fix(anthropic): drop forced tool_choice when thinking is enabled (#35544)

libs/partners/anthropic/langchain_anthropic/chat_models.py

@@ -1567,6 +1567,26 @@ class ChatAnthropic(BaseChatModel):
                 msg,
             )
 
+        # Anthropic API rejects forced tool use when thinking is enabled:
+        # "Thinking may not be enabled when tool_choice forces tool use."
+        # Drop forced tool_choice and warn, matching the behavior in
+        # _get_llm_for_structured_output_when_thinking_is_enabled.
+        if (
+            self.thinking is not None
+            and self.thinking.get("type") == "enabled"
+            and "tool_choice" in kwargs
+            and kwargs["tool_choice"].get("type") in ("any", "tool")
+        ):
+            warnings.warn(
+                "tool_choice is forced but thinking is enabled. The Anthropic "
+                "API does not support forced tool use with thinking. "
+                "Dropping tool_choice to avoid an API error. Tool calls are "
+                "not guaranteed. Consider disabling thinking or adjusting "
+                "your prompt to ensure the tool is called.",
+                stacklevel=2,
+            )
+            del kwargs["tool_choice"]
+
         if parallel_tool_calls is not None:
             disable_parallel_tool_use = not parallel_tool_calls
             if "tool_choice" in kwargs:

libs/partners/anthropic/tests/unit_tests/test_chat_models.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import copy
 import os
+import warnings
 from collections.abc import Callable
 from typing import Any, Literal, cast
 from unittest.mock import MagicMock, patch
@@ -2520,3 +2521,89 @@ def test_context_overflow_error_backwards_compatibility() -> None:
     # Verify it's both types (multiple inheritance)
     assert isinstance(exc_info.value, anthropic.BadRequestError)
     assert isinstance(exc_info.value, ContextOverflowError)
+
+
+def test_bind_tools_drops_forced_tool_choice_when_thinking_enabled() -> None:
+    """Regression test for https://github.com/langchain-ai/langchain/issues/35539.
+
+    Anthropic API rejects forced tool_choice when thinking is enabled:
+    "Thinking may not be enabled when tool_choice forces tool use."
+    bind_tools should drop forced tool_choice and warn.
+    """
+    chat_model = ChatAnthropic(
+        model=MODEL_NAME,
+        anthropic_api_key="secret-api-key",
+        thinking={"type": "enabled", "budget_tokens": 5000},
+    )
+
+    # tool_choice="any" should be dropped with warning
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        result = chat_model.bind_tools([GetWeather], tool_choice="any")
+    assert "tool_choice" not in cast("RunnableBinding", result).kwargs
+    assert len(w) == 1
+    assert "thinking is enabled" in str(w[0].message)
+
+    # tool_choice="auto" should NOT be dropped (auto is allowed)
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        result = chat_model.bind_tools([GetWeather], tool_choice="auto")
+    assert cast("RunnableBinding", result).kwargs["tool_choice"] == {"type": "auto"}
+    assert len(w) == 0
+
+    # tool_choice=specific tool name should be dropped with warning
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        result = chat_model.bind_tools([GetWeather], tool_choice="GetWeather")
+    assert "tool_choice" not in cast("RunnableBinding", result).kwargs
+    assert len(w) == 1
+
+    # tool_choice=dict with type "tool" should be dropped with warning
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        result = chat_model.bind_tools(
+            [GetWeather],
+            tool_choice={"type": "tool", "name": "GetWeather"},
+        )
+    assert "tool_choice" not in cast("RunnableBinding", result).kwargs
+    assert len(w) == 1
+
+    # tool_choice=dict with type "any" should also be dropped
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        result = chat_model.bind_tools(
+            [GetWeather],
+            tool_choice={"type": "any"},
+        )
+    assert "tool_choice" not in cast("RunnableBinding", result).kwargs
+    assert len(w) == 1
+
+
+def test_bind_tools_keeps_forced_tool_choice_when_thinking_disabled() -> None:
+    """When thinking is not enabled, forced tool_choice should pass through."""
+    chat_model = ChatAnthropic(
+        model=MODEL_NAME,
+        anthropic_api_key="secret-api-key",
+    )
+
+    # No thinking — tool_choice="any" should pass through
+    result = chat_model.bind_tools([GetWeather], tool_choice="any")
+    assert cast("RunnableBinding", result).kwargs["tool_choice"] == {"type": "any"}
+
+    # Thinking explicitly None
+    chat_model_none = ChatAnthropic(
+        model=MODEL_NAME,
+        anthropic_api_key="secret-api-key",
+        thinking=None,
+    )
+    result = chat_model_none.bind_tools([GetWeather], tool_choice="any")
+    assert cast("RunnableBinding", result).kwargs["tool_choice"] == {"type": "any"}
+
+    # Thinking explicitly disabled — should NOT drop tool_choice
+    chat_model_disabled = ChatAnthropic(
+        model=MODEL_NAME,
+        anthropic_api_key="secret-api-key",
+        thinking={"type": "disabled"},
+    )
+    result = chat_model_disabled.bind_tools([GetWeather], tool_choice="any")
+    assert cast("RunnableBinding", result).kwargs["tool_choice"] == {"type": "any"}