Description

The onFinish callback is defined in ToolLoopAgentSettings but is not being passed to the underlying streamText()/generateText() calls in ToolLoopAgent methods since version 6.0.135.

Affected Versions

• Working: ai@6.0.116
• Broken: ai@6.0.135 - ai@6.0.157 (and possibly later v6 versions)

Root Cause

PR #13698 ("chore(ai): remove all experimental agent events") was intended to remove experimental callbacks (experimental_onStart, experimental_onStepStart, experimental_onToolCallStart, experimental_onToolCallFinish) that were mistakenly merged into v6. However, the PR also incorrectly removed the onFinish callback usage, which is not an experimental callback.

Evidence

In ai@6.0.116 (working):

// packages/ai/src/agent/tool-loop-agent.ts
async stream({
  onFinish,  // ✅ onFinish parameter exists
  ...options
}) {
  return streamText({
    onFinish: this.mergeCallbacks(this.settings.onFinish, onFinish),  // ✅ passed to streamText
  });
}

In ai@6.0.157 (broken):

// packages/ai/src/agent/tool-loop-agent.ts
async stream({
  onStepFinish,  // ❌ only onStepFinish, no onFinish parameter
  ...options
}) {
  return streamText({
    onStepFinish: this.mergeOnStepFinishCallbacks(onStepFinish),
    // ❌ onFinish is NOT passed to streamText
  });
}

However, onFinish is still defined in the types:

// packages/ai/src/agent/tool-loop-agent-settings.ts (line 110)
onFinish?: ToolLoopAgentOnFinishCallback<NoInfer<TOOLS>>;

Expected Behavior

The onFinish callback should be:

1. Accepted as a parameter in AgentCallParameters and AgentStreamParameters
2. Passed to the underlying generateText() and streamText() calls
3. Merged with the onFinish from ToolLoopAgentSettings (constructor)

Actual Behavior

• onFinish is defined in ToolLoopAgentSettings but never used
• Users cannot pass onFinish to agent.stream() or agent.generate()
• The callback silently does nothing

Suggested Fix

Restore the onFinish parameter and its usage in ToolLoopAgent:

// In agent.ts - add onFinish to AgentCallParameters
export type AgentCallParameters<CALL_OPTIONS, TOOLS extends ToolSet = {}> = ... & {
  onFinish?: ToolLoopAgentOnFinishCallback<TOOLS>;
};

// In tool-loop-agent.ts - restore onFinish handling
async stream({
  onFinish,
  ...options
}) {
  return streamText({
    onFinish: this.mergeCallbacks(this.settings.onFinish, onFinish),
  });
}

Related

• PR chore(ai): remove all experimental agent events #13698 - The PR that introduced this bug
• PR feat(ai): experimental callbacks in ToolLoopAgent #12717 - Original PR that added experimental callbacks (mistakenly merged to v6)

AI SDK Version

ai: 6.0.157

Code of Conduct

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