104002: Run cancel handler failed

A developer-supplied onCancel hook threw while AI Transport was processing a cancel message for a run. The cancel is not carried out, so the run continues as though the message had not arrived.

What you should do

The fault is in your own handler, not the transport: inspect the cause for what it threw and make your onCancel callback handle its own errors so it returns normally.

You will not catch this from a call you made. Because it arises while the SDK is handling an inbound message, it is delivered to the run's onError handler if you supplied one, and otherwise to the session's error handler; handle it in whichever you use.

Why it happens

When a cancel message arrives for a run, the SDK calls that run's onCancel hook to decide whether the cancel is allowed. This error is raised when the hook throws, with the thrown value preserved as the cause. The SDK stops there and never reaches the abort, so the run is not canceled and carries on to completion.

What you'll see

The error is reported with code 104002 and status 500. The message names the run and the hook, in the form unable to process cancel for run ...; onCancel handler threw: ....