Architecture

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Request lifecycle

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Key design decisions

• No LangGraph — orchestration is plain Python asyncio and async generators.
• Context.state is a shared mutable dict across the call tree. Use EventActions.state_delta to persist changes back to the session.
• Context.event_callback enables real-time event streaming from tools. LlmAgent injects an asyncio.Queue-based callback before tool execution; AgentTool uses it to push sub-agent events as they arrive. Any custom tool can use the same mechanism.
• LlmRequest / LlmResponse isolate LangChain types from the rest of the SDK. Swap the LLM provider without touching agent logic.
• Planners are per-turn hooks, not static prompts. They receive the live context and request so they can make dynamic decisions each turn.

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Custom session backend

from langchain_adk.sessions import BaseSessionService, Session

class RedisSessionService(BaseSessionService):
    async def create_session(self, *, app_name, user_id, state=None, session_id=None) -> Session: ...
    async def get_session(self, *, app_name, user_id, session_id) -> Session | None: ...
    async def update_session(self, session_id, *, state) -> Session: ...
    async def delete_session(self, session_id) -> None: ...
    async def list_sessions(self, *, app_name, user_id) -> list[Session]: ...

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Custom planner

from langchain_adk import BasePlanner, ReadonlyContext, LlmRequest

class MyPlanner(BasePlanner):
    def build_planning_instruction(self, ctx: ReadonlyContext, request: LlmRequest) -> str | None:
        return "Always verify your answer before responding."