Better Auth OAuth/OIDC

Implement centralized authentication with Better Auth - either as an auth server or SSO client.

MCP Server Setup

Better Auth provides an MCP server powered by Chonkie for guided configuration:

claude mcp add --transport http better-auth https://mcp.chonkie.ai/better-auth/better-auth-builder/mcp

Or in settings.json:

{
  "mcpServers": {
    "better-auth": {
      "type": "http",
      "url": "https://mcp.chonkie.ai/better-auth/better-auth-builder/mcp"
    }
  }
}

When to Use the MCP

Task	Use MCP?
Initial Better Auth setup	Yes - guided configuration
Adding OIDC provider plugin	Yes - generates correct config
Troubleshooting auth issues	Yes - can analyze setup
Understanding auth flow	Yes - explains concepts
Writing custom middleware	No - use patterns below

---

Architecture Overview

┌─────────────────┐
│ Better Auth SSO │ ← Central auth server (auth-server-setup.md)
│  (Auth Server)  │
└────────┬────────┘
         │
    ┌────┴────┐
    ▼         ▼
┌───────┐  ┌───────┐
│ App 1 │  │ App 2 │ ← SSO clients (sso-client-integration.md)
└───────┘  └───────┘

---

Quick Start: Auth Server Setup

npm install better-auth @better-auth/oidc-provider drizzle-orm

Core Configuration

// src/lib/auth.ts
import { betterAuth } from "better-auth";
import { drizzleAdapter } from "better-auth/adapters/drizzle";
import { oidcProvider } from "better-auth/plugins/oidc-provider";

export const auth = betterAuth({
  database: drizzleAdapter(db, { provider: "pg", schema }),
  emailAndPassword: { enabled: true },
  session: {
    expiresIn: 60 * 60 * 24 * 7, // 7 days
    updateAge: 60 * 60 * 24,     // 1 day
  },
  plugins: [
    oidcProvider({
      loginPage: "/sign-in",
      consentPage: "/consent",
      // PKCE for public clients (recommended)
      requirePKCE: true,
    }),
  ],
});

Register OAuth Clients

// Register SSO client
await auth.api.createOAuthClient({
  name: "My App",
  redirectUris: ["http://localhost:3000/api/auth/callback"],
  type: "public", // Use 'public' for PKCE
});

See references/auth-server-setup.md for complete setup with JWKS, email verification, and admin dashboard.

---

Quick Start: SSO Client Integration

npm install jose

Environment Variables

NEXT_PUBLIC_SSO_URL=http://localhost:3001
NEXT_PUBLIC_SSO_CLIENT_ID=your-client-id

PKCE Auth Flow

// lib/auth-client.ts
import { generateCodeVerifier, generateCodeChallenge } from "./pkce";

export async function startLogin() {
  const verifier = generateCodeVerifier();
  const challenge = await generateCodeChallenge(verifier);

  // Store verifier in cookie
  document.cookie = `pkce_verifier=${verifier}; path=/; SameSite=Lax`;

  const params = new URLSearchParams({
    client_id: process.env.NEXT_PUBLIC_SSO_CLIENT_ID!,
    redirect_uri: `${window.location.origin}/api/auth/callback`,
    response_type: "code",
    scope: "openid profile email",
    code_challenge: challenge,
    code_challenge_method: "S256",
  });

  window.location.href = `${SSO_URL}/oauth2/authorize?${params}`;
}

Token Exchange (API Route)

// app/api/auth/callback/route.ts
export async function GET(request: Request) {
  const { searchParams } = new URL(request.url);
  const code = searchParams.get("code");
  const verifier = cookies().get("pkce_verifier")?.value;

  const response = await fetch(`${SSO_URL}/oauth2/token`, {
    method: "POST",
    headers: { "Content-Type": "application/x-www-form-urlencoded" },
    body: new URLSearchParams({
      grant_type: "authorization_code",
      client_id: process.env.NEXT_PUBLIC_SSO_CLIENT_ID!,
      code: code!,
      redirect_uri: `${process.env.NEXT_PUBLIC_APP_URL}/api/auth/callback`,
      code_verifier: verifier!,
    }),
  });

  const tokens = await response.json();

  // Set httpOnly cookies
  const res = NextResponse.redirect("/dashboard");
  res.cookies.set("access_token", tokens.access_token, { httpOnly: true });
  res.cookies.set("refresh_token", tokens.refresh_token, { httpOnly: true });
  return res;
}

See references/sso-client-integration.md for JWKS verification, token refresh, and global logout.

---

PKCE Utilities

// lib/pkce.ts
export function generateCodeVerifier(): string {
  const array = new Uint8Array(32);
  crypto.getRandomValues(array);
  return base64UrlEncode(array);
}

export async function generateCodeChallenge(verifier: string): Promise<string> {
  const encoder = new TextEncoder();
  const data = encoder.encode(verifier);
  const hash = await crypto.subtle.digest("SHA-256", data);
  return base64UrlEncode(new Uint8Array(hash));
}

function base64UrlEncode(buffer: Uint8Array): string {
  return btoa(String.fromCharCode(...buffer))
    .replace(/\+/g, "-")
    .replace(/\//g, "_")
    .replace(/=+$/, "");
}

---

Key Patterns

1. Token Storage

• Store tokens in httpOnly cookies (not localStorage)
• Use SameSite=Lax for CSRF protection

2. Token Refresh

async function refreshTokens() {
  const response = await fetch(`${SSO_URL}/oauth2/token`, {
    method: "POST",
    body: new URLSearchParams({
      grant_type: "refresh_token",
      client_id: process.env.NEXT_PUBLIC_SSO_CLIENT_ID!,
      refresh_token: currentRefreshToken,
    }),
  });
  return response.json();
}

3. JWKS Verification

import { createRemoteJWKSet, jwtVerify } from "jose";

const JWKS = createRemoteJWKSet(
  new URL(`${SSO_URL}/.well-known/jwks.json`)
);

export async function verifyAccessToken(token: string) {
  const { payload } = await jwtVerify(token, JWKS, {
    issuer: SSO_URL,
    audience: process.env.NEXT_PUBLIC_SSO_CLIENT_ID,
  });
  return payload;
}

4. Global Logout

// Logout from all apps
const logoutUrl = new URL(`${SSO_URL}/oauth2/logout`);
logoutUrl.searchParams.set("post_logout_redirect_uri", window.location.origin);
window.location.href = logoutUrl.toString();

---

Common Pitfalls

Issue	Solution
PKCE verifier lost after redirect	Store in httpOnly cookie before redirect
Token in localStorage	Use httpOnly cookies instead
JWKS fetch fails	Check CORS on auth server
Consent screen loops	Ensure consent page saves decision

---

Verification

Run: python3 scripts/verify.py

Expected: ✓ configuring-better-auth skill ready

If Verification Fails

1. Check: references/ folder has both setup files
2. Stop and report if still failing

References

• references/auth-server-setup.md - Complete auth server with OIDC provider
• references/sso-client-integration.md - Full SSO client implementation