Partial UIMessage collection on abort or error

Description

Currently, createUIMessageStream only invokes the onFinish callback when the stream completes successfully.

In cases where the stream is aborted (e.g., by the user) or an error occurs mid-way, we don’t get access to any partial UIMessages that were emitted before termination.

In long-running sessions that involve side effects (e.g., saving to disk, modifying state, committing to a DB, etc.), losing this in-progress output is problematic and can result in lost work or inconsistent state.

It would be valuable to have:

• An onAbort and/or onError callback that receives the partial messages.
• Or a modified onFinish that is always called, with an additional argument indicating whether the stream was aborted, failed, or completed.

I’m considering maintaining a partialMessages: ResponseMessage[] variable by collecting the output from onStepFinish. However, I’m not sure if there’s a util function available to convert ResponseMessage[] into a valid UIMessage.

+don't want to lose the usage data when error or abort occurs.

thanks!

AI SDK Version

Currently, using v5, but the issue existed in v4 as well.

Start at createUIMessageStream and inspect how onFinish and onStepFinish behave when a stream aborts or errors. Determine the intended callback or completion-state contract, including preservation of partial UI messages and usage data, and consider the issue resolved when both termination paths expose that information consistently.