next-safe-action Better Auth Adapter

Install

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

Import

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

Quick Start

1. Set up Better Auth

Create your Better Auth server instance. Add the nextCookies() plugin if your actions need to set cookies (e.g. signInEmail, signUpEmail):

// 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
  ],
});

2. Enable auth interrupts in Next.js

The default behavior uses unauthorized() from next/navigation, which requires this flag:

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

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

export default nextConfig;

3. Create an authenticated action client

// 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));

4. Use it in your actions

// 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 };
  });

How It Works

betterAuth() creates a pre-validation middleware for the safe action client's .use() chain:

1. Fetches the session by calling auth.api.getSession({ headers: await headers() }) using the request headers from next/headers
2. Blocks unauthenticated requests by calling unauthorized() from next/navigation when no session exists
3. Injects typed context by passing { auth: { user, session } } to next(), merging it into the action context

The context is namespaced under auth to avoid collisions with other middleware context properties.

Type Inference

The middleware infers the exact user and session types from your Better Auth instance, including any fields added by plugins. For example, if you use the organization plugin, ctx.auth.session will include activeOrganizationId. No manual type annotations are needed.

Entry Points

Exported Types

vs. Manual Auth Middleware

If you are using Better Auth, prefer betterAuth(auth) over writing manual auth middleware. The adapter handles session fetching, cookie integration, typing, and unauthorized rejection automatically.

// 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

Supporting Docs

• [Custom authorize patterns (role checks, redirects, org access)](./authorize-patterns.md)

Anti-Patterns

// 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));