vercel__eve/research/frontend-durable-turn-cancellation.md

issue, status, last_updated

issue	status	last_updated
483	implemented	2026-08-14

Durable frontend turn cancellation

Summary

Frontend bindings exposed stop(), which aborted the browser stream without stopping durable server execution. Templates that needed a real Stop button rebuilt cancellation manually by tracking the session ID and waiting for turn.started to reveal a guarded turn ID.

Replace stop() with cancel() on the React, Vue, and Svelte bindings. Put exact-turn cancellation on MessageResponse, the client abstraction that already owns one accepted turn and its stream, so every frontend binding delegates to the same lifecycle.

Authoring API

const agent = useEveAgent();

const sending = agent.send("Run the analysis");
await agent.cancel();
await sending;

Lower-level clients can cancel the exact response while continuing to consume its stream:

const response = await session.send("Run the analysis");
const result = response.result();

await response.cancel();
await result;

Semantics

flowchart LR
  UI["cancel()"] --> Response["Active MessageResponse"]
  Response --> Started["Observe turn.started"]
  Started --> Guarded["session.cancel({ turnId })"]
  Guarded --> Boundary["turn.cancelled then session.waiting"]

• Cancellation can be requested while a send is still submitted. The frontend binding waits for its accepted MessageResponse; the response then waits for its own turn.started event before dispatching one guarded request.
• Concurrent cancellation calls for the same live turn share one request. A failed request can be retried while the turn remains active.
• Cancellation acceptance and turn settlement are separate. The stream remains attached until the durable boundary so the session cursor advances normally.
• Component disposal, page closure, AbortSignal, and reset() detach local transport only. They do not imply durable cancellation.
• A response that reaches a boundary without starting a turn, or a binding with no active response, returns no_active_turn.

Scope

The existing session cancel route and ClientSession.cancel({ turnId }) remain available for consumers that have only a session handle. detach stays an internal framework-adapter operation rather than a second public stopping primitive.

Validation

• Unit coverage exercises queued cancellation, exact turn-ID guarding, single-flight calls, rejection retry, reset races, boundary races, and local detach behavior.
• React, Vue, and Svelte typechecks prove the shared public surface, and their example applications handle cancellation request failures.
• Existing cancellation e2e fixtures continue to prove durable turn.cancelled followed by session.waiting and successful session continuation.