Harness engineering: leveraging Codex in an agent-first world

Over the past five months, our team has been running an experiment: building and shipping an internal beta of a software product with 0 lines of manually-written code.

The product has internal daily users and external alpha testers. It ships, deploys, breaks, and gets fixed. What’s different is that every line of code—application logic, tests, CI configuration, documentation, observability, and internal tooling—has been written by Codex. We estimate that we built this in about 1/10th the time it would have taken to write the code by hand.

Humans steer. Agents execute.

We intentionally chose this constraint so we would build what was necessary to increase engineering velocity by orders of magnitude. We had weeks to ship what ended up being a million lines of code. To do that, we needed to understand what changes when a software engineering team’s primary job is no longer to write code, but to design environments, specify intent, and build feedback loops that allow Codex agents to do reliable work.

This post is about what we learned by building a brand new product with a team of agents—what broke, what compounded, and how to maximize our one truly scarce resource: human time and attention.

We started with an empty git repository

The first commit to an empty repository landed in late August 2025.

The initial scaffold—repository structure, CI configuration, formatting rules, package manager setup, and application framework—was generated by Codex CLI using GPT‑5, guided by a small set of existing templates. Even the initial AGENTS.md file that directs agents how to work in the repository was itself written by Codex.

There was no pre-existing human-written code to anchor the system. From the beginning, the repository was shaped by the agent.

Throughout the development process, humans never directly contributed any code. This became a core philosophy for the team: no manually-written code.

Redefining the role of the engineer

Increasing application legibility

Agents produce: