How to Write a Build Brief for an AI Coding Agent

How to Write a Build Brief for an AI Coding Agent

A useful AI build brief tells the agent who needs what, what observable behavior will count as correct, what it may change, what it must not touch, and what evidence a human will review.

The brief does not need to predict every line of code. It needs to make ambiguity visible before fast generation buries it inside an implementation.

Start with the user job

"Build a task app" names an object. It does not name a job.

A better opening is: "A single user needs to capture a short task, mark it complete, and recover it after refreshing the browser. This build is a private local prototype using synthetic data."

That sentence supplies a user, an outcome, persistence behavior, and a maturity boundary. It also prevents the word "app" from silently expanding into accounts, teams, notifications, analytics, payments, or public hosting.

Current Replit Agent guidance recommends being specific, planning the work, adding context, reviewing and testing, and using checkpoints. Those habits begin with a clear job, but a release-quality brief needs more than a good prompt.

Define the fields that make behavior reviewable

Use the brief as a small contract between the builder, the agent, and the reviewer.

The table is not a request to write a long document. A tiny prototype may answer each field in one sentence. A risky service may need linked specifications, a threat model, a privacy assessment, and domain review.

Describe every state the user can reach

Many generated interfaces look complete because the happy path is visible. Their missing behavior appears when the data is empty, input is wrong, the network is slow, a request fails, or access is denied.

For the small task prototype, the brief should say what appears before any tasks exist, while stored tasks load, after a task saves, when the text is blank, when storage fails, and after a user retries.

This turns "make it work" into a finite set of observable states. It also gives the agent a better basis for a plan and gives the reviewer places to look for invented assumptions.

flowchart TD
    A["User job"] --> B["Scope and boundaries"]
    B --> C["States and data"]
    C --> D["Rules and failure behavior"]
    D --> E["Acceptance evidence"]
    E --> F["Agent plan"]
    F --> G["Human review before implementation"]

Write acceptance cases before implementation

An acceptance case should name the starting state, action, expected result, and forbidden side effect.

"The form validates correctly" is too vague. A stronger version is: "When the task field contains only spaces and the user chooses Add, show an error beside the field, move focus to the error, keep the existing tasks unchanged, and make no storage write."

Another case might say: "When the user marks the second task complete, preserve the original order, expose the completed state to assistive technology, save the change, and show the same state after refresh."

Plain language is enough if the behavior is observable. The agent can propose automated tests, but the builder should not let the implementation define its own acceptance after the fact.

State the agent's authority

A coding agent can often read files, write code, install packages, run commands, use a browser, contact services, modify a database, or deploy. The brief should say which actions are allowed.

For the task prototype, authority might be limited to one local repository and one development command. It might prohibit new dependencies, network requests, secrets, telemetry, account systems, infrastructure changes, and deployment.

If the work later needs one of those actions, the plan should stop and surface the decision. Silence should not become permission.

This matters even when the agent is accurate. A technically correct command can still act on the wrong environment, expose data, create cost, or exceed the builder's authority.

Keep security and accessibility inside the brief

Security is not a scan added after the interface looks finished. The brief should identify trust boundaries, sensitive data, identity, authorization, input handling, secrets, external calls, logging, abuse cases, and recovery in proportion to risk.

The NIST Secure Software Development Framework supports tracking security requirements, design decisions, provenance, review, and vulnerability response across the lifecycle.

Accessibility is also behavior. A web brief should name keyboard operation, focus movement, semantic controls, accessible names, error association, contrast, zoom, and any applicable conformance target. W3C's WCAG 2.2 quick reference is the primary route for the standard, but a qualified review still needs to interpret it for the product.

Ask for a plan and a handoff

Before code changes, ask the agent to restate the job, identify unknowns, name the files and dependencies it expects to change, propose tests, flag risks, and explain anything outside its authority.

After implementation, require a concise handoff. It should identify the actual changes, commands run, test results, failures, human edits, dependencies, remaining risks, and rollback point.

Do not accept "done" as evidence. The agent may have completed its instructions while the product still fails the user's job.

A compact brief for a harmless prototype

The smallest useful version can fit into a page.

The user is one person tracking local tasks. Success means adding, completing, deleting, and recovering tasks after refresh. The build may change only the files inside a clean prototype repository. It may use browser storage and synthetic text. It may not add accounts, network access, analytics, payments, secrets, external services, or deployment.

The interface must represent empty, saved, invalid, storage-failure, and recovered states. Every control must work by keyboard and expose its name and state. Acceptance cases cover blank input, a valid task, two tasks in order, completion, deletion, refresh, simulated storage failure, and retry.

The agent must propose a plan before editing, add no package without approval, run the agreed tests, summarize the diff, name unresolved issues, and leave a reviewed rollback point.

That brief is more useful than a long adjective-filled prompt because it tells both the agent and the reviewer what the build means.

The brief ends when accountability begins

High-stakes, regulated, private, financial, health, safety, or customer-facing systems need qualified product, engineering, security, privacy, legal, accessibility, and domain owners. A template cannot grant authority or replace their judgment.

The purpose of the brief is simpler. It keeps fast generation attached to an explicit user job and makes the human acceptance decision visible.

This page was developed with AI assistance from the E057 transcript and linked primary sources, then structured for human product, engineering, security, accessibility, and editorial review. It does not authorize an agent to change code, data, infrastructure, or a live environment.