One AGENTS.md, no drift: prove CLAUDE.md is a real symlink and the generated files are in sync

The Stack

Steps

1. 1

   Scaffold one source, symlink Claude, generate copilot, then assert no drift

   AGENTS.md is the real file; CLAUDE.md is a symlink to it; the copilot file is generated from it. After committing, the check asserts CLAUDE.md is a symlink in git (mode 120000) pointing at AGENTS.md, and that the committed copilot file equals a fresh regeneration. Hand-edit the copilot file, or commit CLAUDE.md as a copy, and the check trips, which is the whole point.

   rm -rf proj && mkdir -p proj/.github && cd proj
   git init -q
   cat > AGENTS.md <<'MD'
   # Project rules for coding agents
   - Setup: npm install
   - Test: npm test
   - Style: no default exports; named exports only
   MD
   ln -s AGENTS.md CLAUDE.md
   gen() { printf '%s\n' "<!-- generated from AGENTS.md by rulesync; do not edit -->"; cat AGENTS.md; }
   gen > .github/copilot-instructions.md
   git add -A
   git -c user.email=a@b.c -c user.name=x commit -qm init
   
   mode=$(git ls-files -s CLAUDE.md | cut -d' ' -f1)
   if [ "$mode" != "120000" ]; then echo "BAD: CLAUDE.md is committed as a regular file (git mode $mode), not a symlink; on a Windows checkout it drifts from AGENTS.md"; exit 1; fi
   target=$(readlink CLAUDE.md)
   if [ "$target" != "AGENTS.md" ]; then echo "BAD: CLAUDE.md points at [$target], not AGENTS.md"; exit 1; fi
   if [ "$(gen)" != "$(cat .github/copilot-instructions.md)" ]; then echo "BAD: .github/copilot-instructions.md was hand-edited; it has drifted from AGENTS.md, regenerate it"; exit 1; fi
   
   echo "one-source OK: AGENTS.md is the single source; CLAUDE.md is a committed symlink (git mode 120000) to it; the generated copilot file matches a fresh regeneration (no drift). Which agent reads which file, and Claude's layered CLAUDE.md memory, are fenced"

2. 2

   Do it in your real repo (the parts CI cannot do for you)

   Write your actual AGENTS.md, run ln -s AGENTS.md CLAUDE.md, and commit, checking git tracks CLAUDE.md as a symlink. For tools that need their own format or a mixed-OS team, npm i -g rulesync, then rulesync import --targets claudecode and rulesync generate --targets '*' --features '*', and wire the generate step into a pre-commit hook so the outputs never drift. Whether each agent actually reads the file, and whether Claude's layered memory features survive the symlink, are the fenced parts.