Emit agent/node lifecycle events (start/finish) in runner.run_async

[ferponse]

Context

We maintain the AG-UI ↔ ADK middleware (ag-ui-adk, part of ag-ui-protocol/ag-ui). To emit AG-UI's STEP_STARTED / STEP_FINISHED events — which bracket the nodes/phases of a run — from ADK workflows, the middleware must know when each node/agent starts and finishes.

Problem

The runner.run_async stream carries text / tool-call / state / reasoning events, but no explicit "agent X started / finished" boundaries. Consumers that want to bracket work per node/agent (workflows, multi-agent, loops) must infer boundaries from author / branch transitions, which is lossy:

• exact parallel-branch end can't be observed (only inferred at the run boundary), and
• a ParallelAgent inside a LoopAgent can't be split per iteration — the events are identical across iterations (see LoopAgent: surface the current iteration index in the event stream #6266).

ADK already has the lifecycle internally

• before_agent_callback / after_agent_callback fire once per agent invocation (including once per LoopAgent iteration).
• The 2.0 Workflow graph engine has _node_status / node_tracing / _node_runner.

It is simply not surfaced in the public event stream.

Ask

Optionally (behind a flag/config) emit lightweight lifecycle events — agent/node started and finished — into runner.run_async, authored by the agent, so any consumer can bracket nodes exactly: including per LoopAgent iteration and exact parallel-branch boundaries.

Prior art / parity

Other agent frameworks already emit these, and AG-UI integrations map them directly to STEP_STARTED / STEP_FINISHED:

• CrewAI → MethodExecutionStartedEvent / MethodExecutionFinishedEvent
• AWS Strands → multiagent_node_start / multiagent_node_stop

Surfacing lifecycle events in the stream would let the ADK integration reach the same precision, without consumers having to install their own plugin/callback to observe boundaries.

Motivation

We need this to faithfully emit all AG-UI event types from ADK; STEP_STARTED/STEP_FINISHED is currently the one category the ADK integration can only approximate. See the AG-UI PR: ag-ui-protocol/ag-ui#2076.

Related

#6266 is a minimal, loop-scoped subset of this (just expose the loop iteration index). This issue is the general form and would subsume #6266.

[ferponse]

Implemented in PR #6269 (opt-in agent lifecycle events in the stream). Related minimal, loop-scoped subset: issue #6266 → PR #6268.