v0 by Vercel: how to review generated UI code before you paste it

Published 2026-04-26 · 5 min read

v0 by Vercel generates production-ready React components from a text prompt. Describe the UI you want — a login form, a data table, a settings panel — and v0 returns Shadcn UI + Tailwind CSS code that renders immediately in a polished live preview. One click copies the component to your clipboard. Paste it into your project.

The problem is that “renders correctly in the v0 preview” and “is ready to ship” are not the same thing, and v0’s workflow makes it harder than usual to close that gap before the code is already in your codebase.

Why v0’s attention problem is different

Most AI coding tools keep the code in the center of the experience. Aider prints diffs in the terminal. Continue.dev shows inline diffs in your editor. Cline requests approval before each write. Even Bolt.new and Replit Agent generate full applications but expose a Files tab and console output alongside the preview.

v0 centers the preview. The Code tab is available, but the default state — the thing your eye lands on when generation completes — is the rendered component. And the rendered component, styled by Shadcn’s design system and Tailwind’s utility classes, looks exactly like the kind of polished UI you would expect to ship. That visual quality is doing a job that code review is supposed to do, and it’s doing it badly.

The three v0 attention traps

1. The design system makes bad code look good

Shadcn components render consistently and look professional regardless of what’s inside them. A Button with a missing aria-label looks identical to one that has it. A form that submits without client-side validation renders the same as one that validates correctly. A data table that crashes on an empty array looks the same as one that handles it gracefully — because the preview uses the hardcoded sample data that v0 generated alongside the component.

This is the design system trap: visual polish is decoupled from code correctness. The preview confirms that the component renders with the specific data v0 invented for demonstration. It says nothing about null props, empty arrays, very long strings, special characters, missing keys, or error responses from your actual API. Those are the cases where generated code breaks most often, and the preview systematically hides all of them.

2. Copy-paste is not a review decision

When you paste code into a file in your editor, the action feels deliberate — you chose to accept the change. But the cognitive weight of Cmd+C, Cmd+V is approximately zero. There is no diff to scroll, no “approve / request changes” toggle, no commit message to write. The clipboard operation bypasses every review ritual that editors and version control have trained into the workflow.

Compare this to Aider, where the edit appears in your editor as a diff and the next explicit action is accept or reject. With v0, there is no accept step. Pasting is accepting. That asymmetry means the review has to happen before you copy — at precisely the moment when the polished preview is telling your brain the work is already done. Without a deliberate interruption, copy follows preview as a reflex.

3. Refinement loops without a diff baseline

v0 encourages iteration. You paste the first version into your editor, see what’s off, describe the change, and v0 regenerates. The second version replaces the first. The third replaces the second. After four or five iterations the component looks closer to what you wanted — but you have no record of what changed between versions.

v0 does not show diffs between iterations. Each version is a complete file. If iteration three silently dropped the error handling that iteration two introduced — because the refinement prompt moved focus to styling and the model deprioritized the error state — the preview will not tell you. The component will render fine in the preview and break in production on the first empty API response.

Three fixes

Read the imports before copying

Before clicking Copy, scroll to the top of the Code tab and read the import list. Thirty seconds, every time. Each import is a dependency you are accepting. Unknown package names are a red flag — v0 occasionally generates imports for packages that do not exist, or for packages with slightly wrong names that resolve to something else entirely. Known packages you were not planning to add are a scope decision that deserves a conscious choice, not a clipboard operation.

Reading imports also creates a mental map of the component before you read the rest of it — which makes the next two fixes faster to apply.

Name one missing state before copying

Before clicking Copy, name one state the preview did not show you: empty data, error state, or loading state. Pick one, then scroll to the relevant part of the code and check that it handles it.

This is not a full review. It is a forcing function that interrupts the “preview looks good → copy” reflex and puts you in reading mode for at least one specific reason. That single specific check almost always surfaces two or three additional things the preview could not show you — because once you are reading the code rather than watching the preview, the review has actually started.

Search for placeholder text after pasting

After pasting but before running, search the file for TODO, placeholder, example, and your-. v0 sometimes generates structural stubs: comments that read // Add your authentication logic here, event handlers with empty bodies, or hardcoded data arrays that look functional because the preview uses them as-is.

These placeholders are invisible in the preview. The component renders, the interaction works against the sample data, and the stub sits quietly in the code until you hit the case it was supposed to handle. The search takes five seconds and catches the most expensive class of v0 output: the component that looks complete but is actually a scaffold.

v0 generates excellent starting points. Shadcn’s design system means you reach “looks right in the preview” faster than with any other tool in this series. The review work left after that — validating edge cases, replacing placeholder logic, confirming the real API contract — takes longer when the code is already in your project than when you check it first.

Thirty seconds of import-reading and one named missing state turns “paste → debug → fix” into “read → paste → ship.”

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