I have watched teams treat the agent like a goldfish. Every session opens with the same paragraph in the chat: we use Tailwind, no inline styles, single quotes, named exports, please don’t add comments. By the third session someone has pasted it from a Notion page. By the tenth, half the team has a slightly different version. By the thirtieth, two engineers are arguing about which one is right and the agent is following whichever paragraph landed last.

The fix has been sitting in Anthropic’s own engineering writing since 2024. Commit the brief. Put it in the repo, in a file the agent reads at the start of every session, and let it be versioned like everything else the team owns.

Put the brief under 200 lines. Anthropic’s recommendation is load-bearing.

You read the brief or you do not. That is what the line count decides.

A brief that grows into a wall of text gets skimmed the way a new joiner skims a 900-line onboarding doc. The paragraphs that matter end up buried under the ones that do not.

Keeping it short makes the team choose what changes the agent’s output, rather than dumping every convention the codebase has ever had.

The other thing the cap does is make the brief a living document. A short file is cheap to edit, so it gets edited when the convention changes. A long one is not, so it rots, and then someone starts a fresh paragraph in the chat because the committed version no longer reflects reality. That is how you end up with thirty versions again.

The longer version is at https://prickles.org/tenet/persistent-brief/AI2