Vercel AI SDK

Call Knobase beside streamText and keep the model answer unchanged.

This is host-application sample code. The Knobase repository does not depend on the `ai` package. The example uses the current public AI SDK streamText / UIMessage surface.

What the host may do

Call Knobase from the server independently from streamText.
Preserve the normal answer stream.
Attach a structured offer as typed application data or UIMessage metadata/data parts.
Let the existing app decide whether it has an appropriate native UI surface.

Do not

Put offer content into the model system prompt.
Ask the model to produce sponsored text.
Re-feed a sponsored offer tool result to the model as answer content.
Add a required card component.

Independent decide beside streamText

import { streamText } from "ai";

type CommercialAttachment = {
  disclosure: string;
  advertiser: string;
  link: string;
  title?: string;
  description?: string;
  cta?: string;
};

async function decideOffer(message: string): Promise<CommercialAttachment | undefined> {
  const controller = new AbortController();
  const timer = setTimeout(() => controller.abort(), 400);
  try {
    const response = await fetch("https://knobase.com/v1/offers/decide", {
      method: "POST",
      headers: {
        Authorization: `Bearer ${process.env.KNOBASE_API_KEY}`,
        "Content-Type": "application/json",
      },
      body: JSON.stringify({
        message,
        locale: "ko",
        audience: { age_band: "18_plus", contextual_offer_consent: true },
        native_surface: { clickable: true, disclosure_capable: true },
      }),
      signal: controller.signal,
    });
    if (!response.ok) return undefined;
    const decision = await response.json();
    if (!decision.serve) return undefined;
    return {
      disclosure: decision.disclosure,
      advertiser: decision.advertiser,
      link: decision.link,
      title: decision.title,
      description: decision.description,
      cta: decision.cta,
    };
  } catch {
    return undefined;
  } finally {
    clearTimeout(timer);
  }
}

export async function POST(req: Request) {
  const { messages } = await req.json();
  const current = messages.at(-1)?.content ?? "";

  const [result, commercialAttachment] = await Promise.all([
    streamText({
      model: "openai/gpt-4.1-mini",
      messages,
      // Do not put offer copy in the system prompt.
    }),
    decideOffer(typeof current === "string" ? current : ""),
  ]);

  // Return the model stream unchanged. Attach the offer as separate
  // application data or UIMessage metadata — never as answer text.
  return result.toUIMessageStreamResponse({
    messageMetadata: {
      commercialAttachment,
    },
  });
}