AI Coding Fails Without Design First

A lot of AI coding problems start before you ever open the chat. You type "Build a login system for my app", read what comes back, and wonder why it feels generic, why it doesn't fit the project you already have, why it needs so much fixing.

The problem usually isn't the AI. You didn't give it enough to work with.

The Prompt That Guesses

Take a basic one:

Create a user authentication system for my app.

It reads like a finished instruction. It isn't. Every decision that actually shapes the code is missing from it, and the model can't leave those decisions blank — it has to fill them in from the most common patterns it has seen. You get an answer built for an average project, and yours is not the average project.

What The AI Has To Guess

• Which database. Postgres, MySQL, Mongo — the schema, the queries, and half the error handling change with the answer.
• Which backend. A Node and Express codebase and a Django one want different shapes for the same feature.
• How your project is already structured. Where things live, what your modules are called, which conventions the rest of the code already follows.
• What kind of authentication you want. Sessions, tokens, a hosted provider — three different systems wearing the same word.
• What your security requirements are. Password hashing, token lifetimes, how strict the login path has to be.
• What happens at the edges. Refresh tokens, roles, rate limits — the parts that decide whether the thing survives contact with real users.

None of that is exotic. It's just work you hadn't done yet, so the model did it for you, quietly, and you inherited its defaults.

The Same Request, Designed

Now compare it with a prompt written after actually thinking the system through:

Build a JWT-based authentication system for my Node/Express + PostgreSQL app. The users table has email, hashed password, and role (admin/user). Implement signup, login, refresh-token rotation, and rate limiting for login attempts. Follow the existing project structure and keep authentication inside the "/auth" module. Don't use third-party authentication providers.

Same AI. Same model. Completely different result. The second prompt isn't really a request — it's a small system design, written down.

Weak Prompt Versus Designed Prompt

What it settles	"Create a user authentication system"	The designed version
Stack	Left to the model	Node/Express and PostgreSQL, stated
Data model	Invented	Email, hashed password, role
Scope	Open-ended	Signup, login, refresh rotation, rate limiting
Fit with your code	Ignored	Existing structure, inside the "/auth" module
Exclusions	None	No third-party providers

The right-hand column is not longer for the sake of being longer. Every line in it is a decision that would otherwise have been made without you.

How I Approach It Now

1. Think about the system first. Even a rough sketch is enough. What are the components, how do they talk to each other, where does the data go.
2. Understand the things you don't know. If you're unsure about something, research how it applies to your specific system, not just the general definition of it.
3. Give the AI the context. Your stack, your existing structure, your requirements, your constraints, and the things you specifically don't want.
4. Let the AI build. Then compare what it produced against the design you had in mind, instead of accepting it because it runs.

What You Are Not Building

Before you ask for anything, four questions: What am I building. How should the pieces connect. What are my constraints. And what am I not building.

That last one is underrated. Most of the confusion in an AI-written feature comes from scope nobody bounded — extra abstractions, a provider you never wanted, an admin panel you didn't ask for. Naming the exclusions costs one line and saves an afternoon.

AI can give you incredible speed. It can't know the decisions you never made.

Think through the system first, then let AI handle the execution.

So which one are you doing — designing before you prompt, or prompting and hoping?