Fix: drop trailing assistant message when rstrip leaves it empty

python/packages/autogen-ext/src/autogen_ext/models/_utils/rstrip_last_assistant_message.py

@@ -0,0 +1,21 @@
+from typing import Sequence
+
+from autogen_core.models import AssistantMessage, LLMMessage
+
+
+def rstrip_last_assistant_message(messages: Sequence[LLMMessage]) -> Sequence[LLMMessage]:
+    """
+    Remove trailing whitespace from the last assistant message, dropping the
+    message entirely if that leaves it empty. Some providers (e.g. Anthropic)
+    reject text content blocks that are empty or end in whitespace.
+
+    Only the trailing message is affected; earlier messages in the sequence
+    are left untouched even if they are also `AssistantMessage`s.
+    """
+    if messages and isinstance(messages[-1], AssistantMessage):
+        if isinstance(messages[-1].content, str):
+            messages[-1].content = messages[-1].content.rstrip()
+            if messages[-1].content == "":
+                messages = messages[:-1]
+
+    return messages

python/packages/autogen-ext/src/autogen_ext/models/anthropic/_anthropic_client.py

@@ -65,6 +65,7 @@
 from pydantic import BaseModel, SecretStr
 from typing_extensions import Self, Unpack
 
+from .._utils.rstrip_last_assistant_message import rstrip_last_assistant_message
 from . import _model_info
 from .config import (
     AnthropicBedrockClientConfiguration,
@@ -538,17 +539,6 @@ def _get_thinking_config(self, extra_create_args: Mapping[str, Any]) -> Dict[str
 
         return {"thinking": thinking_config}
 
-    def _rstrip_last_assistant_message(self, messages: Sequence[LLMMessage]) -> Sequence[LLMMessage]:
-        """
-        Remove the last assistant message if it is empty.
-        """
-        # When Claude models last message is AssistantMessage, It could not end with whitespace
-        if isinstance(messages[-1], AssistantMessage):
-            if isinstance(messages[-1].content, str):
-                messages[-1].content = messages[-1].content.rstrip()
-
-        return messages
-
     async def create(
         self,
         messages: Sequence[LLMMessage],
@@ -586,7 +576,7 @@ async def create(
 
         # Merge continuous system messages into a single message
         messages = self._merge_system_messages(messages)
-        messages = self._rstrip_last_assistant_message(messages)
+        messages = rstrip_last_assistant_message(messages)
 
         for message in messages:
             if isinstance(message, SystemMessage):
@@ -804,7 +794,7 @@ async def create_stream(
 
         # Merge continuous system messages into a single message
         messages = self._merge_system_messages(messages)
-        messages = self._rstrip_last_assistant_message(messages)
+        messages = rstrip_last_assistant_message(messages)
 
         for message in messages:
             if isinstance(message, SystemMessage):

python/packages/autogen-ext/src/autogen_ext/models/openai/_openai_client.py

@@ -75,6 +75,7 @@
 
 from .._utils.normalize_stop_reason import normalize_stop_reason
 from .._utils.parse_r1_content import parse_r1_content
+from .._utils.rstrip_last_assistant_message import rstrip_last_assistant_message
 from . import _model_info
 from ._transformation import (
     get_transformer,
@@ -487,17 +488,6 @@ def __init__(
     def create_from_config(cls, config: Dict[str, Any]) -> ChatCompletionClient:
         return OpenAIChatCompletionClient(**config)
 
-    def _rstrip_last_assistant_message(self, messages: Sequence[LLMMessage]) -> Sequence[LLMMessage]:
-        """
-        Remove the last assistant message if it is empty.
-        """
-        # When Claude models last message is AssistantMessage, It could not end with whitespace
-        if isinstance(messages[-1], AssistantMessage):
-            if isinstance(messages[-1].content, str):
-                messages[-1].content = messages[-1].content.rstrip()
-
-        return messages
-
     def _process_create_args(
         self,
         messages: Sequence[LLMMessage],
@@ -610,7 +600,7 @@ def _process_create_args(
         # in that case, for ad-hoc, we using startswith instead of model_family for code consistency
         if create_args.get("model", "unknown").startswith("claude-"):
             # When Claude models last message is AssistantMessage, It could not end with whitespace
-            messages = self._rstrip_last_assistant_message(messages)
+            messages = rstrip_last_assistant_message(messages)
 
         oai_messages_nested = [
             to_oai_type(

python/packages/autogen-ext/tests/models/test_anthropic_model_client.py

@@ -18,6 +18,7 @@
 )
 from autogen_core.models._types import LLMMessage
 from autogen_core.tools import FunctionTool
+from autogen_ext.models._utils.rstrip_last_assistant_message import rstrip_last_assistant_message
 from autogen_ext.models.anthropic import (
     AnthropicBedrockChatCompletionClient,
     AnthropicChatCompletionClient,
@@ -828,14 +829,48 @@ def test_mock_rstrip_trailing_whitespace_at_last_assistant_content() -> None:
         AssistantMessage(content="foobar ", source="assistant"),
     ]
 
-    # This will crash if _rstrip_railing_whitespace_at_last_assistant_content is not applied to "content"
-    dummy_client = AnthropicChatCompletionClient(model="claude-3-5-haiku-20241022", api_key="dummy-key")
-    result = dummy_client._rstrip_last_assistant_message(messages)  # pyright: ignore[reportPrivateUsage]
+    # This will crash if rstrip_last_assistant_message is not applied to "content"
+    result = rstrip_last_assistant_message(messages)
 
     assert isinstance(result[-1].content, str)
     assert result[-1].content == "foobar"
 
 
+def test_mock_rstrip_removes_whitespace_only_last_assistant_message() -> None:
+    """If the last assistant message is whitespace-only, rstrip leaves an empty
+    string, which Anthropic's API rejects (text content blocks must be non-empty).
+    The message should be dropped entirely instead."""
+    messages: list[LLMMessage] = [
+        UserMessage(content="foo", source="user"),
+        UserMessage(content="bar", source="user"),
+        AssistantMessage(content="   ", source="assistant"),
+    ]
+
+    result = rstrip_last_assistant_message(messages)
+
+    assert len(result) == 2
+    assert isinstance(result[-1], UserMessage)
+
+
+def test_mock_rstrip_only_affects_trailing_assistant_message() -> None:
+    """An interleaved (non-trailing) assistant message with trailing whitespace
+    must be left untouched, even if it would otherwise be empty after rstrip."""
+    messages: list[LLMMessage] = [
+        UserMessage(content="foo", source="user"),
+        AssistantMessage(content="   ", source="assistant"),
+        UserMessage(content="bar", source="user"),
+        AssistantMessage(content="baz ", source="assistant"),
+    ]
+
+    result = rstrip_last_assistant_message(messages)
+
+    assert len(result) == 4
+    assert isinstance(result[1], AssistantMessage)
+    assert result[1].content == "   "
+    assert isinstance(result[-1], AssistantMessage)
+    assert result[-1].content == "baz"
+
+
 @pytest.mark.asyncio
 async def test_anthropic_tool_choice_with_actual_api() -> None:
     """Test tool_choice parameter with actual Anthropic API endpoints."""

python/packages/autogen-ext/tests/models/test_openai_model_client.py

@@ -23,6 +23,7 @@
 )
 from autogen_core.models._model_client import ModelFamily
 from autogen_core.tools import BaseTool, FunctionTool
+from autogen_ext.models._utils.rstrip_last_assistant_message import rstrip_last_assistant_message
 from autogen_ext.models.openai import AzureOpenAIChatCompletionClient, OpenAIChatCompletionClient
 from autogen_ext.models.openai._model_info import resolve_model
 from autogen_ext.models.openai._openai_client import (
@@ -2635,14 +2636,29 @@ def test_rstrip_railing_whitespace_at_last_assistant_content() -> None:
         AssistantMessage(content="foobar ", source="assistant"),
     ]
 
-    # This will crash if _rstrip_railing_whitespace_at_last_assistant_content is not applied to "content"
-    dummy_client = OpenAIChatCompletionClient(model="claude-3-5-haiku-20241022", api_key="dummy-key")
-    result = dummy_client._rstrip_last_assistant_message(messages)  # pyright: ignore[reportPrivateUsage]
+    # This will crash if rstrip_last_assistant_message is not applied to "content"
+    result = rstrip_last_assistant_message(messages)
 
     assert isinstance(result[-1].content, str)
     assert result[-1].content == "foobar"
 
 
+def test_rstrip_removes_whitespace_only_last_assistant_message() -> None:
+    """If the last assistant message is whitespace-only, rstrip leaves an empty
+    string, which Anthropic's API rejects (text content blocks must be non-empty).
+    The message should be dropped entirely instead."""
+    messages: list[LLMMessage] = [
+        UserMessage(content="foo", source="user"),
+        UserMessage(content="bar", source="user"),
+        AssistantMessage(content="   ", source="assistant"),
+    ]
+
+    result = rstrip_last_assistant_message(messages)
+
+    assert len(result) == 2
+    assert isinstance(result[-1], UserMessage)
+
+
 def test_find_model_family() -> None:
     assert _find_model_family("openai", "gpt-4") == ModelFamily.GPT_4
     assert _find_model_family("openai", "gpt-4-latest") == ModelFamily.GPT_4