Vercel feature flags with Flag Engine

index.ts

import { createFlagEngine } from "@flag-engine/core";
import crypto from "node:crypto";
import { headers } from "next/headers";
import { unstable_flag as flag } from "@vercel/flags/next";

const engine = createFlagEngine([
  {
    key: "summer-sale",
    status: "enabled",
    strategies: [
      {
        name: "All audience",
        rules: [],
        variants: [
          {
            name: "A",
            percent: 50,
          },
          {
            name: "B",
            percent: 50,
          },
        ],
      },
    ],
  },
]);

const hash = (str: string) =>
  crypto.createHash("sha256").update(str).digest("hex");

export const showSummerSale = flag({
  key: "summer-sale",
  decide: () => {
    const headersMap = headers();
    const ua = headersMap.get("user-agent") || "Unknown UA";
    const userId = hash(ua);

    const userCtx = engine.createUserContext({
      __id: userId,
    });

    return userCtx.evaluate("summer-sale");
  },
});