The Codex Build Sheet
Free one-pager · idea → working tool with Codex
Stop renting software. Start building your own.
One page that takes you from “I need a tool” to a working internal tool with OpenAI Codex. The build loop, the exact scoping prompts, the commands that matter, and the 30-second build-vs-buy gut check.
› scope the smallest tool to track my client projects
✓ wrote PLAN.md · 3 screens, SQLite, billing out of scope
› build step 1 from PLAN.md
● creating app/page.tsx …
● creating lib/db.ts …
✓ step 1 ready. review the diff?
You don’t need another $200/month tool.
You need the one thing it does that you actually use. Codex lets you build exactly that. A tracker, a quote builder, an internal dashboard, shaped around how you really work.
This sheet is the shortcut from idea to shipped.
What’s on the sheet
Everything you need on one page.
The 4-step build loop
Scope → plan → build → verify. The repeatable loop that takes you from a vague idea to a working tool without going in circles.
The scoping prompts
Copy-paste prompts that turn “I need a tool” into a clear spec Codex can actually build. The exact words I use.
The commands that matter
Setup, running it, controlling how much it can touch, and the in-session commands you’ll reach for every day. Just the ones you’ll actually use.
Build vs buy gut check
A 30-second test for when to build your own and when to just pay for the tool. Saves you from building the wrong thing.
Pro moves & gotchas
AGENTS.md for project context, commit-often discipline, plan-before-code, and the traps that sink most first builds.
One printable page
Built to live next to your keyboard. Front: the build. Back: the commands. Print it, pin it, ship.
The core of the sheet
The 4-step build loop.
The difference between a tool that ships and a chat that goes in circles is the loop you run. This is it.
Scope
Get clear on the smallest tool that solves the real problem. Build the one thing, not a platform.
Plan
Make Codex write the plan to a file first. Read it, fix it, then let it build. Planning beats prompting.
Build
One feature at a time. Review every diff. Small steps keep you in control instead of chasing a runaway.
Verify
Run it. Break it. Tell Codex exactly what’s wrong and what it should do. Repeat until it’s right.
Also on the sheet
The 30-second build-vs-buy gut check.
Building your own isn’t always right. The sheet gives you a fast way to call it.
Build your own when
- You’re paying per seat for features you never touch
- Your process keeps bending to fit the software
- Nothing off the shelf actually matches how you work
- You need 20% of a $200/mo tool
Just buy it when
- A great tool already fits how you work
- The cost stays predictable as you grow
- It’s a solved problem (auth, payments, email)
- You’d spend more time maintaining than using it
Made by
Aaron Rountree
I run DevLaunch and help businesses stop overpaying for software that fights them. I build simple internal tools with Codex and Claude that match exactly how a team works. This sheet is the condensed version of how I do it.
When you want the full thing
The sheet is the 10-minute version.
The full 60-minute walkthrough is the Build vs Buy masterclass. You audit your process, scope the right tool, and build it live with Codex and Claude. Grab the free sheet first. The class is there when you’re ready.
Quick questions
Yes. The PDF is free and the follow-up emails are free. No paid wall on either.
Codex is OpenAI’s command-line coding agent. You tell it what you want in plain English and it writes and edits the code for you. The sheet covers exactly how to drive it.
No. The whole point is that you direct the agent and it does the building. The sheet is about thinking clearly about what to build, then driving Codex to ship it.
Inbox in about a minute. If you don’t see it, check Promotions or Spam and reply once so future emails land in primary.
No. A few short emails over about ten days, then nothing scheduled. One-click unsubscribe in every email.
There isn’t one. If the sheet clicks and you want the full 60-minute walkthrough that audits your process, scopes the right tool, and builds it live, that’s the Build vs Buy masterclass. Totally optional.
Build the tool you keep paying for.
Grab the free Codex Build Sheet and go from idea to a working internal tool, without writing the code yourself.