The agent rewrote my working page.tsx again

I timed it.

Fourteen minutes. One prompt: disable the save button while the Server Action is pending.

What I got back:

• 'use client' at the top of app/(dashboard)/invoices/page.tsx
• useEffect fetching /api/invoices for data the server already had
• as User on res.json()
• a new useInvoiceStore.ts nobody asked for
• the save button still not disabled

The original file was 78 lines, compiled, and rendered the list. The agent replaced it because a client event handler is illegal in a Server Component and the cheapest local move is "make the whole file a client." That move costs a waterfall, a cache miss, and an auth check that now lives wherever the fetch happens to run.

I started counting the pattern in my own sessions over a week of after-hours work:

Failure	Times I undid it
use client lifted to the page	11
formData.get(id) as string / String(formData.get())	9
Edited components/ui/button.tsx to add a product variant	4
Middleware redirect treated as authorization	3
Deleted a test because it failed after a rename	2

None of those are model-capability problems. They are missing constraints. Cursor will follow a 40-line .mdc file with globs on app/**/*.tsx more faithfully than it will follow a 2,000-word blog post you pasted into the prompt once.

So I wrote the constraints down as files the agent actually loads:

• architecture — RSC default, client at the leaf, no function props across the boundary, schema on searchParams / formData
• typescript — no any, parse unknown, branded ids, discriminated UI state
• ui — wrap shadcn, do not edit components/ui, four states on every list
• testing — getByRole first, no page snapshots, Server Actions tested as functions
• git / PRs — no git add everything from root, conventional titles, .env.example only
• security — secrets not in git, authz inside the action, no localStorage sessions

Plus six skills the agent can run on demand: plan before edit, do not rewrite working files, copy pass, a11y pass, PR description, launch checklist.

Install is not a package. You copy the rules folder and AGENTS.md into the Next.js app. Sixty seconds. The point is not more prose — it is fewer sessions that end with git checkout -- app.

If you want the pack: Cursor Agent Ship Kit (USD 19, personal license, refund via Getly).

If you only want the habit: next time the agent wants to recreate a file that already works, the right patch is three lines on the existing Button, not a new folder.

Comments

[Dmitry Sergeev]

tbh seeing it take 14 mins to just disable a button is kinda wild