How I Built a Free SEO Audit Tool with Next.js, Supabase, and Stripe in 1 Week

Last weekend I started building SiteGrade — a free instant SEO audit tool that gives any website an A–F letter grade and the top fixes ranked by impact. I just launched it. This post is the build log: the architecture decisions that worked, the ones that didn't, and the gotchas you'd save time knowing about. If you're considering building a similar audit/scanner tool, or just want to see what a 2026 Next.js + Supabase + Stripe stack looks like in practice, this should be useful. Why I built it I got tired of family and friends asking me "how's the SEO on my website?" and not having a good answer to send back. Every existing tool I tried either cost $99+/month (Ahrefs, Semrush) or threw 200 metrics at people who just wanted to know if their site was OK. The wedge I built around: one letter grade, three top fixes, plain English, no signup. The paid tier ($29/mo) re-audits weekly and emails the report so non-technical users can track improvements as their developer makes them. The stack Nothing exotic. Everything is the obvious choice for a 2026 indie SaaS, which is the whole point — boring stack means I spent zero time fighting infrastructure and 100% of my time on the actual audit logic. Next.js 14 App Router — frontend + API routes in one repo Supabase — Postgres + auth + RLS + file storage, EU-hosted for GDPR Stripe — subscriptions + Billing Portal + webhooks Resend — transactional email (audit reports, weekly reports, Supabase auth via Custom SMTP) Vercel — hosting + cron jobs for the weekly re-audit cheerio — HTML parsing for the audit checks Google PageSpeed Insights API — Core Web Vitals + mobile performance Total monthly infrastructure cost at zero traffic: ~$0 (everyone on free tiers). At 100 paying customers it'd still be under $30/mo. The audit logic — what 15 checks look like in code Each audit runs 15 checks and produces a score from 0-100, then maps that to a letter grade (A: 90+, B: 75+, C: 60+, D: 45+, F: below 45). The checks are structured as pure fu