Stop an agent run when the user cancels or the client disconnects

Goal

Stop computation when the user is no longer waiting, reducing waste and preventing later side effects.

Steps

1. Identify cancellation sources such as a stop button, request deadline, or closed SSE connection. [abort]
2. Pass asyncio.Event() as abort_signal to runner.run_async(..., abort_signal=signal) and call signal.set() when the user stops. [abort]
3. Poll ctx.is_aborted in long synchronous loops; the SDK cancels child tasks, seals pending tool calls, and ends with an abort event. [abort]
4. Test normal completion, client disconnect, and mid-run cancellation; verify session history remains valid. [abort]

How to verify

Cancellation ends the stream cleanly with an `INVOCATION_ABORTED` event when no tool call is pending, or a synthetic response for dangling calls; the session remains usable.

Troubleshooting

If a tool keeps running, propagate cancellation into downstream I/O; add safe checkpoints for non-cancellable actions.