assistant-ui--assistant-ui/templates/cloud/app/api/chat/route.ts

import { openai } from "@ai-sdk/openai";
import { frontendTools } from "@assistant-ui/react-ai-sdk";
import {
  type JSONSchema7,
  streamText,
  convertToModelMessages,
  type UIMessage,
} from "ai";

export async function POST(req: Request) {
  const {
    messages,
    system,
    tools,
  }: {
    messages: UIMessage[];
    system?: string;
    tools?: Record<string, { description?: string; parameters: JSONSchema7 }>;
  } = await req.json();

  const result = streamText({
    model: openai.responses("gpt-5.4-nano"),
    messages: await convertToModelMessages(messages),
    system,
    tools: {
      ...frontendTools(tools ?? {}),
    },
    providerOptions: {
      openai: {
        reasoningEffort: "low",
        reasoningSummary: "auto",
      },
    },
  });

  return result.toUIMessageStreamResponse({
    sendReasoning: true,
    onError: (error) =>
      error instanceof Error ? error.message : String(error),
    messageMetadata: ({ part }) => {
      if (part.type === "finish") {
        return {
          usage: part.totalUsage,
        };
      }
      if (part.type === "finish-step") {
        return {
          modelId: part.response.modelId,
        };
      }
      return undefined;
    },
  });
}