Add newline separation between Anthropic system prompts (#1638)

Co-authored-by: Douwe Maan <[email protected]>

Author: timesler

pydantic_ai_slim/pydantic_ai/models/anthropic.py

@@ -284,7 +284,7 @@ def _get_tools(self, model_request_parameters: ModelRequestParameters) -> list[T
 
     async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[MessageParam]]:
         """Just maps a `pydantic_ai.Message` to a `anthropic.types.MessageParam`."""
-        system_prompt: str = ''
+        system_prompt_parts: list[str] = []
         anthropic_messages: list[MessageParam] = []
         for m in messages:
             if isinstance(m, ModelRequest):
@@ -293,7 +293,7 @@ async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[Me
                 ] = []
                 for request_part in m.parts:
                     if isinstance(request_part, SystemPromptPart):
-                        system_prompt += request_part.content
+                        system_prompt_parts.append(request_part.content)
                     elif isinstance(request_part, UserPromptPart):
                         async for content in self._map_user_prompt(request_part):
                             user_content_params.append(content)
@@ -333,6 +333,7 @@ async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[Me
                 anthropic_messages.append(MessageParam(role='assistant', content=assistant_content_params))
             else:
                 assert_never(m)
+        system_prompt = '\n\n'.join(system_prompt_parts)
         if instructions := self._get_instructions(messages):
             system_prompt = f'{instructions}\n\n{system_prompt}'
         return system_prompt, anthropic_messages

tests/models/test_anthropic.py

@@ -927,6 +927,22 @@ def simple_instructions():
     )
 
 
+async def test_multiple_system_prompt_formatting(allow_model_requests: None):
+    c = completion_message([TextBlock(text='world', type='text')], AnthropicUsage(input_tokens=5, output_tokens=10))
+    mock_client = MockAnthropic().create_mock(c)
+    m = AnthropicModel('claude-3-5-haiku-latest', provider=AnthropicProvider(anthropic_client=mock_client))
+    agent = Agent(m, system_prompt='this is the system prompt')
+
+    @agent.system_prompt
+    def system_prompt() -> str:
+        return 'and this is another'
+
+    await agent.run('hello')
+    completion_kwargs = get_mock_chat_completion_kwargs(mock_client)[0]
+    assert 'system' in completion_kwargs
+    assert completion_kwargs['system'] == 'this is the system prompt\n\nand this is another'
+
+
 def anth_msg(usage: AnthropicUsage) -> AnthropicMessage:
     return AnthropicMessage(
         id='x',