A micro-level look at how AI coding agents write, run, test, verify and debug code: the edit loop, output acceptance, the exec tool, and why some agents feel like native command-line programmers while others feel like scripters.

The first four posts in this series took the telescope view of AI agents. This one takes the microscope: how a coding agent turns an instruction into a working, tested change — how it edits, compiles, runs, tests and debugs — and why Claude Code behaves like a native *nix programmer while others reach for a throwaway script.

How a Coding Agent Actually Writes Code

A strengths-and-weaknesses comparison of the open coding agents Codex, Pi, OpenCode, Kimi Code and Gemini CLI, plus why Claude Code arguably has the best harness despite being closed.

After reading the source of five open coding agents — OpenAI's Codex, Pi, OpenCode, Kimi Code and Gemini CLI — here is how they actually stack up, strengths and weaknesses, which one wins, and why the best harness of all belongs to a tool we deliberately left out of the code study: Claude Code.

Tag: Claude Code

Tag: Claude Code

How a Coding Agent Actually Writes Code

The Coding Agent Shootout and the Best Harness We Could Not Read