Python: Honor allow_multiple_tool_calls in AnthropicClient when tool_choice is unset

python/packages/anthropic/agent_framework_anthropic/_chat_client.py

@@ -1144,14 +1144,18 @@ def _prepare_tools_for_anthropic(
             request_state.tool_name_aliases = {}
 
         # Process tool choice
+        allow_multiple = options.get("allow_multiple_tool_calls")
         if options.get("tool_choice") is None:
+            # Anthropic only exposes parallel tool use through tool_choice, so carry the setting
+            # on its default "auto" mode (e.g. after the function loop resets tool_choice="required").
+            if allow_multiple is not None and result.get("tools"):
+                result["tool_choice"] = {"type": "auto", "disable_parallel_tool_use": not allow_multiple}
             return result or None
         tool_mode = validate_tool_mode(options.get("tool_choice"))
         if tool_mode is None:
             return result or None
         if "allowed_tools" in tool_mode:
             logger.warning("allowed_tools is not supported by Anthropic; the setting will be ignored")
-        allow_multiple = options.get("allow_multiple_tool_calls")
         match tool_mode.get("mode"):
             case "auto":
                 tool_choice: dict[str, Any] = {"type": "auto"}

python/packages/anthropic/tests/test_anthropic_client.py

@@ -1417,6 +1417,82 @@ async def test_prepare_options_with_tool_choice_auto(
     assert "allow_multiple_tool_calls" not in run_options
 
 
+async def test_prepare_options_allow_multiple_tool_calls_without_tool_choice(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """allow_multiple_tool_calls=False must disable parallel tool use even when tool_choice is unset."""
+    client = create_test_anthropic_client(mock_anthropic_client)
+
+    @tool(approval_mode="never_require")
+    def get_weather(city: str) -> str:
+        """Get the weather."""
+        return f"sunny in {city}"
+
+    messages = [Message(role="user", contents=["Weather in Paris and Rome?"])]
+    run_options = client._prepare_options(messages, {"tools": [get_weather], "allow_multiple_tool_calls": False})
+
+    assert run_options["tool_choice"]["type"] == "auto"
+    assert run_options["tool_choice"]["disable_parallel_tool_use"] is True
+
+
+async def test_function_loop_keeps_parallel_tool_use_disabled_after_required_tool_choice(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """After the loop resets tool_choice='required', later requests must still disable parallel tool use."""
+    client = create_test_anthropic_client(mock_anthropic_client)
+
+    @tool(approval_mode="never_require")
+    def get_weather(city: str) -> str:
+        """Get the weather."""
+        return f"sunny in {city}"
+
+    usage = BetaUsage(input_tokens=10, output_tokens=5)
+    mock_anthropic_client.beta.messages.create.side_effect = [
+        BetaMessage(
+            id="msg_1",
+            type="message",
+            role="assistant",
+            model="claude-test",
+            content=[BetaToolUseBlock(type="tool_use", id="toolu_1", name="get_weather", input={"city": "Paris"})],
+            stop_reason="tool_use",
+            usage=usage,
+        ),
+        BetaMessage(
+            id="msg_2",
+            type="message",
+            role="assistant",
+            model="claude-test",
+            content=[BetaToolUseBlock(type="tool_use", id="toolu_2", name="get_weather", input={"city": "Rome"})],
+            stop_reason="tool_use",
+            usage=usage,
+        ),
+        BetaMessage(
+            id="msg_3",
+            type="message",
+            role="assistant",
+            model="claude-test",
+            content=[BetaTextBlock(type="text", text="Sunny in both.")],
+            stop_reason="end_turn",
+            usage=usage,
+        ),
+    ]
+
+    await client.get_response(
+        [Message(role="user", contents=["Weather in Paris and Rome?"])],
+        options={"tools": [get_weather], "tool_choice": "required", "allow_multiple_tool_calls": False},
+    )
+
+    calls = mock_anthropic_client.beta.messages.create.call_args_list
+    assert len(calls) == 3
+    assert calls[0].kwargs["tool_choice"]["type"] == "any"
+    assert calls[0].kwargs["tool_choice"]["disable_parallel_tool_use"] is True
+    for call in calls[1:]:
+        tool_choice = call.kwargs.get("tool_choice")
+        assert tool_choice is not None
+        assert tool_choice["type"] == "auto"
+        assert tool_choice["disable_parallel_tool_use"] is True
+
+
 async def test_prepare_options_with_tool_choice_required(
     mock_anthropic_client: MagicMock,
 ) -> None: