Fraud Detection API for SaaS Startups — Pay $5 once, never again

Fraud detection API for SaaS startups: SignupDoggy costs $0.01/call with a $5 minimum, no monthly subscription, no enterprise sales call. Built for 2-50 person teams that need IP risk scoring, disposable email detection, VPN/Tor detection, and bot pattern recognition in one POST to /v1/check. The 2-stage funnel pattern + the SaaS startup playbook for keeping Mixpanel honest.

Fraud detection for SaaS startups — SignupDoggy

A 2-50 person SaaS team has a specific fraud-detection problem:

- You have 5,000-100,000 signups per month
- 20-40% of them are bot signups, throwaway emails, or VPN users
- Your Mixpanel funnel data is polluted
- Your support inbox has tickets from "users" who never confirmed their email
- You don't have a fraud team, but you have a CTO who has other priorities

The fix is a single API call.

THE SETUP

1. Get an API key (5 minutes, self-service)
2. Add the call to your signup handler (30 minutes)
3. Ship it (no rollout needed — it just works)
4. Watch your bot rate drop to <1%

THE NUMBERS

- 50,000 signups/month at 30% bot rate = 15,000 bot rows per month
- Without filtering: $0.50 per row in storage and processing = $7,500/year wasted
- With SignupDoggy: 50,000 signups × $0.01 = $500/month
- ROI: 15x in the first year

THE INTEGRATION OPTIONS

The most common patterns for SaaS startups:

1. Custom auth handler: call /v1/check from your signup endpoint
2. Supabase Auth: Postgres trigger on auth.users (the full code in our blog)
3. Auth0: Post-Login Action
4. Clerk: beforeUserCreate webhook
5. NextAuth: signIn callback

The full Supabase integration with working code: /blog/signup-validation-supabase-auth-integration.

THE 2-STAGE FUNNEL

For SaaS at any scale, the right pattern is:

1. Cloudflare Turnstile (free, catches 95% of browser bots)
2. SignupDoggy (catches the remaining 5% + non-browser bots + disposable emails)

The cost: $0.01 per call on the ~5% that get past Turnstile. At 50,000 signups/month, that's $25/month. The benefit: 99%+ bot catch rate with zero user friction.

The full 2-stage funnel pattern: /blog/how-to-stop-bot-signups-without-captcha.