Agent instructions file
Also called AGENTS.md, CLAUDE.md, context file, rules file
A file in your repository that a coding agent reads every session: the commands, conventions and gotchas it can't work out from the code.
A new agent session knows only your code and what it loads at the start. The instructions file is the part you control: a short note in the repository that tells the agent what it would otherwise get wrong, such as the test command, the package manager, the folder nobody edits by hand.
Every line goes into every request, so length has a cost twice over: more to pay for, and more for the important line to get lost in. The useful test for each line is whether removing it would cause a mistake. The best files grow from the agent's real repeat mistakes, not from a description of the project.
For example
After the agent runs npm install for the third time in a pnpm repo, a developer adds one line to the file: "Use pnpm; npm rewrites the lockfile."
Put it to use
Where you'll meet agent instructions file in ChatGPT, Gemini and Claude, how to use it there, and the mix-up most people make.
Daily Bytes Plus is in the Android app.