The first real fight I had with a teammate over AI tooling was not about which model to use. It was about which config file we were supposed to be editing.
We had a CLAUDE.md at the repo root, written by me back in February. We had a .cursorrules file from the engineer who joined in March, who only used Cursor and refused to even open Claude Code. We had a copilot-instructions.md under .github/, dropped in by whoever turned on Copilot for the org. And we had a half-empty AGENTS.md that someone had heard about at a conference. Four files. Four different versions of the truth. Three subtly different opinions on how to write a database migration. Zero shared understanding.
The bug that triggered the argument was small. A junior engineer asked Cursor to write a Postgres migration. Cursor produced something fine by its own rules. The PR review pointed out that "our convention is to always wrap migrations in a transaction and write a rollback." Junior engineer pointed at .cursorrules. Nothing about transactions. We checked CLAUDE.md. It had the rule. We checked copilot-instructions.md. Different wording, same intent. We checked AGENTS.md. Empty.
Nobody had written the rule into Cursor's file because nobody used Cursor when the rule was added. The model did exactly what the file told it to. The file was just wrong by neglect.
