Allow empty messages when using GoogleModel

pydantic_ai_slim/pydantic_ai/models/google.py

@@ -346,8 +346,10 @@ async def _map_messages(self, messages: list[ModelMessage]) -> tuple[ContentDict
                     else:
                         assert_never(part)
 
-                if message_parts:  # pragma: no branch
-                    contents.append({'role': 'user', 'parts': message_parts})
+                # Google GenAI requires at least one part in the message.
+                if not message_parts:
+                    message_parts = [{'text': ''}]
+                contents.append({'role': 'user', 'parts': message_parts})
             elif isinstance(m, ModelResponse):
                 contents.append(_content_model_response(m))
             else:

tests/models/cassettes/test_google/test_google_model_empty_user_prompt.yaml

@@ -0,0 +1,68 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - '*/*'
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '173'
+      content-type:
+      - application/json
+      host:
+      - generativelanguage.googleapis.com
+    method: POST
+    parsed_body:
+      contents:
+      - parts:
+        - text: ''
+        role: user
+      generationConfig: {}
+      systemInstruction:
+        parts:
+        - text: You are a helpful assistant.
+        role: user
+    uri: https://generativelanguage.googleapis.com/v1beta/models/gemini-1.5-flash:generateContent
+  response:
+    headers:
+      alt-svc:
+      - h3=":443"; ma=2592000,h3-29=":443"; ma=2592000
+      content-length:
+      - '736'
+      content-type:
+      - application/json; charset=UTF-8
+      server-timing:
+      - gfet4t7; dur=361
+      transfer-encoding:
+      - chunked
+      vary:
+      - Origin
+      - X-Origin
+      - Referer
+    parsed_body:
+      candidates:
+      - avgLogprobs: -0.20726837430681502
+        content:
+          parts:
+          - text: |
+              Please provide me with a question or task. I need some information to be able to help you.
+          role: model
+        finishReason: STOP
+      modelVersion: gemini-1.5-flash
+      responseId: y5BBaPCQELGqmecP68OBiAc
+      usageMetadata:
+        candidatesTokenCount: 21
+        candidatesTokensDetails:
+        - modality: TEXT
+          tokenCount: 21
+        promptTokenCount: 7
+        promptTokensDetails:
+        - modality: TEXT
+          tokenCount: 7
+        totalTokenCount: 28
+    status:
+      code: 200
+      message: OK
+version: 1

tests/models/test_google.py

@@ -581,3 +581,13 @@ async def test_google_model_safety_settings(allow_model_requests: None, google_p
 
     with pytest.raises(UnexpectedModelBehavior, match='Safety settings triggered'):
         await agent.run('Tell me a joke about a Brazilians.')
+
+
+async def test_google_model_empty_user_prompt(allow_model_requests: None, google_provider: GoogleProvider):
+    m = GoogleModel('gemini-1.5-flash', provider=google_provider)
+    agent = Agent(m, instructions='You are a helpful assistant.')
+
+    result = await agent.run()
+    assert result.output == snapshot(
+        'Please provide me with a question or task. I need some information to be able to help you.\n'
+    )