worktree-stale-local-main-ref-inflates-pr-diff

Worktree: a stale local main ref inflates git diff main...branch

Problem

In a multi-worktree repo, git diff main...<your-branch> (or --stat) reports far more files and lines than your commit actually changed — e.g. 92 files / 8,000+ insertions when your one commit touched 6 files. The instinct is to panic that your PR is silently reverting or re-touching a pile of upstream work (the stale-base-pr-silently-reverts-upstream-content trap) and to block your own merge or demand a rebase. Almost always, nothing is wrong with the PR — the local main ref is stale.

Context / Trigger Conditions

All of:

• You're working inside a git worktree (git rev-parse --is-inside-work-tree; path contains .claude/worktrees/<name>/ or git worktree list shows 2+ paths).
• git diff main...HEAD --stat (3-dot) shows tens/hundreds of files, but git show --stat HEAD (your actual commit) shows only a few.
• You branched off what you thought was current main, and gh pr view <N> reports the PR as mergeable: MERGEABLE / mergeStateStatus: CLEAN.

Why it happens: main...branch (3-dot) diffs from the merge-base of the two refs. The merge-base uses your LOCAL main ref. In a multi-worktree setup the local main ref is whatever the primary worktree last checked out / pulled — frequently many commits behind origin/main because sibling sessions merge via GitHub (squash) and nobody runs git pull on the parked primary checkout. So the merge-base is ancient, and the 3-dot diff includes every file that landed on origin/main since that old point — none of which your branch is actually reverting. (This is the REF being stale, distinct from git-diff-2dot-vs-3dot-merge-safety, which is about 2-dot-vs-3-dot operator semantics against a correct origin/main.)

Solution

Diff against the remote ref, not the local main ref:

git fetch origin main -q

# The TRUE PR diff (what GitHub will merge):
git diff origin/main...HEAD --stat

# Confirm divergence is small:
git rev-list --count HEAD..origin/main   # commits origin/main has, your branch doesn't
git rev-list --count origin/main..HEAD   # YOUR commits

# Is your branch's base actually on origin/main's history? (necessary, NOT sufficient
# — see the Verification note below; this does not rule out a revert on its own)
git merge-base --is-ancestor <your-branch-base-sha> origin/main && echo "clean base"

# The check that DOES rule out a revert — deletions, from the merge-base:
git diff --diff-filter=D --name-only origin/main...HEAD   # empty ⇒ deletes nothing

# Ground truth from GitHub — the actual file list the PR will change:
gh pr view <N> --json files -q '.files[].path'
gh pr view <N> --json mergeable,mergeStateStatus