Don't send empty messages to Anthropic (#1027)

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

Author: oscar-broman

pydantic_ai_slim/pydantic_ai/models/anthropic.py

@@ -373,7 +373,8 @@ async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[Be
                                 is_error=True,
                             )
                         user_content_params.append(retry_param)
-                anthropic_messages.append(BetaMessageParam(role='user', content=user_content_params))
+                if len(user_content_params) > 0:
+                    anthropic_messages.append(BetaMessageParam(role='user', content=user_content_params))
             elif isinstance(m, ModelResponse):
                 assistant_content_params: list[
                     BetaTextBlockParam

tests/models/cassettes/test_anthropic/test_anthropic_model_empty_message_on_history.yaml

@@ -0,0 +1,73 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '281'
+      content-type:
+      - application/json
+      host:
+      - api.anthropic.com
+    method: POST
+    parsed_body:
+      max_tokens: 1024
+      messages:
+      - content:
+        - text: Hello, how can I help you?
+          type: text
+        role: assistant
+      - content:
+        - text: I need a potato!
+          type: text
+        role: user
+      model: claude-3-5-sonnet-latest
+      stream: false
+      system: |+
+        You are a helpful assistant.
+
+    uri: https://api.anthropic.com/v1/messages?beta=true
+  response:
+    headers:
+      connection:
+      - keep-alive
+      content-length:
+      - '671'
+      content-type:
+      - application/json
+      strict-transport-security:
+      - max-age=31536000; includeSubDomains; preload
+      transfer-encoding:
+      - chunked
+    parsed_body:
+      content:
+      - text: |-
+          I can't physically give you a potato since I'm a computer program. However, I can:
+
+          1. Help you find recipes that use potatoes
+          2. Give you tips on how to select, store, or cook potatoes
+          3. Share information about different potato varieties
+          4. Provide guidance on growing potatoes
+
+          What specifically would you like to know about potatoes?
+        type: text
+      id: msg_01UjnDmX3B57Drosu49sMteT
+      model: claude-3-5-sonnet-20241022
+      role: assistant
+      stop_reason: end_turn
+      stop_sequence: null
+      type: message
+      usage:
+        cache_creation_input_tokens: 0
+        cache_read_input_tokens: 0
+        input_tokens: 41
+        output_tokens: 82
+        service_tier: standard
+    status:
+      code: 200
+      message: OK
+version: 1

tests/models/test_anthropic.py

@@ -1042,6 +1042,34 @@ def test_usage(message_callback: Callable[[], BetaMessage | BetaRawMessageStream
     assert _map_usage(message_callback()) == usage
 
 
+@pytest.mark.vcr()
+async def test_anthropic_model_empty_message_on_history(allow_model_requests: None, anthropic_api_key: str):
+    """The Anthropic API will error if you send an empty message on the history.
+
+    Check <https://github.com/pydantic/pydantic-ai/pull/1027> for more details.
+    """
+    m = AnthropicModel('claude-3-5-sonnet-latest', provider=AnthropicProvider(api_key=anthropic_api_key))
+    agent = Agent(m, instructions='You are a helpful assistant.')
+
+    result = await agent.run(
+        'I need a potato!',
+        message_history=[
+            ModelRequest(parts=[], instructions='You are a helpful assistant.', kind='request'),
+            ModelResponse(parts=[TextPart(content='Hello, how can I help you?')], kind='response'),
+        ],
+    )
+    assert result.output == snapshot("""\
+I can't physically give you a potato since I'm a computer program. However, I can:
+
+1. Help you find recipes that use potatoes
+2. Give you tips on how to select, store, or cook potatoes
+3. Share information about different potato varieties
+4. Provide guidance on growing potatoes
+
+What specifically would you like to know about potatoes?\
+""")
+
+
 @pytest.mark.vcr()
 async def test_anthropic_web_search_tool(allow_model_requests: None, anthropic_api_key: str):
     m = AnthropicModel('claude-3-5-sonnet-latest', provider=AnthropicProvider(api_key=anthropic_api_key))