Record Token Usage After Streaming Object

When you're streaming structured data with streamText and Output, you may want to record the token usage for billing purposes.

onFinish Callback

You can use the onFinish callback to record token usage. It is called when the stream is finished.

import { streamText, Output } from 'ai';
import { z } from 'zod';


const result = streamText({
  model: 'openai/gpt-4.1',
  output: Output.object({
    schema: z.object({
      recipe: z.object({
        name: z.string(),
        ingredients: z.array(z.string()),
        steps: z.array(z.string()),
      }),
    }),
  }),
  prompt: 'Generate a lasagna recipe.',
  onFinish({ usage }) {
    console.log('Token usage:', usage);
  },
});

usage Promise

The streamText result contains a usage promise that resolves to the total token usage.

import { streamText, Output, LanguageModelUsage } from 'ai';
import { z } from 'zod';


const result = streamText({
  model: 'openai/gpt-4.1',
  output: Output.object({
    schema: z.object({
      recipe: z.object({
        name: z.string(),
        ingredients: z.array(z.string()),
        steps: z.array(z.string()),
      }),
    }),
  }),
  prompt: 'Generate a lasagna recipe.',
});


function recordUsage({
  inputTokens,
  outputTokens,
  totalTokens,
}: LanguageModelUsage) {
  console.log('Prompt tokens:', inputTokens);
  console.log('Completion tokens:', outputTokens);
  console.log('Total tokens:', totalTokens);
}


result.usage.then(recordUsage);


recordUsage(await result.usage);


for await (const partialObject of result.partialOutputStream) {
}