Cursor vibe coding and the verify-before-commit loop every other guide skips
Most guides on this topic hand you a prompt library, a .cursorrules template, or a list of composer shortcuts. None of them deal with the structural reason vibe coding breaks in Cursor: the agent cannot open the running app and confirm it works. This page walks through the missing loop with real file paths from a working implementation.
The part every Cursor vibe coding guide skips
If you line up the most popular articles on this topic, they cover the same four things: a shortlist of prompt patterns for agent mode, a .cursorrules file you paste at the top of the project, advice to break the work into small steps, and a reminder to commit often. Good advice, all of it. None of it answers the one question that actually breaks the vibe: after the agent writes the code, who checks that the screen still works?
In Cursor the answer is you. The agent writes files, the diff gets accepted, the dev server hot-reloads, and the next step in the loop is your eyes on the preview. When the preview is wrong, you paste the error back into chat. When it is right, you type the next thing. The vibe is only as long as your tolerance for round-tripping errors through a composer.
The fix is not a better prompt. The fix is an agent that can reach its own output. The rest of this page shows how that works, with file names, ports, and the exact sentence in the agent's config that enforces it.
The vibe loop, drawn two ways
Agent writes files in your repo, then stops. You open the preview on your laptop, see the error, copy it, paste it back into the composer. The agent cannot see what you saw. Every broken turn costs you a context switch.
- Agent writes, you run
- Preview lives on your laptop, out of agent reach
- Errors travel via copy-paste
- No verified state between turns
The exact sentence that closes the loop
When an mk0r session starts, the VM template injects two CLAUDE.md files into the sandbox. The global one lives at /root/.claude/CLAUDE.md, the project one at /app/CLAUDE.md. Both are defined as template literals in src/core/vm-claude-md.ts and baked into the E2B template image, so the contract is the same on every cold boot.
The relevant passage is short. It is the thing that turns a normal chat agent into one that refuses to say done on a broken page.
That last line is the whole difference. It is not a prompt you supply at session start; it is a contract the agent reads through Claude Code's memory discovery on every turn. Playwright MCP on port 3001 is not a tool the agent can forget to call, because the contract says the turn does not end until the snapshot passes.
The ports that make the contract enforceable
A contract is only as good as the wiring behind it. In Cursor the contract cannot exist because the preview is on your laptop and the agent has no address to reach it. In mk0r every service the contract mentions is already running inside the same VM when the agent wakes up, because /opt/startup.sh booted them before the ACP bridge ever accepted a connection.
How one turn flows from prompt to git SHA
This is the piece that no Cursor article can draw, because the corresponding wires do not exist on your laptop. Read it once top to bottom. Every arrow is a real call; every actor is a real process listed in the terminal above.
One turn, from composer to commit
One turn, step by step
The commit function, in full
The verify step is the interesting half; the commit step is the part that makes the verification durable. The full function lives at src/core/e2b.ts starting at line 1588. The shell script it runs inside the VM is six lines long, and every line matters.
Notice the NOCHANGE branch. If the agent's turn did not actually touch any file, the function returns null instead of an empty commit. That means your undo stack stays honest: every SHA in historyStack corresponds to a real, verified change the agent just proved worked in the browser.
The friction math
port exposed from the VM (3000)
second cold boot from pooled snapshot
installs on your laptop to start
Cursor asks your machine for a dev environment. mk0r rents one, already warm, every time you open the URL.
When Cursor is still the right call
This page is not arguing Cursor is bad. It is arguing that Cursor is optimized for a different phase than vibe coding. Here is how the split lands in practice.
Cursor wins at line-level edits
You have a repo, tests, a CI pipeline, and a pull request mindset. You want AI to refactor one function, rename a symbol across files, or explain a stack trace. The editor form factor is doing exactly what it should.
mk0r wins at the first twenty turns
The shape of the app is still unclear. You want to describe a screen, see it, click around, describe the next screen. You do not want to think about imports. The agent's verify-then-commit loop turns a loose idea into a working preview fast.
Hand-off works in one direction
Vibe the first version in mk0r, export the /app project as a standard Vite + React + TypeScript repo, continue in Cursor when the work becomes code-review-shaped. Going the other way is harder.
Demoing to a non-coder
mk0r is a URL. You can paste a link, the recipient watches the build happen, and they can click inside the live preview. Cursor cannot do that without a deploy step.
Production code review
Neither is a substitute for human review on auth, payments, or anything touching real users. That has not changed.
What the first minute looks like
The whole point is that there is no setup. You open a URL and a VM is already warm. The list below is every action that happens between your first prompt and a verified first commit.
From prompt to verified commit
Open mk0r.com and describe the app
No account wall. Type or speak. Be concrete about who the user is and what they do, not about the implementation.
Warm VM resumes in ~3 seconds
A pooled E2B snapshot with Chromium on CDP 9222, Playwright MCP on 3001, Vite on 5173, and the ACP bridge on 3002. startup.sh has already run.
Agent edits /app/src
Writes a component, wires it into App.tsx, saves. The dev server HMR-reloads localhost:5173 inside the VM's Chromium.
Agent snapshots its own preview
Via Playwright MCP on 3001, bound to the same CDP endpoint as the user's live view. Reads the DOM, checks browser_console_messages, fixes any error it sees in the same turn.
commitTurn captures the working state
cd /app, git add -A, git commit -q, git rev-parse HEAD. The SHA is appended to historyStack in Firestore. You can undo to the previous SHA with one click if the next prompt goes sideways.
What runs inside the VM, at a glance
The moving parts are small and named. Every piece below has a line in startup.sh, or a file in docker/e2b/files/opt/, or a function in src/core/. No magic.
The quiet takeaway
Cursor's agent mode is good at a specific job: editing code in a repo you already own. Vibe coding is a different job. It is about describing behavior and demanding a working preview, not a clean diff. The two jobs need different loops. A verify-before-commit loop needs three things Cursor cannot give you on a laptop: a browser the agent can reach, a test tool already wired to that browser, and a git layer that checkpoints only after the agent has confirmed the screen. Everything on this page is a concrete answer to those three requirements, with file paths you can open and read.
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What is Cursor vibe coding, in one paragraph?
Cursor vibe coding is the practice of using Cursor (a VS Code fork with a strong agent mode) to build working software by describing behavior in chat instead of writing code line by line. You stay in the composer, you accept or reject diffs, and the agent writes the code. It works well until an iteration produces a broken preview; at that point the agent cannot check its own work, so the loop stalls on you.
Why does vibe coding feel fragile around turn five or six in Cursor?
Because Cursor's agent stops at save. It can write ten files in one turn, format them, and say 'done', but it does not open the running app to confirm the DOM rendered or the console is clean. So every few turns you hit a bad build, switch back to the preview, manually copy the error back to the chat, and lose the vibe. The fix is not a better prompt: the agent needs a way to test what it just wrote.
What does mk0r do differently on the same loop?
The agent in an mk0r VM runs Playwright MCP (port 3001) bound to the same Chromium the user watches over CDP on port 9222. Its injected CLAUDE.md in /root/.claude/CLAUDE.md instructs it, verbatim, to 'Navigate to http://localhost:5173 via Playwright MCP, take a snapshot to verify the DOM rendered correctly, check browser_console_messages for runtime errors' and ends with 'Do not report completion until the browser shows the expected result.' The turn only ends after a snapshot passes.
Where do the per-turn git commits come from?
From commitTurn in /Users/matthewdi/appmaker/src/core/e2b.ts (lines 1588 to 1642). When a turn reports completion, the host runs a shell script inside the VM: cd /app, git add -A, if git diff --cached --quiet exit with NOCHANGE, git commit -q -m '<turn message>', then git rev-parse HEAD. The resulting SHA is appended to the session's historyStack in Firestore. Undo/redo moves an activeIndex across that stack.
Can I still use Cursor if I like it? Do I have to pick one?
You can use both. mk0r produces a standard Vite + React + TypeScript project in /app with real git history. Once the shape of the app is locked in, export it and open it in Cursor for line-level edits, PR review, or integration work. mk0r is optimized for the early vibe phase where verification matters most; Cursor is optimized for the late-stage code phase where you care about diffs.
Why is the verification loop so hard to build on top of Cursor?
Cursor runs on your laptop, so the preview is your laptop's browser, not an environment the agent can reach. Giving the agent CDP access to your local Chrome is doable but invasive (requires launching Chrome with --remote-debugging-port, exposing it, and trusting the agent with your real browser profile). mk0r sidesteps this by running Chromium inside the E2B sandbox on an Xvfb display and exposing exactly one port (3000) back to the user's tab. The agent talks to that Chromium directly; the user's laptop is never part of the trust boundary.
What exact ports run inside an mk0r VM for this to work?
Documented in docker/e2b/files/opt/startup.sh: Xvfb on display :99, Chromium on 9222 (CDP), x11vnc on 5900 and websockify on 5901, Playwright MCP on 3001 (pointed at --cdp-endpoint http://127.0.0.1:9222), Claude ACP bridge on 3002, Vite dev server on 5173, and the reverse proxy at /opt/proxy.js on 3000, which is the only port the outside world can reach.
Does this mean I can vibe code without installing Cursor at all?
Yes. mk0r is a URL. Open mk0r.com, type or speak the first prompt, and the VM wakes from a pooled E2B snapshot in about three seconds with Chromium, Vite, Playwright MCP, and the Claude ACP bridge already running. No VS Code fork, no Node toolchain, no .cursorrules file. The CLAUDE.md contracts are injected into the snapshot at template build time so they apply on turn one.