How can I add lifecycle hooks or callbacks to a PydanticAI Agent?

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Summary

Adding lifecycle hooks or callbacks to a PydanticAI Agent requires overriding the run methods or leveraging event handlers. While direct hooks are not explicitly documented, inheritance and method overriding are the recommended approaches.

Root Cause

  • Lack of built-in lifecycle hooks: PydanticAI does not provide native lifecycle hooks for before_run or after_run.
  • Reliance on method overriding: Developers must inherit from the Agent class and override methods like run or run_sync to inject custom logic.

Why This Happens in Real Systems

  • Flexibility vs. simplicity: Frameworks often prioritize simplicity over exhaustive features, leaving advanced customization to developers.
  • Event-driven architecture: PydanticAI relies on event handlers (e.g., event_stream_handler) for post-run actions, but pre-run hooks require manual intervention.

Real-World Impact

  • Increased boilerplate code: Developers must write additional code to override methods, reducing readability.
  • Potential for errors: Overriding methods incorrectly can lead to unexpected behavior or bugs.
  • Limited reusability: Custom logic is tightly coupled to the overridden methods, making it harder to reuse across agents.

Example or Code

from pydanticai import Agent

class CustomAgent(Agent):
    def run(self, *args, **kwargs):
        # Pre-run hook: Check credits
        self.check_credits()
        return super().run(*args, **kwargs)

    def check_credits(self):
        # Custom logic to check credits
        pass

How Senior Engineers Fix It

  • Encapsulate logic in mixins: Create reusable mixins with pre/post-run hooks to avoid duplicating code.
  • Use decorators: Implement decorators to wrap run methods and inject custom logic dynamically.
  • Leverage event handlers: For post-run actions, utilize event_stream_handler to keep the code clean and modular.

Why Juniors Miss It

  • Overlooking inheritance: Juniors may not realize they can inherit from the Agent class to customize behavior.
  • Misunderstanding event handlers: They might confuse event_stream_handler as a solution for pre-run hooks, which it is not.
  • Fear of overriding core methods: Juniors may hesitate to override methods like run, fearing they might break functionality.

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