ADR active-turn-steering: Bind Composer steering to the active durable turn
- Status: Accepted
- Date: 2026-09-12
- Issue: https://github.com/vastsa/PI-Desktop/issues/164
Context
The Composer queues follow-ups, but cannot redirect a running turn. A second prompt is correctly rejected with AGENT_BUSY. pi-agent-core already supplies Agent.steer; Codex's turn/steer contract provides the expected-turn admission model.
Decision
Keep Send/Enter as follow-up. Alt+Enter (Option+Enter on macOS) uses an additive agent/steer channel with expectedTurnId. The existing queue slice submits optimistic input and restores rejected drafts. The agent IPC handler reuses attachment validation against the running model and workspace; the sidecar rechecks the target after asynchronous preparation and calls Agent.steer.
Steering keeps the active configuration and durable turn. Started tools finish before the next model request consumes input. The runtime handles admission at its closing boundary and while awaiting delegates. Stop closes admission and retains accepted input as history without independently replaying it.
The existing event-persistence module journals input through the outbox. precedingAssistant reserves an unfinished reply's position before the user row. Host append permits a terminal assistant to replace its own streaming reservation, retaining sequence and turn ownership; completed rows remain immutable on replay. Recovery updates the reservation in place. The persisted UiMessage.steering marker protects input from Smart Stop, including after renderer reload; no separate renderer submission registry is needed. Host provenance treats steering into a claimed collaboration delivery turn as extra human input: it must target that delivery's session, does not inherit the delivery's agent origin, and strips any client-supplied session_message (D597).
Consequences and validation
No new provider transport, host protocol version, or storage migration is required. Streaming reservation updates are the sole exception to append-only message persistence. Existing regression suites cover surrounding behavior; E2E-AGENT-alt-enter-steers-active-turn specifies the full journey and remains Draft until rendered E2E validation is performed.