Better Auth is a comprehensive, open-source authentication framework for TypeScript. It is designed to be framework agnostic, but integrates well with Next.js and provides a lot of features out of the box.

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1. Swap out the auth package dependencies

Uninstall the existing Clerk dependencies from the auth package…
Terminal
pnpm remove @clerk/nextjs @clerk/themes @clerk/types --filter @repo/auth
…and install the Better Auth dependencies:
Terminal
pnpm add better-auth next --filter @repo/auth
Additionally, add @repo/database to the auth package dependencies.

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2. Update your environment variables

Generate a secret with the following command to add it to the .env.local file in each Next.js application (app, web and api):
Terminal
npx @better-auth/cli secret
This will add a BETTER_AUTH_SECRET environment variable to the .env.local file.

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3. Setup the server and client auth

Update the auth package files with the following code: 
import { betterAuth } from 'better-auth';
import { nextCookies } from "better-auth/next-js";
import { prismaAdapter } from "better-auth/adapters/prisma";
import { database } from "@repo/database"

export const auth = betterAuth({
  database: prismaAdapter(database, {
    provider: 'postgresql',
  }),
  plugins: [
    nextCookies()
    // organization() // if you want to use organization plugin
  ],
  //...add more options here
});
Read more in the Better Auth installation guide.

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4. Update the auth components

Update both the sign-in.tsx and sign-up.tsx components in the auth package to use the signIn and signUp functions from the client file. 
"use client";

import { signIn } from '../client';
import { useState } from 'react';

export const SignIn = () => {
  const [email, setEmail] = useState("");
  const [password, setPassword] = useState("");

  return (
    <form
      onSubmit={async (e) => {
        e.preventDefault();
        await signIn.email({
          email,
          password,
        })
      }}
    >
      <input
        type="email"
        value={email}
        onChange={(e) => setEmail(e.target.value)}
      />
      <input
        type="password"
        value={password}
        onChange={(e) => setPassword(e.target.value)}
      />
      <button type="submit">Sign in</button>
    </form>
  );
}
You can use different sign-in methods like social providers, phone, username etc. Read more about Better Auth basic usage.

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5. Generate Prisma Models

From the root folder, generate Prisma models for Better Auth by running the following command:
Terminal
npx @better-auth/cli generate --output ./packages/database/prisma/schema.prisma --config ./packages/auth/server.ts
You may have to comment out the server-only directive in packages/database/index.ts temporarily. Ensure you have environment variables set.

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6. Update the Provider file

Better Auth has no concept of a Provider as a higher-order component, so you can either remove it entirely or just replace it with a stub, like so:
packages/auth/provider.tsx
import type { ReactNode } from 'react';

type AuthProviderProps = {
  children: ReactNode;
};

export const AuthProvider = ({ children }: AuthProviderProps) => children;

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7. Change Middleware

Change the middleware in the auth package to the following:
packages/auth/middleware.ts
import type { NextRequest } from "next/server";
import { NextResponse } from 'next/server';

const isProtectedRoute = (request: NextRequest) => {
  return request.url.startsWith("/dashboard"); // change this to your protected route
}

export const authMiddleware = async (request: NextRequest) => {
  const url = new URL('/api/auth/get-session', request.nextUrl.origin);
  const response = await fetch(url, {
    headers: {
      cookie: request.headers.get('cookie') || '',
    },
  });

  const session = await response.json();
  
  if (isProtectedRoute(request) && !session) {
    return NextResponse.redirect(new URL("/sign-in", request.url));
  }
  
  return NextResponse.next();
}

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8. Update your apps

From here, you’ll need to replace any remaining Clerk implementations in your apps with Better Auth. Here is some inspiration: 
const user = await currentUser();
const { redirectToSignIn } = await auth();

// to

const session = await auth.api.getSession({
  headers: await headers(), // from next/headers
});
if (!session?.user) {
  return redirect('/sign-in'); // from next/navigation
}
// do something with session.user

const { orgId } = await auth();

// to

const h = await headers(); // from next/headers
const session = await auth.api.getSession({
  headers: h,
});
const orgId = session?.session.activeOrganizationId;
const fullOrganization = await auth.api.getFullOrganization({
  headers: h,
  query: { organizationId: orgId },
});
webhooks/stripe/route.ts
import { clerkClient } from '@repo/auth/server';

const clerk = await clerkClient();
const users = await clerk.users.getUserList();
const user = users.data.find(
  (user) => user.privateMetadata.stripeCustomerId === customerId
);

// to

import { database } from '@repo/database';

const user = await database.user.findFirst({
  where: {
    privateMetadata: {
      contains: { stripeCustomerId: customerId },
    },
  },
});
For using organization, check organization plugin and more from the Better Auth documentation.