AI doesn't write bad code. It writes plausible code — so I tried to break my own AI-built app

Disclosure: I work on one of the tools in this post (create-microservices-app). But the experiment, commands, and outputs below are real, and the pattern at the end works no matter what stack you're on — that's the part I actually want you to take.

If you ship with Claude Code, Cursor, or Codex, you know the feeling. The agent gets you 70% of the way in minutes. It compiles. The diff looks reasonable. You merge it.

And then there's the quiet doubt: did it actually get the hard 30% right — auth boundaries, payments, tenant isolation, the booking logic that stops two people taking the same slot? Because AI doesn't usually write obviously bad code. It writes plausible code. And plausible-but-wrong is the expensive kind — it passes review and breaks in production on day three.

(The data backs the doubt: 84% of devs use AI tools, only 29% trust the output, and 45% of AI-generated apps ship an exploitable vulnerability — Veracode, 2025.)

So I ran an experiment: build a real app with an agent, then deliberately make the mistake an agent makes every day, and see what — if anything — catches it.

Build the app (one command)

npm create microservices-app@latest booking-demo -- --template booking-sveltekit

A full Cloudflare SvelteKit booking app — public flow, admin, D1, auth. The detail that matters for this experiment: it ships its own contract into the repo — README.agent.md, docs/api-boundary.md, and an executable spec, microservices.check.mjs. The layering rule is one line: routes are thin adapters; domain logic lives in verified modules, not in your handlers.

Baseline:

$ microservices check
Template checks: pass

Now break it the way an agent would

The request an agent gets constantly: "simplify the bookings endpoint." So I did the eager-agent thing — inlined the write straight to the DB and dropped the module:

// src/routes/api/bookings/+server.ts — the "simplified" version
export const POST: RequestHandler = async ({ request, locals }) => {
  const body = await request.json();
  await locals.bookingRepository.insert({
    serviceId: body.serviceId,
    startsAt: body.startsAt,
    customerId: body.customerId
  });
  return json({ ok: true });
};

It type-checks. It runs. It would pass review. And it silently drops the slot-conflict guard the verified createBooking use case enforced — a double-booking waiting to happen. Classic plausible-but-wrong.

Then I ran the check:

$ microservices check
Error: One or more generated app checks failed.

$ microservices check --json
FAIL: spec:src/routes/api/bookings/+server.ts
      — Booking API route stays a thin adapter over createBooking and injected repositories.

It named the exact file and the exact contract I broke — not a vague lint warning, but "you bypassed the verified booking use case." Restore the delegation to the module, and:

$ microservices check
Template checks: pass

Green. The slot-conflict protection is back where it belongs.

The pattern (this is the part that's yours, tool or not)

Forget my tool for a second — the transferable idea is this:

The fix for plausible-but-wrong isn't a smarter model. It's a boundary your agent can't cross without a named, machine-readable failure.

Three moves you can apply on any stack:

1. Push the dangerous 30% behind a real boundary — auth, payments, domain integrity in a module/package with a tested API, not regenerated inline every time.
2. Write a contract check that asserts the boundary held — "this route only calls the use case," "this handler imports the verified module." A few assertions beat a thousand eyeballed diffs.
3. Put that check in the agent loop — run it after every edit. Let the agent move fast on the safe 70% (the adapter/UI layer); make the 30% fail loudly when it's touched.

You can roll this yourself with a test file and a grep. I happen to ship it as a contract + check for Cloudflare apps — but the move is the move.

What I verified vs. what you'd run

I ran the scaffold → contract → check → break → fix loop above for real. The parts that need your own machine — npm install, npm run dev, a deploy — are yours to run; I'm not going to claim outputs I didn't produce:

npm create microservices-app@latest booking-demo -- --template booking-sveltekit
cd booking-demo && npm install
npm run microservices -- check     # the gate — wire it into your agent loop
npm run dev

(If you ship apps for clients on Cloudflare, the same gate is what lets you hand the result to a security review without the 2am call — but that's a different post.)

Repo + the rest of the modules: https://microservices.sh

Genuinely curious: how are you keeping your agent from quietly rewriting the dangerous 30%? Contract tests, review checklists, just vibes? What's caught a plausible-but-wrong change for you — and what slipped through?