The loop exposed durable turn and step boundaries through both `SessionEvent` and live `agent/*` mirrors. Consumers had to choose between two sources for the same fact and reconcile their timing. The stdio UI was the only remaining mirror consumer; ACP and persistence already used the session log.
This duplication is not free. Every lifecycle change had to update the session event, the mirror event, docs, invariants, tests, and snapshot expectations. The duplicate boundary events also made failure ordering subtle: a turn can be durably closed before a live `agent/turn-end` listener runs, so a post-boundary listener failure has no valid in-log position left and must be reported out of band.
## Decision
Make `session/event` the single live boundary/transcript stream. Consumers that render turns, tool calls, tool results, assistant messages, and durable boundaries subscribe to `session/event` and derive their UI from the same event vocabulary persistence uses.
Remove `agent/turn-start`, `agent/turn-end`, `agent/step-start`, and `agent/step-end`. Boundary consumers subscribe to `session/event`. A UI that also needs an agent id maintains a session-to-agent map from `agent/created` and `agent/disposed`.
This decision covers only durable turn and step boundaries. Steering and stream mirrors have separate decisions: [steering](2026-07-04-remove-agent-steering-mirror.md) and [stream chunks](2026-07-02-remove-stream-chunk-mirror.md). `agent/created`, `agent/disposed`, `agent/status`, `agent/error`, and `agent/queued` remain live lifecycle or control events rather than transcript mirrors.
A plugin can no longer observe turn/step boundaries from a convenient `Agent`-first event. It must either subscribe to `session/event` or maintain a session-to-agent association. That is an acceptable trade: boundary consumers should not depend on a second event feed that can drift from the durable log.
