AI Agents for Web Development: Specs, Changes, and Checks

AI agents can turn a page requirement into a working implementation, help with component refactoring, and flag obvious accessibility or performance problems. Getting consistent results from session to session means giving each agent the same design constraints and task context it needs to make good decisions. Tools like fenbs keep that project record outside any one chat session, so when a tool or model changes, the agreed requirements travel with the work.

Web development involves many small decisions: which component handles state, how an API error is displayed, whether a layout uses grid or flex, which breakpoints apply to a specific module. Those decisions accumulate. An agent that does not know them will re-derive them from general practice, which may conflict with what is already in the codebase.

Starting from page requirements

Write the page requirement before the agent touches any code. A requirement describes the user goal ("A visitor can submit a contact form and receive a confirmation"), the constraints ("no more than three fields, no CAPTCHA"), and the acceptance criteria ("the form clears and shows a success message within two seconds of submission"). That description is what the agent works toward, and it is what you use to verify the result.

State what already exists and what the agent should leave alone. If the form component already handles validation, tell the agent so. If the API endpoint exists and should not be changed, say that explicitly. Agents that are not told what is settled will sometimes re-implement or restructure things that were working, which introduces risk without a clear benefit.

Separate requirements from design notes. A requirement is a condition the implementation must meet. A design note is context that helps the agent understand the product ("this is a B2B SaaS, so the form is used by people who know the domain"). Both are useful, but mixing them makes requirements harder to check.

Scoping changes to one component at a time

Give each agent session a single component or a single behavior to implement. "Build the contact form" is better than "build the contact page." "Fix the mobile layout for the navigation menu" is better than "fix responsive issues across the site." Focused sessions are easier to review, easier to test, and easier to revert if the result is wrong.

Provide the agent with the relevant existing code: the component it will modify, the parent it sits in, and the data shape it receives. Asking an agent to work on a component it cannot see will produce code that makes assumptions about the surrounding context, and those assumptions may be wrong.

After each scoped change, run the development server and check the component in a browser. Check it on a phone screen size as well as a desktop. A layout that looks correct in a tool preview may have spacing or overflow problems at smaller widths.

Browser checks and visual review

An agent cannot see what the browser renders. Visual review is a step only a person can do. Check the output against the acceptance criteria: does the form clear after submission? Does the confirmation message appear? Is the layout correct at the sizes the requirement specifies? Write down what you checked and what the result was.

Also check what the agent did not mention. Tab through the form with a keyboard. Check the color contrast of text over a background. Resize the window slowly and look for layout breaks. Agents produce code that works in the common case; edge cases in accessibility and responsive behavior need deliberate review.

Managing design constraints that must not drift

Design decisions that apply across the project — the color system, the component library in use, the spacing scale, the breakpoints — need to be written down somewhere an agent can read them. If they live only in a design file, the agent will not see them unless someone pastes the relevant details into the session.

Write a short constraints document in the project record: the component library and version, the naming conventions for CSS classes or design tokens, the file structure rules, and any patterns the team is actively migrating away from. Update it when the team makes a decision that changes it.

When an agent suggests an approach that conflicts with a constraint — for example, using a layout method the team agreed to phase out — reject the suggestion and add the constraint to the shared record if it is not already there. That is the most efficient way to enforce constraints across sessions.

Handing off work between sessions

Web development work rarely finishes in one session. Before you stop, ask the agent to update the task with what it completed, what it left open, and what the next step is. Include any edge cases it noticed but did not address, and any decision points that need input from a designer or a product owner.

When the next session starts — whether it is a different tool, a different model, or just a fresh chat — ask the agent to read the project context and state where it will start before it changes any code. That check catches misunderstandings before they result in a component being rebuilt from scratch.

Managing build configuration and CSS tooling

Build configuration changes — adding a new bundler rule, adjusting a PostCSS plugin, modifying a Vite or webpack config — affect every file in the project, not just the one you are working on. Before asking an agent to modify the build config, describe what the change should accomplish and what the current setup already handles. An agent that rewrites a config to add one option may inadvertently remove settings that were serving other purposes.

CSS tooling choices accumulate into decisions that are hard to reverse: whether the project uses a CSS framework, a design token system, a preprocessor, or utility classes. If those choices are not in the session instructions, the agent may suggest a CSS approach that conflicts with what is already in the codebase. Write the CSS tooling choice in the project record once and reference it across sessions rather than explaining it from scratch each time.

Dependency updates on the web front end carry the same risks as elsewhere. A major version bump to a UI library may change component APIs, default behaviors, or how the library handles accessibility. Ask the agent to summarize the relevant changes before suggesting how to update your usage, then review the diff against both the old API and the new one to confirm that the behavior matches expectations.

When a build change causes an unexpected problem — an asset fails to load, a chunk boundary shifts, a source map stops working — tell the agent exactly what worked before and what the error is now. Include the specific error message and the relevant section of the build output. A focused description of what broke produces more useful suggestions than a general request to fix the build error. Record the root cause and fix in the project record so a later session does not repeat the same investigation.

How to do this with fenbs

Start with one page and one page requirement. Write it in a fenbs task, connect your coding agent, and run the check-then-save loop for one complete feature. Use that experience to define the constraints and process for the rest of the project.

1. Create a free account. Add the web project with a short description of its stack, the component library in use, and the main pages or sections.
2. Add the first page requirement as a task. Include the user goal, the constraints, and the acceptance criteria. Add any open questions as sub-notes rather than leaving them implicit.
3. Write the design constraints in Decisions and rules: component library, naming conventions, layout patterns, responsive breakpoints, and patterns the team avoids. Mark each as a standing rule.
4. Connect your coding agent at Integrations. Ask it to read the project context before starting. Check that it describes the constraints correctly before it touches any code.
5. After each component session, run a browser check against the acceptance criteria. Ask the agent to update the task with what changed and what is next. Repeat the loop for each subsequent task.