RFC: Keep one public stop primitive

Status: implemented

Implementation note: Only abort() was removed. whenIdle() remains because it is the public quiescence signal and safely handles waiter settlement and replacement-turn races; consumers should not reconstruct that behavior from status transitions.

Problem

The public Agent handle exposed two overlapping ways to stop in-flight work: abort(reason?) and cancel(reason?). abort() killed only the in-flight step and left queued work alone; cancel() clears queued and steering work, aborts the running step, and handles the pre-step race. In production, ACP uses cancel() for session/cancel, while lifecycle owners tear down agents through AgentHandle.dispose(). No production caller needed bare abort().

The abort()/cancel() distinction is real — abort() preserves queued prompts and steering while cancel() drops them — but no shipping code called the public abort() verb. The loop's own stop paths (cancel() and disposal) abort the current AbortController directly rather than routing through Agent.abort(). Most tests that called abort() interrupt an empty queue and switch to cancel(reason); the steering re-delivery test that deliberately depends on queue preservation drives the in-flight AbortController directly, because cancel() would drop the queued steering it is trying to prove survives a step abort. The no-argument abort() default reason ('aborted') is deleted with the verb rather than preserved by accident; cancel() keeps its own 'cancelled' default.

The extra surface area made the loop carry a public verb that is mostly a teardown internal: abort() had to be documented as distinct from queue-aware cancellation even though a UI cancellation almost always wants the broader operation.

Decision

cancel() is the only public stop primitive on Agent. Lifecycle owners use AgentHandle.dispose() to stop and unregister an agent; non-owners use cancel() to abandon current and queued work. The implementation keeps a private abort controller, but it is not part of the plugin-facing Agent contract.

whenIdle() is retained as the public quiescence-observation primitive (resolve once the agent settles out of running, resolve immediately when already idle, await the loop exit when disposed). It is not a stop verb; it is how a non-owner observes the stop completing without disposing the agent. Its live consumers are ACP and agent tests that await settlement through this public seam (packages/ui/acp/tests, packages/core/agent-loop/tests); the production ACP bridge owns its agents and tears them down through AgentHandle.dispose(), so packages/ui/acp/src itself has no whenIdle() call.

Public abort() is deleted, with the tests that exercised it as standalone API and the docs that described step-only abort as an embedding feature. Empty-queue abort tests migrated to cancel(reason) where they still prove cancellation behavior; tests whose subject is the loop's internal AbortController drive that controller directly via an in-package typed cast to the private field; tests that only pinned the removed no-arg abort() default went with the method. The disposer remains async and still waits for the loop to stop.

Alternatives considered

Removing whenIdle() too — the original proposal's shape, reversed on validating the premise against the code (the implementation note above carries the full record): it is a load-bearing quiescence primitive, and pushing consumers onto hand-observed running→idle transitions is exactly the brittle path the defensive patterns warn against.

Verification

Agent exposes no public abort() while cancel(), whenIdle(), and steer() remain; ACP cancellation calls cancel(); teardown awaits quiescence through handle disposal, with whenIdle() resolving on quiescence for non-owner observers; and the suites cover cancellation and disposal as the two supported stop paths.

Consequences

A future plugin cannot abort only the current model/tool step while preserving queued prompts through the public interface. If that use case becomes real, it should return with a named consumer and a narrower contract. Today it is latent generality that keeps a private loop mechanic public.

This RFC only removes the redundant stop verb. Mid-turn steering remains an intentional message path; quiescence observation remains via whenIdle(). The resulting public surface is send(), steer(), inject(), cancel(), whenIdle(), status, options, session, and identity.

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