feat(ai): add onStepFinish to agent.generate and agent.stream#11980
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feat(ai): add onStepFinish to agent.generate and agent.stream#11980
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…#11980) ## Background For things such as token tracking, it would be helpful if callbacks can be registered on a per-call basis. See vercel#11468 ## Summary Add `onStepFinish` callback support to `Agent.generate()` and `Agent.stream()` ## Related Issues Resolves vercel#11468
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This changes the type of the `onStepFinish` callback from `ToolLoopAgentOnStepFinishCallback<TOOLS>` to `UIMessageStreamOnStepFinishCallback<UI_MESSAGE>` for methods which create a `UIMessageChunk` stream for an Agent, giving the implementation access to the generated`UIMessage` so far instead of just the raw parts. This partly reverts vercel#11980, which explicitly added the `onStepFinish` callback to the method signatures. Instead, `onStepFinish` is now added to the `UIMessageStreamOptions` type in the same way as `onFinish` already was, and is passed through via `options`. Affected methods are: - `createAgentUIStream()` - `createAgentUIStreamResponse()` - `pipeAgentUIStreamToResponse()` For even more consistency, the `onStepFinish` callback is also added to: - `DefaultStreamTextResult.toUIMessageStream()` (used by the methods above) - `DefaultStreamTextResult.toUIMessageStreamResponse()` - `DefaultStreamTextResult.pipeUIMessageStreamToResponse()` Callers which want to receive the `ToolLoopAgentOnStepFinishCallback` can easily do that with the `onStepFinish` callback to the constructor of the `ToolLoopAgent` that they're passing to the methods above. Fixes vercel#12841.
rovo89
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Feb 25, 2026
This changes the type of the `onStepFinish` callback from `ToolLoopAgentOnStepFinishCallback<TOOLS>` to `UIMessageStreamOnStepFinishCallback<UI_MESSAGE>` for methods which create a `UIMessageChunk` stream for an Agent, giving the implementation access to the generated`UIMessage` so far instead of just the raw parts. This partly reverts vercel#11980, which explicitly added the `onStepFinish` callback to the method signatures. Instead, `onStepFinish` is now added to the `UIMessageStreamOptions` type in the same way as `onFinish` already was, and is passed through via `options`. Affected methods are: - `createAgentUIStream()` - `createAgentUIStreamResponse()` - `pipeAgentUIStreamToResponse()` For even more consistency, the `onStepFinish` callback is also added to: - `DefaultStreamTextResult.toUIMessageStream()` (used by the methods above) - `DefaultStreamTextResult.toUIMessageStreamResponse()` - `DefaultStreamTextResult.pipeUIMessageStreamToResponse()` Callers which want to receive the `ToolLoopAgentOnStepFinishCallback` can easily do that with the `onStepFinish` callback to the constructor of the `ToolLoopAgent` that they're passing to the methods above. Fixes vercel#12841.
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Background
For things such as token tracking, it would be helpful if callbacks can be registered on a per-call basis. See #11468
Summary
Add
onStepFinishcallback support toAgent.generate()andAgent.stream()Related Issues
Resolves #11468