Python: re-role trailing assistant message to user for Anthropic compatibility (fixes #5008)

python/packages/anthropic/agent_framework_anthropic/_chat_client.py

@@ -696,11 +696,26 @@ def _prepare_messages_for_anthropic(self, messages: Sequence[Message]) -> list[d
 
         This skips the first message if it is a system message,
         as Anthropic expects system instructions as a separate parameter.
+
+        Anthropic's API requires that the conversation ends with a user message.
+        If the last message is from the assistant, its role is changed to user
+        to satisfy this constraint.
         """
         # first system message is passed as instructions
         if messages and isinstance(messages[0], Message) and messages[0].role == "system":
-            return [self._prepare_message_for_anthropic(msg) for msg in messages[1:]]
-        return [self._prepare_message_for_anthropic(msg) for msg in messages]
+            msgs = list(messages[1:])
+        else:
+            msgs = list(messages)
+
+        result = [self._prepare_message_for_anthropic(msg) for msg in msgs]
+
+        # Anthropic requires the conversation to end with a user message.
+        # Re-role a trailing assistant message as user so chained agent
+        # outputs work as valid context for the next agent.
+        if result and result[-1].get("role") == "assistant":
+            result[-1] = {**result[-1], "role": "user"}
+
+        return result
 
     def _prepare_message_for_anthropic(self, message: Message) -> dict[str, Any]:
         """Prepare a Message for the Anthropic client.

python/packages/core/agent_framework/_tools.py

@@ -636,7 +636,7 @@ async def invoke(
                     parsed_arguments = dict(arguments)
                     if self.input_model is not None and not self._schema_supplied:
                         parsed_arguments = self.input_model.model_validate(parsed_arguments).model_dump(
-                            exclude_none=True
+                            exclude_none=False
                         )
                 elif isinstance(arguments, BaseModel):
                     if (
@@ -645,7 +645,7 @@ async def invoke(
                         and not isinstance(arguments, self.input_model)
                     ):
                         raise TypeError(f"Expected {self.input_model.__name__}, got {type(arguments).__name__}")
-                    parsed_arguments = arguments.model_dump(exclude_none=True)
+                    parsed_arguments = arguments.model_dump(exclude_none=False)
                 else:
                     raise TypeError(
                         f"Expected mapping-like arguments for tool '{self.name}', got {type(arguments).__name__}"
@@ -1492,7 +1492,7 @@ async def _auto_invoke_function(
         runtime_kwargs["session"] = invocation_session
     try:
         if not cast(bool, getattr(tool, "_schema_supplied", False)) and tool.input_model is not None:
-            args = tool.input_model.model_validate(parsed_args).model_dump(exclude_none=True)
+            args = tool.input_model.model_validate(parsed_args).model_dump(exclude_none=False)
         else:
             args = dict(parsed_args)
         args = _validate_arguments_against_schema(

python/packages/core/tests/core/test_tools.py

@@ -1,4 +1,5 @@
 # Copyright (c) Microsoft. All rights reserved.
+import asyncio
 from typing import Annotated, Any, Literal, get_args, get_origin
 from unittest.mock import Mock
 
@@ -1462,4 +1463,20 @@ def test_skip_parsing_is_singleton() -> None:
     assert repr(SKIP_PARSING) == "SKIP_PARSING"
 
 
+def test_invoke_preserves_explicit_none_arguments() -> None:
+    """Optional parameters explicitly set to None must not be stripped before invocation."""
+
+    @tool
+    def greet(name: str, greeting: str | None = None) -> str:
+        return f"{greeting or 'Hello'}, {name}!"
+
+    result = asyncio.run(greet.invoke(arguments={"name": "World", "greeting": None}))
+    assert isinstance(result, list)
+    assert result[0].text == "Hello, World!"
+
+    result = asyncio.run(greet.invoke(arguments={"name": "World"}))
+    assert isinstance(result, list)
+    assert result[0].text == "Hello, World!"
+
+
 # endregion