def build_planning_instruction(
      self,
      readonly_context: ReadonlyContext,
      llm_request: LlmRequest,
  ) -> Optional[str]:
    """Builds the system instruction to be appended to the LLM request for planning.

    Args:
        readonly_context: The readonly context of the invocation.
        llm_request: The LLM request. Readonly.

    Returns:
        The planning system instruction, or None if no instruction is needed.
    """
    pass
Builds the system instruction to be appended to the LLM request for planning.

    Args:
        readonly_context: The readonly context of the invocation.
        llm_request: The LLM request. Readonly.

    Returns:
        The planning system instruction, or None if no instruction is needed.
    
src/google/adk/planners/base_planner.py