Make user_prompt optional

pydantic_ai_slim/pydantic_ai/_agent_graph.py

@@ -79,7 +79,7 @@ class GraphAgentDeps(Generic[DepsT, ResultDataT]):
 
     user_deps: DepsT
 
-    prompt: str | Sequence[_messages.UserContent]
+    prompt: str | Sequence[_messages.UserContent] | None
     new_message_index: int
 
     model: models.Model
@@ -124,7 +124,7 @@ def is_agent_node(
 
 @dataclasses.dataclass
 class UserPromptNode(AgentNode[DepsT, NodeRunEndT]):
-    user_prompt: str | Sequence[_messages.UserContent]
+    user_prompt: str | Sequence[_messages.UserContent] | None
 
     system_prompts: tuple[str, ...]
     system_prompt_functions: list[_system_prompt.SystemPromptRunner[DepsT]]
@@ -151,7 +151,7 @@ async def _get_first_message(
 
     async def _prepare_messages(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None,
         message_history: list[_messages.ModelMessage] | None,
         run_context: RunContext[DepsT],
     ) -> tuple[list[_messages.ModelMessage], _messages.ModelRequest]:
@@ -166,16 +166,18 @@ async def _prepare_messages(
                 messages = ctx_messages.messages
                 ctx_messages.used = True
 
+        parts: list[_messages.ModelRequestPart] = []
         if message_history:
             # Shallow copy messages
             messages.extend(message_history)
             # Reevaluate any dynamic system prompt parts
             await self._reevaluate_dynamic_prompts(messages, run_context)
-            return messages, _messages.ModelRequest([_messages.UserPromptPart(user_prompt)])
         else:
-            parts = await self._sys_parts(run_context)
+            parts.extend(await self._sys_parts(run_context))
+
+        if user_prompt is not None:
             parts.append(_messages.UserPromptPart(user_prompt))
-            return messages, _messages.ModelRequest(parts)
+        return messages, _messages.ModelRequest(parts)
 
     async def _reevaluate_dynamic_prompts(
         self, messages: list[_messages.ModelMessage], run_context: RunContext[DepsT]

pydantic_ai_slim/pydantic_ai/agent.py

@@ -242,7 +242,7 @@ def instrument_all(instrument: InstrumentationSettings | bool = True) -> None:
     @overload
     async def run(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         result_type: None = None,
         message_history: list[_messages.ModelMessage] | None = None,
@@ -257,7 +257,7 @@ async def run(
     @overload
     async def run(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         result_type: type[RunResultDataT],
         message_history: list[_messages.ModelMessage] | None = None,
@@ -271,7 +271,7 @@ async def run(
 
     async def run(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         result_type: type[RunResultDataT] | None = None,
         message_history: list[_messages.ModelMessage] | None = None,
@@ -335,7 +335,7 @@ async def main():
     @asynccontextmanager
     async def iter(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         result_type: type[RunResultDataT] | None = None,
         message_history: list[_messages.ModelMessage] | None = None,
@@ -503,7 +503,7 @@ async def main():
     @overload
     def run_sync(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         message_history: list[_messages.ModelMessage] | None = None,
         model: models.Model | models.KnownModelName | str | None = None,
@@ -517,7 +517,7 @@ def run_sync(
     @overload
     def run_sync(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         result_type: type[RunResultDataT] | None,
         message_history: list[_messages.ModelMessage] | None = None,
@@ -531,7 +531,7 @@ def run_sync(
 
     def run_sync(
         self,
-        user_prompt: str | Sequence[_messages.UserContent],
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         result_type: type[RunResultDataT] | None = None,
         message_history: list[_messages.ModelMessage] | None = None,

pydantic_ai_slim/pydantic_ai/tools.py

@@ -48,7 +48,7 @@ class RunContext(Generic[AgentDepsT]):
     """The model used in this run."""
     usage: Usage
     """LLM usage associated with the run."""
-    prompt: str | Sequence[_messages.UserContent]
+    prompt: str | Sequence[_messages.UserContent] | None
     """The original user prompt passed to the run."""
     messages: list[_messages.ModelMessage] = field(default_factory=list)
     """Messages exchanged in the conversation so far."""

tests/models/cassettes/test_openai/test_openai_model_without_system_prompt.yaml

@@ -0,0 +1,78 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '101'
+      content-type:
+      - application/json
+      host:
+      - api.openai.com
+    method: POST
+    parsed_body:
+      messages:
+      - content: You are a potato.
+        role: system
+      model: o3-mini
+      n: 1
+      stream: false
+    uri: https://api.openai.com/v1/chat/completions
+  response:
+    headers:
+      access-control-expose-headers:
+      - X-Request-ID
+      alt-svc:
+      - h3=":443"; ma=86400
+      connection:
+      - keep-alive
+      content-length:
+      - '906'
+      content-type:
+      - application/json
+      openai-organization:
+      - pydantic-28gund
+      openai-processing-ms:
+      - '8045'
+      openai-version:
+      - '2020-10-01'
+      strict-transport-security:
+      - max-age=31536000; includeSubDomains; preload
+      transfer-encoding:
+      - chunked
+    parsed_body:
+      choices:
+      - finish_reason: stop
+        index: 0
+        message:
+          annotations: []
+          content: That's right—I am a potato! A spud of many talents, here to help you out. How can this humble potato be
+            of service today?
+          refusal: null
+          role: assistant
+      created: 1744099208
+      id: chatcmpl-BJyAKqCjJI3mIdQmTSW6UlG6NKpjm
+      model: o3-mini-2025-01-31
+      object: chat.completion
+      service_tier: default
+      system_fingerprint: fp_617f206dd9
+      usage:
+        completion_tokens: 809
+        completion_tokens_details:
+          accepted_prediction_tokens: 0
+          audio_tokens: 0
+          reasoning_tokens: 768
+          rejected_prediction_tokens: 0
+        prompt_tokens: 11
+        prompt_tokens_details:
+          audio_tokens: 0
+          cached_tokens: 0
+        total_tokens: 820
+    status:
+      code: 200
+      message: OK
+version: 1

tests/models/test_openai.py

@@ -1204,3 +1204,13 @@ class MyModel(BaseModel):
             '$ref': '#/$defs/MyModel',
         }
     )
+
+
+@pytest.mark.vcr
+async def test_openai_model_without_system_prompt(allow_model_requests: None, openai_api_key: str):
+    m = OpenAIModel('o3-mini', provider=OpenAIProvider(api_key=openai_api_key))
+    agent = Agent(m, system_prompt='You are a potato.')
+    result = await agent.run()
+    assert result.data == snapshot(
+        "That's right—I am a potato! A spud of many talents, here to help you out. How can this humble potato be of service today?"
+    )