Add system prompts independently of history of messages

[josead]

Hello! I wanted to know how to make the @agent.system_prompt decorator being called on every run, when there is history of messages added. I'm basically trying to evaluate the dependencies on each run (the dependencies will change independently of the run and I don't want to use a tool for this because is not needed)
Here is how to reproduce:

 # where I declare the agent
 chat_agent = Agent(
        'openai:gpt-4o',
        system_prompt="Some prompt",
        deps_type=ChatAgentDeps,
    )
    
   ...
    # where I run the agent on each message from the user received
    @chat_agent.system_prompt
    def get_current_status_of_tasks(ctx: RunContext[ChatAgentDeps]):
          print("System Prompt:", ctx.deps.aa_agent.pending_tasks)
          print("System Prompt:", ctx.deps.aa_agent.done_tasks)
          return f"""Pending Action Items: {ctx.deps.aa_agent.get_pending_tasks()}
      Done Action Items: {ctx.deps.aa_agent.get_done_tasks()}"""

    result = await chat_agent.run(message, deps=ChatAgentDeps(
        aa_agent=session["agent"]
    ), message_history=session.get("history", []))
    
    session['history'] = result.all_messages()

The function get_current_status_of_tasks only gets called once.

Am I doing something wrong here? Is this expected? Thanks in advance!

[HamzaFarhan]

Yes as of now, self._sys_parts is not called if message_history is passed:

https://github.com/pydantic/pydantic-ai/blob/629495cd1083b74680858f0ce44cc8ba9e19e48a/pydantic_ai_slim/pydantic_ai/agent.py#L835C1-L847C24

async def _prepare_messages(
    self, user_prompt: str, message_history: list[_messages.ModelMessage] | None, run_context: RunContext[AgentDeps]
) -> list[_messages.ModelMessage]:
    if message_history:
        # shallow copy messages
        messages = message_history.copy()
        messages.append(_messages.ModelRequest([_messages.UserPromptPart(user_prompt)]))
    else:
        parts = await self._sys_parts(run_context)
        parts.append(_messages.UserPromptPart(user_prompt))
        messages: list[_messages.ModelMessage] = [_messages.ModelRequest(parts)]


    return messages

[josead]

Hey @HamzaFarhan, thanks for your quick answer. Is this to prevent the parts to repeat? Is there a simpler way to inject a system prompt into messages? I could do workarounds here, to accept system events to be injected as part of the conversation, do you see the Agents supporting this in the future?

Regards

[HamzaFarhan]

@josead I'm not sure actually, I did try manually calling self._sys_parts and replacing the system prompt in the message_history when passing it in and it worked.
Looks like @samuelcolvin is open to altering the logic: #496