def canonical_before_agent_callbacks(self) -> list[_SingleAgentCallback]:
    """The resolved self.before_agent_callback field as a list of _SingleAgentCallback.

    This method is only for use by Agent Development Kit.
    """
    if not self.before_agent_callback:
      return []
    if isinstance(self.before_agent_callback, list):
      return self.before_agent_callback
    return [self.before_agent_callback]
The resolved self.before_agent_callback field as a list of _SingleAgentCallback.

    This method is only for use by Agent Development Kit.
    
src/google/adk/agents/base_agent.py