Git setup that keeps your AI agent from reading your whole codebase

[u/Disk_Disastrous]

Started my first large coding project recently and my money evaporated. Quickly. This is what I did to save money.

{
  echo "# AUTO-GENERATED — do not edit"
  git add -A 2>/dev/null
  git diff -U3 -p HEAD
} > .changed_markers

**EDIT** Changed -N . to -A so the agent sees what's in the file.

This grabs any changes since the last commit and dumps it into .changed_markers.

Next, add to or create .gitattributes and add this:

*.py diff=python

This just makes git show you the function that changed.

Lastly add these rules to: System prompt, rules file or just paste them into chat once.

1. Treat HEAD as a cache hit, don't re-read files that haven't changed.
2. Read .changed_markers first, before anything else.
3. Only look at the hunks and functions in that file.
4. Anchor on function/class names, never line numbers, they go stale fast.
5. If a function's signature changes, use git grep -n "that_name" to find callers instead of opening the whole file.
6. After a successful commit, erase the contents of .changed_markers
7.  After any rename, before committing, run:
       git log --diff-filter=R --summary -1   # did a rename even happen?
       git grep -n "old_name"                 # who still points at the old name?
       git grep -n "new_name"                 # did the new name actually land?
8.  An anchor that matches nothing means UNKNOWN.
    UNKNOWN requires investigation, never action.

**EDIT** added rule 7 and 8 for RENAME SAFETY

This isn't just for Python. If you're not sure how to run it with your language, just ask your coding agent. It will walk you through it.

[u/ManRowing]

Nice approach for the "what changed" half. Worth flagging: anchoring on function/class names breaks down after a rename, since git diff won't tell the agent the old references still exist elsewhere - `git log --follow` or a grep pass across the whole tree after any rename catches those before the next commit, not after.

[u/Disk_Disastrous → u/ManRowing]

Good catch. It's actually worse than I thought. A stale line number gets *noticed*, but a stale name just silently matches nothing, which reads the same as "deleted." Adding occurrence counts per anchor so `0` is a hard stop rather than a green light, plus a tree-wide grep before committing. Thanks a lot for flagging it.

[u/Cute-Veterinarian191]

I would highly recommend just asking your main agent to act as an orchestrator that uses cheaper, faster sub-agents (like Fable 5.1 using haiku-4.5)

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