npm install @next-safe-action/adapter-better-auth better-auth

import { betterAuth } from "@next-safe-action/adapter-better-auth";

// src/lib/auth.ts
import { betterAuth } from "better-auth";
import { nextCookies } from "better-auth/next-js";

export const auth = betterAuth({
  // ...your config (database, plugins, etc.)
  plugins: [
    // ...other plugins
    nextCookies(), // must be the last plugin in the array
  ],
});

// next.config.ts
import type { NextConfig } from "next";

const nextConfig: NextConfig = {
  experimental: {
    authInterrupts: true,
  },
};

export default nextConfig;

// src/lib/safe-action.ts
import { createSafeActionClient } from "next-safe-action";
import { betterAuth } from "@next-safe-action/adapter-better-auth";
import { auth } from "./auth";

// Public action client (no auth required)
export const actionClient = createSafeActionClient();

// Authenticated action client
export const authClient = actionClient.use(betterAuth(auth));

// src/app/actions.ts
"use server";

import { z } from "zod";
import { authClient } from "@/lib/safe-action";

export const updateProfile = authClient
  .inputSchema(z.object({ name: z.string().min(1) }))
  .action(async ({ parsedInput, ctx }) => {
    // ctx.auth.user and ctx.auth.session are fully typed,
    // including fields from Better Auth plugins
    const userId = ctx.auth.user.id;

    await db.user.update({
      where: { id: userId },
      data: { name: parsedInput.name },
    });

    return { success: true };
  });

// Manual — don't do this if you have @next-safe-action/adapter-better-auth installed
const authClient = actionClient.use(async ({ next }) => {
  const session = await auth.api.getSession({ headers: await headers() });
  if (!session) {
    throw new Error("Unauthorized");
  }
  return next({ ctx: { userId: session.user.id } });
});

// With adapter — do this instead
const authClient = actionClient.use(betterAuth(auth));
// ctx.auth.user and ctx.auth.session are fully typed automatically

// BAD: Missing nextCookies() plugin — cookies won't be set in Server Actions
// Session will silently be null when actions try to set cookies
import { betterAuth } from "better-auth";

export const auth = betterAuth({
  plugins: [/* no nextCookies() */],
});

// GOOD: Add nextCookies() as the last plugin
import { betterAuth } from "better-auth";
import { nextCookies } from "better-auth/next-js";

export const auth = betterAuth({
  plugins: [
    // ...other plugins
    nextCookies(), // must be last
  ],
});

// BAD: Missing authInterrupts flag — unauthorized() will throw a runtime error
// next.config.ts
const nextConfig: NextConfig = {};

// GOOD: Enable authInterrupts
const nextConfig: NextConfig = {
  experimental: {
    authInterrupts: true,
  },
};

// BAD: Re-fetching session inside authorize — it's already pre-fetched as authData
actionClient.use(
  betterAuth(auth, {
    authorize: async ({ next }) => {
      const session = await auth.api.getSession({ headers: await headers() }); // Redundant!
      if (!session || session.user.role !== "admin") {
        unauthorized();
      }
      return next({ ctx: { auth: session } });
    },
  })
);

// GOOD: Use the pre-fetched authData directly
actionClient.use(
  betterAuth(auth, {
    authorize: ({ authData, next }) => {
      if (!authData || authData.user.role !== "admin") {
        unauthorized();
      }
      return next({ ctx: { auth: authData } });
    },
  })
);

// BAD: Writing manual Better Auth middleware when the adapter is installed
import { auth } from "./auth";

const authClient = actionClient.use(async ({ next }) => {
  const session = await auth.api.getSession({ headers: await headers() });
  if (!session) throw new Error("Unauthorized");
  return next({ ctx: { user: session.user } });
});

// GOOD: Use the adapter — handles typing, cookies, and unauthorized() automatically
import { betterAuth } from "@next-safe-action/adapter-better-auth";
import { auth } from "./auth";

const authClient = actionClient.use(betterAuth(auth));