Connect Client

Product clients use User City. The common paths are city.ai.* for AIService and city.service(id).action(name).invoke() for custom services.

import { City } from "@downcity/federation/legacy";

const city = new City({
  federation_url: "https://base.example.com",
  user_token: "ub_xxx",
});

const catalog = await city.ai.catalog();
const model = catalog.get("gpt-5.4");
if (!model) throw new Error("model not found");

const result = await city.ai.text({
  model,
  prompt: "Write a welcome message for the product",
});

If you already know the target model, select it directly from the directory:

const body = await city.ai.stream({
  model: catalog.get("gpt-5.4"),
  prompt: "Stream a short paragraph",
});

ai.text() returns an AI SDK UIMessage. ai.stream() uses CityModel's LanguageModelV3 transport and converts it client-side into a ReadableStream<UIMessageChunk>.

One Federation can serve many client surfaces:

• https://app.example.com
• a Chrome extension
• a desktop tool
• a mobile app
• an internal operations tool

Those clients do not need to manage provider keys, model routing, or usage tracking on their own.