Quiz - Text, objects, and the chat surface

A reviewer flags this streaming chat handler: the daily token counter is recorded, but the numbers are always wrong.

export const POST = authedRoute('member', schema, async ({ messages, userId }) => {
  const result = streamText({
    model: chatModel,
    system: SYSTEM_PROMPT,
    messages: convertToModelMessages(messages),
    maxOutputTokens: 1000,
  });

  incrementDailyTokens(userId, result.usage.totalTokens);
  return result.toUIMessageStreamResponse();
});

What is the fix?

Move the incrementDailyTokens call into an onFinish callback on streamText — that callback is the only place that runs after the tokens are actually counted.

await the streamText call before reading result.usage, then increment as written.

Read the count from toUIMessageStreamResponse()’s return value instead of from result.

You are extracting an invoice number the model generates, and it should look like INV-0001. The model usually complies but occasionally returns INV/0001 or 2026-INV-1. Where does the format constraint belong?

As a hint in the prompt (“invoice numbers use the format INV-0001”), with the field left as a plain z.string() in the schema.

As a z.string().refine((s) => s.startsWith('INV-')) in the schema, so a near-miss is rejected and corrected automatically.

As both — the .refine enforces it and the prompt hint reduces how often the refine fails.

Your chat needs to survive a page reload. A teammate proposes saving the conversation from useChat’s client-side onFinish callback. Why is that the wrong place for the durable write?

The client’s onFinish is for UI reactions and doesn’t reliably run — a user who closes the tab mid-stream never fires it, so the save is lost. The durable write belongs in the handler’s server-side onFinish.

The client only has access to ModelMessage[], so saving from there would persist the lossy shape and lose tool calls and metadata.

useChat doesn’t expose an onFinish callback at all, so the code wouldn’t compile.

Quiz - Text, objects, and the chat surface

Quiz complete