Python: fix(anthropic): harden tool-choice reset and tool-call argument serialization

Fixes #9853

- Keep Anthropic tool definitions and set tool_choice to {"type":"none"} when auto-invoke retries are exhausted
- Add explicit Anthropic tool_choice validation with clear allowed values and invalid-type guard
- Serialize ToolUseBlock input with an is-not-None check so empty dict arguments are preserved as "{}"
- Add regression tests for exhausted auto-invoke payload behavior, empty-argument serialization, and invalid tool_choice handling

Author: AdithyaReddyGeeda

python/semantic_kernel/connectors/ai/anthropic/prompt_execution_settings/anthropic_prompt_execution_settings.py

@@ -5,7 +5,9 @@
 
 from pydantic import Field, model_validator
 
+from semantic_kernel.connectors.ai.function_choice_type import FunctionChoiceType
 from semantic_kernel.connectors.ai.prompt_execution_settings import PromptExecutionSettings
+from semantic_kernel.exceptions import ServiceInvalidExecutionSettingsError
 
 logger = logging.getLogger(__name__)
 
@@ -44,5 +46,25 @@ class AnthropicChatPromptExecutionSettings(AnthropicPromptExecutionSettings):
 
     @model_validator(mode="after")
     def validate_tool_choice(self) -> "AnthropicChatPromptExecutionSettings":
-        """Validate tool choice payload."""
+        """Validate tool choice payload.
+
+        Anthropic supports disabling tool calls by setting {"type": "none"},
+        which is used when auto-invocation attempts are exhausted.
+        """
+        if self.tool_choice is None:
+            return self
+
+        allowed_tool_choice_types = {
+            FunctionChoiceType.AUTO.value,
+            FunctionChoiceType.NONE.value,
+            FunctionChoiceType.REQUIRED.value,
+            "any",
+            "tool",
+        }
+        tool_choice_type = self.tool_choice.get("type")
+        if tool_choice_type not in allowed_tool_choice_types:
+            raise ServiceInvalidExecutionSettingsError(
+                f"Invalid Anthropic tool_choice type '{tool_choice_type}'. "
+                f"Expected one of: {sorted(allowed_tool_choice_types)}."
+            )
         return self

python/semantic_kernel/connectors/ai/anthropic/services/anthropic_chat_completion.py

@@ -144,6 +144,8 @@ def _update_function_choice_settings_callback(
     @override
     def _reset_function_choice_settings(self, settings: "PromptExecutionSettings") -> None:
         if hasattr(settings, "tool_choice") and getattr(settings, "tools", None):
+            # Anthropic supports disabling tool calls while keeping tool definitions:
+            # https://docs.anthropic.com/en/api/messages#body-tool_choice
             settings.tool_choice = {"type": FunctionChoiceType.NONE.value}
 
     @override
@@ -302,7 +304,7 @@ def _create_streaming_chat_message_content(
                     id=tool_use_block.id,
                     index=stream_event.index,
                     name=tool_use_block.name,
-                    arguments=json.dumps(tool_use_block.input) if tool_use_block.input else None,
+                    arguments=json.dumps(tool_use_block.input) if tool_use_block.input is not None else None,
                 )
             )
         elif isinstance(stream_event, RawMessageDeltaEvent):

python/tests/unit/connectors/ai/anthropic/services/test_anthropic_chat_completion.py

@@ -645,3 +645,33 @@ def test_get_tool_calls_from_message_serializes_arguments(
 
     assert len(tool_calls) == 1
     assert tool_calls[0].arguments == '{"input": 3, "amount": 3}'
+
+
+def test_get_tool_calls_from_message_serializes_empty_arguments_dict(
+    mock_anthropic_client_completion: MagicMock,
+):
+    chat_completion = AnthropicChatCompletion(
+        ai_model_id="test_model_id", service_id="test", api_key="", async_client=mock_anthropic_client_completion
+    )
+    message = Message(
+        id="test_message_id",
+        content=[
+            ToolUseBlock(
+                id="test_tool_use_block_id",
+                input={},
+                name="math-Add",
+                type="tool_use",
+            ),
+        ],
+        model="claude-3-opus-20240229",
+        role="assistant",
+        stop_reason="tool_use",
+        stop_sequence=None,
+        type="message",
+        usage=Usage(input_tokens=100, output_tokens=100),
+    )
+
+    tool_calls = chat_completion._get_tool_calls_from_message(message)
+
+    assert len(tool_calls) == 1
+    assert tool_calls[0].arguments == "{}"

python/tests/unit/connectors/ai/anthropic/test_anthropic_request_settings.py

@@ -1,10 +1,13 @@
 # Copyright (c) Microsoft. All rights reserved.
 
+import pytest
+
 from semantic_kernel.connectors.ai.anthropic.prompt_execution_settings.anthropic_prompt_execution_settings import (
     AnthropicChatPromptExecutionSettings,
 )
 from semantic_kernel.connectors.ai.function_choice_behavior import FunctionChoiceBehavior
 from semantic_kernel.connectors.ai.prompt_execution_settings import PromptExecutionSettings
+from semantic_kernel.exceptions import ServiceInvalidExecutionSettingsError
 
 
 def test_default_anthropic_chat_prompt_execution_settings():
@@ -124,3 +127,14 @@ def test_tool_choice_none():
         function_choice_behavior=FunctionChoiceBehavior.NoneInvoke(),
     )
     assert settings.tool_choice == {"type": "none"}
+
+
+def test_tool_choice_invalid_type_raises():
+    with pytest.raises(ServiceInvalidExecutionSettingsError, match="Invalid Anthropic tool_choice type"):
+        AnthropicChatPromptExecutionSettings(
+            service_id="test_service",
+            extension_data={
+                "tool_choice": {"type": "invalid"},
+                "messages": [{"role": "system", "content": "Hello"}],
+            },
+        )