Runner and InMemoryRunner

Runner is the top-level execution engine ADK uses to manage session lifecycles, resolve persistent state, dispatch agent invocations, and stream events back to callers. InMemoryRunner is the default in-memory implementation suitable for local development, CLI applications, and unit testing.

Introduction

Executing an LLM agent or workflow involves coordinating session storage, artifact management, plugin callbacks, and multi-turn message state. Directly instantiating flows or managing raw session objects mixes execution infrastructure with agent business logic.

Runner acts as the execution boundary between external callers and internal ADK agent trees. A runner binds a root agent or App container to session, memory, artifact, and credential services. It provides standard execution methods (run_async, run, run_live, run_debug) that handle session lookup/creation, user event append, context assembly, plugin hook dispatch, and structured event streaming (Event).

ADK provides InMemoryRunner for development with built-in in-memory session and artifact services, while production deployments use Runner with persistent services (e.g. database-backed session stores).

Get started

Wrap a root agent in an App, attach it to an InMemoryRunner, create a session, and invoke run_async with a structured types.Content message:

root_agent = LlmAgent(
    name="greeter",
    instruction="Greet users politely and answer their questions.",
)

app = App(
    name="greeter_app",
    root_agent=root_agent,
)

runner = InMemoryRunner(app=app)

# In an async function:
# 1. Create a session explicitly using the session service
session = await runner.session_service.create_session(
    app_name=app.name,
    user_id="user_123",
    session_id="session_456",
)

# 2. Run agent turn with the created session
async for event in runner.run_async(
    user_id="user_123",
    session_id=session.id,
    new_message=types.Content(
        role="user",
        parts=[types.Part.from_text(text="Hello, ADK!")],
    ),
):
  if event.content and event.content.parts:
    for part in event.content.parts:
      if part.text:
        print(event.author, part.text)

The runner retrieves session session_456 under application greeter_app, appends the user message, executes root_agent, and yields generated Event objects containing model responses and tool outputs.

How it works

The execution lifecycle coordinates between Runner, BaseSessionService, PluginManager, InvocationContext, and the root BaseAgent:

sequenceDiagram
    autonumber
    participant Caller
    participant Runner as Runner / InMemoryRunner
    participant Session as BaseSessionService
    participant Plugins as PluginManager
    participant Agent as Root Agent / Workflow

    Caller->>Runner: run_async(user_id, session_id, new_message, run_config)
    Runner->>Session: get_session(app_name, user_id, session_id)
    alt Session Not Found & auto_create_session=True
        Runner->>Session: create_session(app_name, user_id, session_id)
    end
    Runner->>Runner: Append new_message to session events
    Runner->>Runner: Construct InvocationContext & apply RunConfig
    Runner->>Plugins: run_before_run_callback()
    Runner->>Agent: run_async(invocation_context)
    loop Stream Events
        Agent-->>Runner: Yield Event
        Runner->>Plugins: run_on_event_callback()
        Runner-->>Caller: Yield Event
    end
    Runner->>Plugins: run_after_run_callback()
    Runner->>Session: append_events(session_id, new_events)

1. Session Resolution & Normalization: Runner normalizes its root target to an App. On run_async, it retrieves the active Session from session_service using app.name, user_id, and session_id. If the session is missing and auto_create_session=True, a new session is created automatically; otherwise SessionNotFoundError is raised.
2. Context & Event Ingestion: The caller's new_message is appended as a user Event. Runner constructs an InvocationContext linking session state (app:, user:, temp:), artifact service, memory service, and plugin manager.
3. Execution & Event Streaming: The runner executes the root agent generator. Generated Event instances pass through PluginManager.run_on_event_callback() before being yielded to the caller.
4. Session Persistence & Compaction: Produced events are persisted to session_service. If events_compaction_config is set on App, event compaction runs after iteration completes.

Configuration options

Runner and RunConfig introduce the following options:

Runner Options

Constructor arguments passed when initializing Runner(...) or InMemoryRunner(...):

RunConfig Options

Passed per-invocation to runner.run_async(..., run_config=RunConfig(...)):

Advanced applications

Custom persistent runner

In production, pass database-backed session and memory services to Runner to serve persistent sessions:

from google.adk.apps import App
from google.adk.runners import Runner
from google.adk.sessions import DatabaseSessionService

app = App(name="customer_support", root_agent=root_agent)
session_service = DatabaseSessionService(db_url="postgresql://...")

runner = Runner(
    app=app,
    session_service=session_service,
)

Automatic session creation

By default, calling runner.run_async with a non-existent session_id raises SessionNotFoundError. To automatically create a new session when one is missing without an explicit create_session call, set auto_create_session=True when instantiating Runner or InMemoryRunner:

runner = InMemoryRunner(app=app, auto_create_session=True)

Limitations

• App versus Bare Agent: Runner(agent=...) wraps the agent in an unvalidated App without context_cache_config, events_compaction_config, or resumability_config. Always pass an App via app= for production applications.
• InMemoryRunner Volatility: InMemoryRunner uses InMemorySessionService by default. Session state is stored in memory and lost when the process terminates.

Related guides & samples

• App Container — Guide on App configuration, plugins, and cross-cutting features.
• Runner Live Streaming — Guide on real-time bidirectional streaming with run_live and LiveRequestQueue.
• Session and BaseSessionService — Guide on session storage backends and state scoping.
• Agent-to-Agent Sample — Multi-agent application executed via Runner.