Fix NextAuth.js (Auth.js) configuration and callback errors
NextAuth errors typically stem from misconfigured providers, missing environment variables, or incorrect callback URL settings. The error messages can be cryptic, but they usually point to specific configuration issues.
Error messages you might see
Error [OAUTH_CALLBACK_ERROR]: There is a problem with the server configuration.Error [CLIENT_FETCH_ERROR]: Unexpected token < in JSON at position 0Why this happens in AI-generated code
Missing NEXTAUTH_SECRET or NEXTAUTH_URL
AI-generated NextAuth configurations often omit the required NEXTAUTH_SECRET and NEXTAUTH_URL environment variables, causing cryptic errors in production.
Provider configuration mismatch
AI tools generate OAuth provider configs with incorrect client IDs, secrets, or scopes. Even small mismatches in the provider settings cause callback failures.
Database adapter misconfigured
AI-generated NextAuth setups include a database adapter (Prisma, Drizzle) but the schema doesn't match what NextAuth expects, causing session creation to fail.
How to fix it
Set all required environment variables
Ensure NEXTAUTH_SECRET (generate with `openssl rand -base64 32`) and NEXTAUTH_URL (your app's canonical URL) are set in all environments. In Vercel, NEXTAUTH_URL is automatically set.
Verify provider credentials and callback URLs
Double-check your OAuth provider's client ID and secret. Ensure the callback URL in the provider's dashboard exactly matches: {NEXTAUTH_URL}/api/auth/callback/{provider}.
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Still stuck? Our engineers can configure NextAuth with any provider. Visit /products to get started.
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Frequently asked questions
Should I use NextAuth v4 or Auth.js v5?
For new projects, use Auth.js v5 (the latest). It has better App Router support, edge compatibility, and a simpler API. NextAuth v4 is in maintenance mode but still widely used in existing projects.
How do I debug NextAuth errors?
Set debug: true in your NextAuth config to enable verbose logging. Check browser cookies for the next-auth session cookie, and inspect the /api/auth/session endpoint to see the current session state.
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