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

python/packages/anthropic/agent_framework_anthropic/_chat_client.py

@@ -696,11 +696,34 @@ 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, a synthetic user turn is
+        appended when it will not break Anthropic tool_use/tool_result pairing.
         """
         # 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.
+        # Append a synthetic user turn so chained agent outputs work as
+        # valid context for the next agent without rewriting the assistant message.
+        if result and result[-1].get("role") == "assistant" and not self._message_has_tool_use(result[-1]):
+            result.append({"role": "user", "content": "Continue"})
+
+        return result
+
+    def _message_has_tool_use(self, message: dict[str, Any]) -> bool:
+        """Return whether an Anthropic message contains tool_use blocks."""
+        content = message.get("content")
+        return isinstance(content, list) and any(
+            isinstance(item, dict) and item.get("type") in {"tool_use", "mcp_tool_use", "server_tool_use"}
+            for item in content
+        )
 
     def _prepare_message_for_anthropic(self, message: Message) -> dict[str, Any]:
         """Prepare a Message for the Anthropic client.

python/packages/anthropic/tests/test_anthropic_client.py

@@ -618,9 +618,37 @@ def test_prepare_messages_for_anthropic_without_system(
 
     result = client._prepare_messages_for_anthropic(messages)
 
-    assert len(result) == 2
+    assert len(result) == 3
     assert result[0]["role"] == "user"
     assert result[1]["role"] == "assistant"
+    assert result[2]["role"] == "user"
+    assert result[2]["content"] == "Continue"
+
+
+def test_prepare_messages_for_anthropic_does_not_append_after_tool_use(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """Do not append plain user text after assistant tool_use blocks."""
+    client = create_test_anthropic_client(mock_anthropic_client)
+    messages = [
+        Message(role="user", contents=["What's the weather?"]),
+        Message(
+            role="assistant",
+            contents=[
+                Content.from_function_call(
+                    call_id="call_123",
+                    name="get_weather",
+                    arguments={"location": "Seattle"},
+                )
+            ],
+        ),
+    ]
+
+    result = client._prepare_messages_for_anthropic(messages)
+
+    assert len(result) == 2
+    assert result[1]["role"] == "assistant"
+    assert result[1]["content"][0]["type"] == "tool_use"
 
 
 # Tool Conversion Tests