crbnos pr-splitter — break a big PR into reviewable pieces
The original branch is source material, never a casualty: snapshot it, build smaller PRs from it deliberately, and keep a local ledger of what went where.
Announce at start: "Using the pr-splitter skill — splitting {branch} into smaller PRs."
Never use interactive git commands (git restore -p, git add -i,
git rebase -i) — they hang in this environment. Every extraction below is
non-interactive.
Step 1: Snapshot before touching anything
git status --porcelain # must be empty; if not, STOP — commit or stash first
BASE=$(git merge-base origin/main HEAD)
git branch backup/original-$(git branch --show-current)
Do not delete or rewrite the original branch until the split has fully shipped.
Step 2: Inventory the original PR
git diff --stat $BASE...HEAD
git diff --name-only $BASE...HEAD
git log --oneline $BASE..HEAD
Classify every changed file into exactly one review unit:
| Unit | Examples |
|---|---|
| prep / refactor | renames, extractions, no behavior change |
| API / type changes | signatures, models, generated types |
| behavior | the actual feature/fix logic |
| tests | new or changed tests |
| docs / metadata | AGENTS.md, docs, changelogs |
| generated / lockfiles | pnpm-lock.yaml, generated types |
Step 3: Create the scratchpad
Write .ai/scratch/pr-split.md (gitignored — never commit it):
# PR split — {original branch}
Backup: backup/original-{branch} · Base: {BASE sha}
## Planned PRs
1. {branch-name} — scope: … — files/hunks: … — verify: … — status: …
## Remaining original intent
- …
## Drift notes (date / branch / what changed and why)
- …
Step 4: Choose the split shape
| Situation | Shape |
|---|---|
| Later work depends on earlier work | Stacked PRs (each branched off the previous) |
| Changes are truly independent | Parallel PRs off main |
| One shared prep change unlocks independent work | Foundation PR + parallel follow-ups |
| Unsure | Stacked — dependency mistakes surface as conflicts, not broken builds |
Rules: never separate tests from the code they verify; never split one behavior across two PRs by file boundary; each PR must build and pass its tests alone.
Step 5: Extract non-interactively
Start each PR from the correct base (main, or the previous PR in the stack):
git checkout -b {pr-branch} {base}
Whole files (the file belongs entirely to this PR):
git checkout backup/original-{branch} -- path/to/file.ts
Part of a file (the file mixes changes for different PRs):
git diff {base} backup/original-{branch} -- path/to/file.ts > .ai/scratch/extract.patch
# Edit .ai/scratch/extract.patch: keep the file header lines (---/+++),
# DELETE every @@-hunk that belongs to a different PR, keep the hunks you want.
git apply .ai/scratch/extract.patch
If git apply fails (context mismatch), do not fight it: open the file and
make the wanted changes by hand, using the patch as the reference. Then delete
the patch file.
Commit each extraction with a conventional message and check it off in the scratchpad, noting exactly which files/hunks moved.
Step 6: Verify each PR independently
For every PR branch, before opening it:
pnpm exec biome check --write --no-errors-on-unmatched <changed paths>
pnpm exec turbo run typecheck --filter=<pkg> # per touched package, never whole-repo
pnpm --filter <pkg> test
A PR that only compiles on top of an unmerged sibling is stacked — say so in its description and set its GitHub base branch accordingly.
Step 7: PR descriptions
## Summary
PR {N} of {M} split from {original branch}.
## Scope
- …
## Intentionally excluded (follow-up PRs)
- …
## Verification
- {commands run + results}
Keep the detailed extraction ledger in .ai/scratch/pr-split.md, not in PR
bodies.
Step 8: Manage drift as reviews land
- Reviewer-approved changes are the new source of truth — when an earlier PR changes, rebase its dependents onto it and resolve conflicts in favor of the reviewed direction, never blindly back toward the original branch.
- After any rebase/force-push:
git range-diff {old-tip}...{new-tip}and summarize meaningful differences for reviewers. - Periodically diff the stack against the backup to find remaining intent (work not yet shipped in any PR) — not to force byte-for-byte equality.
- Record every intentional divergence in the scratchpad's drift notes.
Done when
- Every hunk of
git diff $BASE...backup/original-{branch}is either shipped in a PR or explicitly listed in the scratchpad as dropped (with a reason) - Each PR builds and passes its scoped gates independently
- Tests shipped in the same PR as the code they verify
- Backup branch still intact
Failure modes to avoid
Splitting by file when behavior spans files · extracting tests without their code · a follow-up PR that doesn't compile · force-pushing without a range-diff summary · deleting the backup early · resolving stack conflicts by reverting reviewer feedback.