dynamicSystemPromptMiddleware

import { z } from "zod";
import { dynamicSystemPrompt } from "langchain";
import { createAgent, SystemMessage } from "langchain";

const contextSchema = z.object({ region: z.string().optional() });

const middleware = dynamicSystemPrompt<z.infer<typeof contextSchema>>(
  (_state, runtime) => `You are a helpful assistant. Region: ${runtime.context.region ?? "n/a"}`
);

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  contextSchema,
  middleware: [middleware],
});

await agent.invoke({ messages }, { context: { region: "EU" } });