AI coding tools such as Cursor and Claude Code speed up website work, but the most common mistakes come from treating their output as finished code. Teams skip review, let the agent touch too many files at once, ignore performance and SEO side effects, and work without version control. Each of these is avoidable with a few working habits.
Cursor is an AI-first code editor built on the VS Code codebase, with chat, inline edits and an agent mode that can change several files. Claude Code is Anthropic’s agentic coding tool that runs in the terminal, reads a repository, runs commands and proposes edits. Both are capable. Both will also confidently produce code that looks right and quietly breaks something you did not ask about.
Why do AI-built websites break in production?
Most failures are not syntax errors. The code runs. The problem is what the change did around the edges: a template that lost its canonical tag, a hero image that now loads without dimensions, a form that stopped sending its tracking event. An agent optimises for the task you described, so anything you did not describe is unprotected.
The second cause is scope. A request like “clean up the header” can turn into edits across twelve files. When the diff is that large, nobody reads it properly, and small regressions slip through.
Mistake 1: working without a safety net
The single most expensive mistake is letting an agent edit a project that is not under Git, or committing everything in one giant commit at the end of the day. If the change breaks the checkout page, you need to know exactly which edit did it.
- Commit before every agent session, so you always have a clean point to return to.
- Ask for one change per session or per commit, with a short message describing it.
- Keep a staging copy of the site; never point an agent at the live server.
- In Claude Code, keep permission prompts on for shell commands that write outside the project or touch deployment.
Mistake 2: accepting large diffs without reading them
Cursor shows proposed edits inline, and Claude Code shows diffs before applying them. The temptation is to accept because the summary sounds right. Summaries describe intent, not the actual change. Read the diff, especially deletions.
A practical rule we use: if a diff is too long to read in five minutes, it is too long. Ask the tool to split the work into smaller steps. It will, and the review becomes realistic.
Mistake 3: forgetting performance budgets
Agents add libraries readily. Ask for a carousel and you may get a 90 KB dependency where twenty lines of CSS would do. They also tend to write images without width and height attributes and load fonts without thinking about render blocking.
Google’s Core Web Vitals give a clear target: Largest Contentful Paint under 2.5 seconds, Interaction to Next Paint under 200 milliseconds and Cumulative Layout Shift under 0.1. Put those numbers in your project instructions file (Cursor rules or a CLAUDE.md file for Claude Code) so the tool sees them on every task, and run Lighthouse or PageSpeed Insights before merging.
Mistake 4: breaking SEO without noticing
Search problems rarely show up in a visual check. The page looks identical; the head section does not. Common regressions after an AI edit include:
- A noindex meta tag copied from a staging template into production.
- Canonical URLs pointing to the wrong path after a routing change.
- Headings reshuffled so a page has two H1 elements or none.
- Structured data removed when a component was rewritten.
- Internal links changed from real anchors to JavaScript click handlers that crawlers cannot follow.
The fix is a short automated check. A script that fetches key templates and asserts the title, canonical, robots tag and JSON-LD are present catches most of these in seconds.
Mistake 5: trusting invented APIs and outdated syntax
Language models sometimes call functions that do not exist, or use a framework API that changed two versions ago. In WordPress this can look like a hook name that is almost right; in Shopify it can be deprecated Liquid. The code may even fail silently.
Ask the tool to cite the documentation it relies on, run the linter or theme checker, and test on a real browser. If an answer includes a function you do not recognise, search for it before accepting it.
Mistake 6: pasting secrets and client data into prompts
API keys, database dumps and customer exports do not belong in a chat window. Keep secrets in environment files that are excluded from the context the tool reads, use test data for debugging, and review the tool’s data settings. Both Cursor and Anthropic publish privacy and retention options at the time of writing; read them for your plan rather than assuming.
How Alien Road applies AI coding tools on client websites
In a typical web build, we start with discovery that is not about the tool at all: page inventory, current Core Web Vitals from field data in Search Console, tracking events that must survive, and SEO elements per template. That list becomes the project instructions file the agent reads on every task.
Setup comes next. The repository goes under Git with a staging environment, a linter, and a small check script for titles, canonicals, robots tags and schema. Agent sessions are scoped to one ticket. A developer reviews each diff and runs the checks before anything merges.
Measurement is the part clients see. We compare Lighthouse scores and field data before and after, confirm analytics events still fire in the debug view, and crawl staging with Screaming Frog to compare indexable pages. Reports list what changed, what was tested and what was left alone on purpose. The tool writes code faster; the process is what keeps the site stable.
A short pre-merge checklist
- Diff read in full, including deleted lines.
- Lighthouse run on the changed templates, with no Core Web Vitals regressions.
- Title, meta description, canonical, robots and structured data present.
- Forms submitted on staging and tracking events visible in analytics debug mode.
- No new dependency added without a stated reason.
- Accessibility basics checked: alt text, focus states, colour contrast.
Where to start this week
Pick one small, real ticket, such as fixing layout shift on the blog template. Commit first, write a short instructions file with your performance and SEO rules, and run the task in Cursor or Claude Code with diff review on. Time the whole thing, including review and checks. That single measured run tells you more about how these tools fit your team than any demo.
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