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The Invisible Bugs in AI-Generated Code

What actually goes wrong inside the MVPs that "just worked" on day one.

Ask a non-technical founder who built their MVP with an AI coding assistant how it went, and you'll hear some version of the same story: astonishingly fast at first, then increasingly weird. A feature that used to ship in an hour takes a day. A customer reports something that shouldn't be possible. The AI is still happy to write more code — but every new thing it adds seems to break something older.

This isn't an argument against AI-assisted coding. It's an argument for understanding how it tends to fail, so you can spot the failures before they find your customers.

The thesis

AI coding assistants don't usually write broken code. They write plausible code. The difference only shows up under stress: real users, real data, real scale, real security. By then, the code looks fine — which is precisely why it takes longer to debug.

The assumption worth interrogating

Most founders, including ones who know better, quietly assume:

If it runs, and it passes the happy path, the AI got it right.

This is reasonable in any other context. A compiler error, a runtime crash, a failing test — those are honest signals. LLM-generated code is dishonest in a specific way: it's trained to produce outputs that look like known-good code, because that's what's in its training data. Looking like known-good code and being known-good code are not the same thing.

Five patterns we find over and over

These are the issues that show up when an audit lands on an MVP that was built primarily with AI assistance. None of them are exotic. All of them are silent until they aren't.

1. Authentication that's almost right

The AI writes login, signup, password reset, session handling. The UX looks great. Then somewhere in the middle of the flow there's a missing check — a token that isn't validated on a critical endpoint, a cookie without the HttpOnly flag, a password reset that doesn't invalidate the old session. Each of these is a one-line fix in isolation. Together they mean any determined person can impersonate a user.

The founder usually doesn't know this is happening because authentication works. You can log in. You can log out. The hole is in what happens in between.

2. Data models that can't evolve

The AI designs a schema based on the prompt it was given. That prompt described the product today. The schema reflects today. When a founder tries to add the "one more thing" a customer asked for three months in, it turns out the original table structure assumed things that aren't true anymore — one user per account, one product per order, one currency, one language.

You can always migrate. But the migration, for a live product with data in it, is the opposite of the "two-hour AI feature" the founder got used to.

3. Over-specific code that pretends to be general

LLMs love to write helpers. Functions with names like getUserOrdersByStatusAndDate. They look reusable. They're not — they only handle the exact combination the founder asked about. The next time someone needs orders by status without by date, the AI writes a second almost-identical function. After a few months the codebase is a museum of near-duplicates, each subtly different.

The bug surface of this isn't a crash. It's that a fix to one copy doesn't fix the others, and the founder has no way to know which copy is running in production for which feature.

4. "Defensive" code that hides bugs

Ask an AI to make code more robust and it will happily wrap things in try/except blocks that swallow errors silently. Failures that should page someone instead get logged to the console — or worse, not logged at all. Data that should cause a hard stop quietly becomes null and flows downstream. The product "never crashes." It also never tells you when something is wrong.

5. Third-party integrations held together with string

Stripe, SendGrid, Supabase, OpenAI — AI assistants are great at scaffolding the first integration. But retries, idempotency, webhook verification, rate limiting, error handling when the provider is down — these are the parts that get skipped because the prompt didn't ask for them. The founder doesn't find out until the first duplicate charge, the first missed signup email, or the first outage that cascades through their product because one upstream service hiccupped.

The possible solution: a review pattern, not a rewrite

You don't need to rewrite an AI-generated MVP to make it safe. You need a review pattern — a short list of questions someone technically competent runs through every few weeks, plus once before any significant customer or investor milestone.

A practical rhythm:

This isn't a full audit every time. It's a routine — the equivalent of getting the oil changed on a car you drive every day. Skipped routinely, it produces the exact failure mode most AI-generated MVPs end up in: fine, fine, fine, catastrophe.

A short worked example

A founder built a B2B tool with an AI assistant in three weekends. It landed five paying customers in six weeks. On week eleven, one of those customers — by accident, through a browser bookmark — loaded another customer's dashboard. The fix was two lines. The damage was the email the founder had to send.

The review that would have caught it would have taken ninety minutes and cost less than the founder's monthly coffee budget.

Summary

AI-generated MVPs aren't a bad idea. They're a great way to get to product-market signal cheaply. But they fail in a specific direction: plausible-looking code that breaks under real conditions. The cost of a light, scheduled review is trivial. The cost of the first avoidable incident is not.

If you've built your MVP primarily with AI help, the single most valuable thing you can do this month is have someone who isn't the AI — and isn't you — read it.

// about the author

Jacek Różański

Senior backend engineer with 18+ years of production experience. Founder of The AI Mechanic — a practice focused on auditing and stabilizing MVPs built by non-technical founders and AI-assisted teams.

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