Half the team uses Claude Code. A third uses Cursor. Two engineers swear by Aider. One was running codex out of curiosity for a sprint. Five different agents reading what should have been five copies of the same conventions; predictably, they were not. The Claude Code users had the most complete CLAUDE.md. The Cursor users had .cursor/rules files that overlapped with it. The Aider users had an .aider.conf.yml plus a CONVENTIONS.md they had drifted from the others.
The team was getting drift across tools, not because the engineers disagreed about how to write code, but because each agent had a slightly different copy of the rules and nobody was syncing them.
The fix was the one nobody wanted to do first: a single source of truth, and a strategy for how each agent's tool-specific config pointed at it. That is the post. What goes in the shared file, what stays per-tool, and where the harness has to fork.
What translates cleanly
Most of the harness is plain English. "When you touch the migrations directory, run the test suite against a real Postgres, not the mocked client." That sentence is meaningful to Claude Code, to Cursor, to Aider, to any agent that reads natural language and acts on it. The rule is portable because the rule is about the codebase, not about the tool.
