modelRouterMiddleware

import { createAgent, modelRouterMiddleware } from "langchain";

const router = modelRouterMiddleware({
  choices: {
    fast: {
      model: "openai:gpt-5-mini",
      criteria: "Simple, well-scoped tasks.",
    },
    powerful: {
      model: powerfulModel,
      criteria: "Complex tasks requiring deeper reasoning.",
    },
  },
  instructions: "Choose the least costly model suited to the task.",
  classifier,
});
const agent = createAgent({
  model: "openai:gpt-5-mini",
  middleware: [router],
});