Prevent Anthropic API errors from empty message content (#1934)

Co-authored-by: Claude <noreply@anthropic.com>

Author: mike-luabase

pydantic_ai_slim/pydantic_ai/models/anthropic.py

@@ -276,7 +276,7 @@ def _get_tools(self, model_request_parameters: ModelRequestParameters) -> list[B
             tools += [self._map_tool_definition(r) for r in model_request_parameters.output_tools]
         return tools
 
-    async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[BetaMessageParam]]:
+    async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[BetaMessageParam]]:  # noqa: C901
         """Just maps a `pydantic_ai.Message` to a `anthropic.types.MessageParam`."""
         system_prompt_parts: list[str] = []
         anthropic_messages: list[BetaMessageParam] = []
@@ -315,7 +315,8 @@ async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[Be
                 assistant_content_params: list[BetaTextBlockParam | BetaToolUseBlockParam] = []
                 for response_part in m.parts:
                     if isinstance(response_part, TextPart):
-                        assistant_content_params.append(BetaTextBlockParam(text=response_part.content, type='text'))
+                        if response_part.content:  # Only add non-empty text
+                            assistant_content_params.append(BetaTextBlockParam(text=response_part.content, type='text'))
                     else:
                         tool_use_block_param = BetaToolUseBlockParam(
                             id=_guard_tool_call_id(t=response_part),
@@ -324,7 +325,8 @@ async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[Be
                             input=response_part.args_as_dict(),
                         )
                         assistant_content_params.append(tool_use_block_param)
-                anthropic_messages.append(BetaMessageParam(role='assistant', content=assistant_content_params))
+                if len(assistant_content_params) > 0:
+                    anthropic_messages.append(BetaMessageParam(role='assistant', content=assistant_content_params))
             else:
                 assert_never(m)
         system_prompt = '\n\n'.join(system_prompt_parts)
@@ -337,11 +339,13 @@ async def _map_user_prompt(
         part: UserPromptPart,
     ) -> AsyncGenerator[BetaContentBlockParam]:
         if isinstance(part.content, str):
-            yield BetaTextBlockParam(text=part.content, type='text')
+            if part.content:  # Only yield non-empty text
+                yield BetaTextBlockParam(text=part.content, type='text')
         else:
             for item in part.content:
                 if isinstance(item, str):
-                    yield BetaTextBlockParam(text=item, type='text')
+                    if item:  # Only yield non-empty text
+                        yield BetaTextBlockParam(text=item, type='text')
                 elif isinstance(item, BinaryContent):
                     if item.is_image:
                         yield BetaImageBlockParam(

tests/models/test_anthropic.py

@@ -1063,3 +1063,53 @@ async def test_anthropic_model_empty_message_on_history(allow_model_requests: No
 
 What specifically would you like to know about potatoes?\
 """)
+
+
+async def test_anthropic_empty_content_filtering(env: TestEnv):
+    """Test the empty content filtering logic directly."""
+
+    from pydantic_ai.messages import (
+        ModelMessage,
+        ModelRequest,
+        ModelResponse,
+        SystemPromptPart,
+        TextPart,
+        UserPromptPart,
+    )
+
+    # Initialize model for all tests
+    env.set('ANTHROPIC_API_KEY', 'test-key')
+    model = AnthropicModel('claude-3-5-sonnet-latest', provider='anthropic')
+
+    # Test _map_message with empty string in user prompt
+    messages_empty_string: list[ModelMessage] = [
+        ModelRequest(parts=[UserPromptPart(content='')], kind='request'),
+    ]
+    _, anthropic_messages = await model._map_message(messages_empty_string)  # type: ignore[attr-defined]
+    assert anthropic_messages == snapshot([])  # Empty content should be filtered out
+
+    # Test _map_message with list containing empty strings in user prompt
+    messages_mixed_content: list[ModelMessage] = [
+        ModelRequest(parts=[UserPromptPart(content=['', 'Hello', '', 'World'])], kind='request'),
+    ]
+    _, anthropic_messages = await model._map_message(messages_mixed_content)  # type: ignore[attr-defined]
+    assert anthropic_messages == snapshot(
+        [{'role': 'user', 'content': [{'text': 'Hello', 'type': 'text'}, {'text': 'World', 'type': 'text'}]}]
+    )
+
+    # Test _map_message with empty assistant response
+    messages: list[ModelMessage] = [
+        ModelRequest(parts=[SystemPromptPart(content='You are helpful')], kind='request'),
+        ModelResponse(parts=[TextPart(content='')], kind='response'),  # Empty response
+        ModelRequest(parts=[UserPromptPart(content='Hello')], kind='request'),
+    ]
+    _, anthropic_messages = await model._map_message(messages)  # type: ignore[attr-defined]
+    # The empty assistant message should be filtered out
+    assert anthropic_messages == snapshot([{'role': 'user', 'content': [{'text': 'Hello', 'type': 'text'}]}])
+
+    # Test with only empty assistant parts
+    messages_resp: list[ModelMessage] = [
+        ModelResponse(parts=[TextPart(content=''), TextPart(content='')], kind='response'),
+    ]
+    _, anthropic_messages = await model._map_message(messages_resp)  # type: ignore[attr-defined]
+    assert len(anthropic_messages) == 0  # No messages should be added