use") is not False:
+            return tool_choice
+        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)
+        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/model_prices_and_context_window_backup.json

@@ -13028,6 +13028,47 @@
         "supports_native_structured_output": true,
         "source": "https://docs.anthropic.com/en/docs/about-claude/models/overview"
     },
+    "claude-fable-5-1": {
+        "deprecation_date": "2027-09-01",
+        "cache_creation_input_token_cost": 1.25e-05,
+        "cache_creation_input_token_cost_above_1hr": 2e-05,
+        "cache_read_input_token_cost": 2.5e-07,
+        "input_cost_per_token": 1e-05,
+        "litellm_provider": "anthropic",
+        "max_input_tokens": 1000000,
+        "max_output_tokens": 128000,
+        "max_tokens": 128000,
+        "mode": "chat",
+        "output_cost_per_token": 5e-05,
+        "search_context_cost_per_query": {
+            "search_context_size_high": 0.01,
+            "search_context_size_low": 0.01,
+            "search_context_size_medium": 0.01
+        },
+        "supports_adaptive_thinking": true,
+        "thinking_always_on": true,
+        "supports_mid_conversation_system": true,
+        "supports_assistant_prefill": false,
+        "supports_computer_use": true,
+        "supports_forced_tool_use": false,
+        "supports_function_calling": true,
+        "supports_pdf_input": true,
+        "supports_prompt_caching": true,
+        "supports_reasoning": true,
+        "supports_response_schema": true,
+        "supports_sampling_params": false,
+        "supports_tool_choice": true,
+        "supports_vision": true,
+        "supports_xhigh_reasoning_effort": true,
+        "supports_max_reasoning_effort": true,
+        "provider_specific_entry": {
+            "us": 1.1
+        },
+        "supports_output_config": true,
+        "prompt_cache_min_tokens": 512,
+        "supports_native_structured_output": true,
+        "source": "https://platform.claude.com/docs/en/models/fable-5-1/overview"
+    },
     "claude-opus-5": {
         "deprecation_date": "2027-07-24",
         "cache_creation_input_token_cost": 6.25e-06,

litellm/setup_wizard.py

@@ -53,11 +53,12 @@
     {
         "id": "anthropic",
         "name": "Anthropic",
-        "description": "Claude Fable 5, Opus 5, Opus 4.8, Opus 4.7, Opus 4.6, Sonnet 5, Sonnet 4.6, Haiku 4.5",
+        "description": "Claude Fable 5.1, Fable 5, Opus 5, Opus 4.8, Opus 4.7, Opus 4.6, Sonnet 5, Sonnet 4.6, Haiku 4.5",
         "env_key": "ANTHROPIC_API_KEY",
         "key_hint": "sk-ant-...",
         "test_model": "claude-haiku-4-5-20251001",
         "models": [
+            "claude-fable-5-1",
             "claude-fable-5",
             "claude-opus-5",
             "claude-sonnet-5",

model_prices_and_context_window.json

@@ -13028,6 +13028,47 @@
         "supports_native_structured_output": true,
         "source": "https://docs.anthropic.com/en/docs/about-claude/models/overview"
     },
+    "claude-fable-5-1": {
+        "deprecation_date": "2027-09-01",
+        "cache_creation_input_token_cost": 1.25e-05,
+        "cache_creation_input_token_cost_above_1hr": 2e-05,
+        "cache_read_input_token_cost": 2.5e-07,
+        "input_cost_per_token": 1e-05,
+        "litellm_provider": "anthropic",
+        "max_input_tokens": 1000000,
+        "max_output_tokens": 128000,
+        "max_tokens": 128000,
+        "mode": "chat",
+        "output_cost_per_token": 5e-05,
+        "search_context_cost_per_query": {
+            "search_context_size_high": 0.01,
+            "search_context_size_low": 0.01,
+            "search_context_size_medium": 0.01
+        },
+        "supports_adaptive_thinking": true,
+        "thinking_always_on": true,
+        "supports_mid_conversation_system": true,
+        "supports_assistant_prefill": false,
+        "supports_computer_use": true,
+        "supports_forced_tool_use": false,
+        "supports_function_calling": true,
+        "supports_pdf_input": true,
+        "supports_prompt_caching": true,
+        "supports_reasoning": true,
+        "supports_response_schema": true,
+        "supports_sampling_params": false,
+        "supports_tool_choice": true,
+        "supports_vision": true,
+        "supports_xhigh_reasoning_effort": true,
+        "supports_max_reasoning_effort": true,
+        "provider_specific_entry": {
+            "us": 1.1
+        },
+        "supports_output_config": true,
+        "prompt_cache_min_tokens": 512,
+        "supports_native_structured_output": true,
+        "source": "https://platform.claude.com/docs/en/models/fable-5-1/overview"
+    },
     "claude-opus-5": {
         "deprecation_date": "2027-07-24",
         "cache_creation_input_token_cost": 6.25e-06,

model_prices_and_context_window.schema.json

@@ -662,6 +662,9 @@
         "supports_embedding_image_input": {
           "type": "boolean"
         },
+        "supports_forced_tool_use": {
+          "type": "boolean"
+        },
         "supports_function_calling": {
           "type": "boolean"
         },

tests/llm_translation/reasoning_effort_grid/grid_spec.py

@@ -163,6 +163,13 @@ def expected(route_name: str, model: ModelEntry, effort: str) -> CellExpectation
 
 
 ANTHROPIC_DIRECT_MODELS: Tuple[ModelEntry, ...] = (
+    ModelEntry(
+        alias="claude-fable-5-1",
+        model="anthropic/claude-fable-5-1",
+        mode="adaptive",
+        required_env=_ANTHROPIC_REQ,
+        caps=_CAPS_XHIGH_MAX,
+    ),
     ModelEntry(
         alias="claude-fable-5",
         model="anthropic/claude-fable-5",

tests/test_litellm/litellm_core_utils/test_fallback_generalizations.py

@@ -366,7 +366,7 @@ def test_shipped_rules_stack_adaptive_and_mid_conversation_flags(shipped_cost_ma
 def test_shipped_rules_flag_unmapped_fable_as_always_on_thinking(shipped_cost_map):
     """An unmapped Fable/Mythos id picks up ``thinking_always_on`` from the
     claude-always-on-thinking rule, while other unmapped Claudes stay unflagged."""
-    model = "claude-fable-5-1"
+    model = "claude-fable-5-2"
     assert model not in litellm.model_cost
     info = litellm.get_model_info(model, custom_llm_provider="anthropic")
     assert info["thinking_always_on"] is True
@@ -419,7 +419,7 @@ def test_shipped_rules_cover_new_families_like_fable_at_5_plus(shipped_cost_map)
     """Both version gates accept any claude-<family>- id at major 5 or higher, bare
     major or major-minor, so a new family shaped like claude-fable-5 gets adaptive
     thinking and mid-conversation system support without a cost-map entry."""
-    model = "claude-fable-5-1"
+    model = "claude-fable-5-2"
     assert model not in litellm.model_cost
     info = litellm.get_model_info(model, custom_llm_provider="anthropic")
     assert info["supports_mid_conversation_system"] is True

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

@@ -6176,9 +6176,10 @@ def test_is_anthropic_usage_object_rejects_responses_api_usage():
     [
         # always-on-thinking models reject thinking.type=disabled with a 400
         ("claude-fable-5", True),
+        ("claude-fable-5-1", True),
         ("claude-mythos-5", True),
         # unmapped future family member -> claude-always-on-thinking fallback rule
-        ("claude-fable-5-1", True),
+        ("claude-fable-5-2", True),
         # adaptive-capable models that ACCEPT disabled must keep it verbatim
         ("claude-opus-5", False),
         ("claude-sonnet-5", False),
@@ -6207,3 +6208,99 @@ def test_disabled_thinking_omitted_only_for_always_on_models(
         assert "thinking" not in request
     else:
         assert request["thinking"] == {"type": "disabled"}
+
+
+@pytest.mark.parametrize(
+    "tool_choice",
+    ["required", {"type": "required"}, {"type": "function", "function": {"name": "get_weather"}}],
+)
+def test_forced_tool_choice_raises_clean_error_on_fable_5_1_without_drop_params(tool_choice, monkeypatch):
+    """Fable 5.1 400s on tool_choice type any/tool (thinking is always on and a
+    forced call would skip it); without drop_params the caller gets a clean
+    client-side 400 that explains the workaround, not a provider error."""
+    monkeypatch.setattr(litellm, "drop_params", False)
+    config = AnthropicConfig()
+
+    with pytest.raises(litellm.utils.UnsupportedParamsError, match="forced tool use"):
+        config.map_openai_params(
+            non_default_params={"tool_choice": tool_choice},
+            optional_params={},
+            model="claude-fable-5-1",
+            drop_params=False,
+        )
+
+
+@pytest.mark.parametrize(
+    "tool_choice",
+    ["required", {"type": "required"}, {"type": "function", "function": {"name": "get_weather"}}],
+)
+def test_forced_tool_choice_downgraded_to_auto_on_fable_5_1_with_drop_params(tool_choice):
+    config = AnthropicConfig()
+
+    result = config.map_openai_params(
+        non_default_params={"tool_choice": tool_choice},
+        optional_params={},
+        model="claude-fable-5-1",
+        drop_params=True,
+    )
+
+    assert result["tool_choice"] == {"type": "auto"}
+
+
+def test_forced_tool_choice_downgrade_keeps_parallel_tool_calls_flag():
+    config = AnthropicConfig()
+
+    result = config.map_openai_params(
+        non_default_params={"tool_choice": "required", "parallel_tool_calls": False},
+        optional_params={},
+        model="claude-fable-5-1",
+        drop_params=True,
+    )
+
+    assert result["tool_choice"] == {"type": "auto", "disable_parallel_tool_use": True}
+
+
+@pytest.mark.parametrize("tool_choice, expected_type", [("auto", "auto"), ("none", "none")])
+def test_unforced_tool_choice_forwarded_on_fable_5_1(tool_choice, expected_type, monkeypatch):
+    monkeypatch.setattr(litellm, "drop_params", False)
+    config = AnthropicConfig()
+
+    result = config.map_openai_params(
+        non_default_params={"tool_choice": tool_choice},
+        optional_params={},
+        model="claude-fable-5-1",
+        drop_params=False,
+    )
+
+    assert result["tool_choice"]["type"] == expected_type
+
+
+@pytest.mark.parametrize("model", ["claude-fable-5", "claude-opus-5", "claude-sonnet-5"])
+def test_forced_tool_choice_forwarded_on_models_that_support_it(model, monkeypatch):
+    monkeypatch.setattr(litellm, "drop_params", False)
+    config = AnthropicConfig()
+
+    result = config.map_openai_params(
+        non_default_params={"tool_choice": "required"},
+        optional_params={},
+        model=model,
+        drop_params=True,
+    )
+
+    assert result["tool_choice"] == {"type": "any"}
+
+
+def test_forced_tool_choice_gating_driven_by_model_map_flag(monkeypatch):
+    """The gate must read ``supports_forced_tool_use`` from the model map, not
+    the model name: a flagged entry gates a model whose name says nothing."""
+    monkeypatch.setitem(litellm.model_cost, "claude-zeta-9", {"supports_forced_tool_use": False})
+    config = AnthropicConfig()
+
+    result = config.map_openai_params(
+        non_default_params={"tool_choice": "required"},
+        optional_params={},
+        model="claude-zeta-9",
+        drop_params=True,
+    )
+
+    assert result["tool_choice"] == {"type": "auto"}