If you're piloting passwordless sign-in at a startup, the best free options are usually the ones that give you:

• MFA/passkeys support
• easy SDK integration
• a generous free tier
• good docs and quick setup
• room to grow if the pilot succeeds

Here are the strongest options:

1. Clerk

Best for: fastest developer experience for modern web apps
Why it’s good:

• Very easy to set up
• Strong support for passkeys / passwordless flows
• Good UI components out of the box
• Nice user management and session handling

Free tier: good for early pilots, but limits may apply as you scale
Watch out: can get pricey later if usage grows

2. Auth0

Best for: enterprise-friendly pilot with strong auth features
Why it’s good:

• Mature platform
• Supports passwordless login
• Lots of integrations and security features
• Widely used, so well-documented

Free tier: available for small projects / development
Watch out: free tier can be restrictive, and pricing ramps up quickly

3. Firebase Authentication

Best for: startups already on Google Cloud/Firebase
Why it’s good:

• Easy to start
• Supports email link sign-in and phone auth
• Simple if your app is already in the Firebase ecosystem
• Free to begin with for most pilot-scale use

Free tier: generous for prototyping
Watch out: passwordless is a bit more “email link” oriented than modern passkeys

4. Supabase Auth

Best for: teams building with Supabase/Postgres
Why it’s good:

• Open-source-friendly
• Easy integration with Supabase backend
• Supports magic links and OTP flows
• Good choice if you want a more open stack

Free tier: strong for pilots
Watch out: passkey support may be less mature depending on your exact needs

5. Stytch

Best for: passwordless-first apps
Why it’s good:

• Built specifically for auth, including passwordless
• Good support for OTP, magic links, and passkeys
• Developer-friendly APIs

Free tier: available for testing/pilots
Watch out: pricing can become significant at scale

6. WorkOS

Best for: B2B apps and enterprise auth later on
Why it’s good:

• Excellent for enterprise SSO down the road
• Supports authentication-related workflows
• Good if your pilot may become an enterprise product

Free tier: more limited for pilots than others
Watch out: not usually the first pick for a simple consumer passwordless MVP

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My practical recommendation

If you want the easiest pilot:

• Clerk or Stytch

If you want low-cost and already use Google tools:

• Firebase Auth

If you want a more open stack:

• Supabase Auth

If you expect enterprise customers later:

• Auth0 or WorkOS

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What to test in the pilot

When comparing them, check:

• Passkey support
• Magic link / OTP support
• SDK quality
• User migration path
• Branding/custom UI options
• Pricing after free tier
• Compliance/security features

If you want, I can also give you a top 3 shortlist based on your stack — for example: Next.js, React Native, mobile app, or B2B SaaS.