I built a €10 AI service that brutally roasts your website

#webdev #sideprojects #ai #buildinpublic

I've been building websites for clients for years. And every time
I looked at their sites, I saw the same problems — unclear CTAs,
hero sections that try to say everything at once, cookie-cutter
layouts that put the booking form three scrolls deep.

The feedback I'd give them privately was always more honest than
what anyone would write in a formal audit. So I built a tool that
does exactly that.

What it does

knallhart.dev does one thing:

You submit a URL and your email
Pay €10 via Stripe
Our service takes a full-page Playwright screenshot
Sends it to Gemini 2.5 Flash with a brutally honest prompt
You get an email with 3 specific critiques + actionable fixes

No dashboards, no subscriptions, no 47-point audit template.
Just three things that are actually wrong and how to fix them.

The tech stack

Next.js on Vercel (frontend + webhook handling)
Playwright for full-page screenshots (headless Chromium)
Gemini 2.5 Flash for the AI analysis
Stripe for payments
Resend for email delivery

The hardest part was getting Playwright to reliably handle
lazy loading on modern sites. The trick: scroll until
window.scrollY stops changing rather than calculating
scrollHeight upfront — that value is unreliable on
too many sites.

Real example output

Here's what it said about basecamp.com:

Welcome to Visual Overload — Seriously, did a screenshot
gallery throw up on your hero section? "Refreshingly
straightforward" indeed, if "straightforward" means immediately
overwhelming me with a static, unreadable dump of your entire
application UI before I even know what Basecamp is.

Fix: Replace the dense UI screenshots with a clean,
aspirational visual that highlights benefits, not features.

The Great Wall of Founder's Text — A full-page novel
from the co-founder right after being visually assaulted.
This isn't a fireside chat; it's a product page.

Fix: Break it into scannable sections. Move the full letter
to an About page.

Testimonial Tsunami — Nine testimonials in a 3x3 grid
and then a button promising a thousand more. This isn't
social proof; it's a marathon reading challenge.

Fix: 2-3 high-impact quotes max, placed near relevant features.

What I learned

Gemini 2.5 Flash is surprisingly good at visual critique
when you give it proper context. The prompt matters a lot —
telling it this is a full-page desktop screenshot, that fixed
elements may be mispositioned, and to focus on the first 900px
as the critical first impression made a huge difference in
output quality.

Bot protection is a real problem. Large e-commerce sites
(Otto, Zalando) detect headless browsers immediately and serve
a gray placeholder. I handle this by checking screenshot file
size before sending to Gemini — anything suspiciously small
triggers an automatic Stripe refund.

Serverless + async don't mix the way you think.
On Vercel, fire-and-forget async operations get killed the
moment you return a response. You have to await the service
call even if the service itself handles things asynchronously.