How to migrate off of `format:'aisdk'` with custom Next route?

// /api/chat
export async function POST(req: Request) {
  // lots of business logic...

  const stream = await myAgent.stream(messages, {
    format: "aisdk", // Enable AI SDK v5 compatibility

export const mastra = new Mastra({
  server: {
    apiRoutes: [
      chatRoute({
        path: "/chat",
        agent: "weatherAgent",
      }),
    ],
  },
});