Stack Changes

Splitting Changes Into PRs

Overview

One diff, one thesis. Each change should make exactly one argument — and that argument should be buildable and testable by itself. "One thing" isn't a size rule; it's a communication rule — if a reviewer can't state the change's thesis in one sentence, it's carrying more than one. Size follows from that.

This skill is about decomposition: given a change that already exists (or is fully designed), how do you carve it into a stack where every change stands on its own and the ordering is obvious to a reviewer. The core move is finding the natural seams between theses — the lines along which one change honestly becomes two. The most common and highest-leverage seam, when a feature builds on existing code, is refactor-first: the reshaping ships before the feature, as its own changes, so the feature itself lands as a small, obvious diff. It's the most useful technique here — but it is a technique in service of the principle, not the principle itself. A change that doesn't sit on existing code splits along other seams entirely.

Why small single-thesis changes matter — empathy for the reviewer, plus review latency, CI cost, and clean rollback/bisection at org scale: reference/why.md.

Procedure

Run this top to bottom; the rest of the document is the reference for each step.

1. Detect the review system — run scripts/detect-review-system.sh. If you can't run it (no shell / sandbox / permissions), ask the fallback questions in Detecting Your Review System instead of guessing.
2. Inspect local context — the change shape, recent history + merge base, ownership/conventions, and build/test hints. See Inspect Local Context.
3. Emit a Split Plan — the proposed stack, ordered refactor-first — and persist it to STACK_PLAN.md so a multi-session stack survives interruptions.
4. Confirm the plan with the user before touching git. Revise until they agree on the shape.
5. Execute the mechanics for the detected system (Stacking in Your Review System). Confirm before any history rewrite or force-push (see the Safety note there).
6. Verify — run the per-revision loop (checkout → build → test each node) and confirm the Verification checklist.
7. Communicate the dependency in every title and description.

A Note on Terminology

This skill says "change" for the unit of review — a PR (GitHub), diff (Phabricator/Sapling), or CL (Critique/Gerrit); a stack is the dependent series. Full mapping table: reference/terminology.md.

When to Use

• A change is over ~400 lines, or touches many files for more than one reason
• A new feature requires changing or reshaping existing code first
• A change mixes concerns: refactor + feature, behavior change + formatting, two features
• You need to land dependent (stacked) changes and want the dependency to be unambiguous
• A monorepo change spans multiple ownership boundaries (OWNERS / CODEOWNERS)
• A change would make presubmit build/test a huge set of targets
• Anything that makes a reviewer say "this is a lot — where do I start?"

When NOT to use: A genuinely atomic, single-purpose change. Don't split a 30-line bug fix into a three-change stack — that's review theater. The goal is one thing per change, not minimum lines per change.

The One-Thing Rule (One Diff, One Thesis)

A change carries exactly one thesis when it is the smallest chunk that can be understood, tested, and rolled back on its own. Concretely, it passes these acceptance tests:

• Statable in one sentence — you can write its thesis without "and." If the honest title is "Add caching and refactor the client," that's two theses → two changes.
• Understandable alone — a reviewer gets the point from this diff, without reading the changes above or below it in the stack.
• Buildable alone — checked out on its own, the revision compiles. No references to code that only exists in a sibling change.
• Testable alone — the change ships the tests for its own behavior, and the suite is green at that change's revision (and passes presubmit/CI independently, at its position in the stack).
• Revertable alone — it can be rolled back without dragging unrelated work with it.

A change that builds but has no way to prove it works is not done — "buildable and testable by itself" is one requirement, not two optional ones.

Atomic commits are not atomic reviews. The unit of review is the diff a reviewer opens — the whole PR/diff/CL. Tidy 80-line commits inside one 900-line PR still ask the reviewer to evaluate 900 lines at once. Split the reviewable unit: open a stack of small changes.

Right-Sizing: Don't Over-Split

"One thing" is a single concern, not a minimum line count. Over-splitting has real costs — every change is a review round-trip, a CI run, and a rebase when its parent lands — so a stack of twenty interdependent tiny pieces is worse than one well-organized change. Aim for the fewest changes that each still do one thing and stand on their own:

• Don't split a single concern to hit a line target. If change 1 can't be understood, tested, or motivated without change 2, fold them.
• Each change must be motivatable alone — its description justifies landing it on its own terms, not "setup for the next one." No independent value and no test of its own = a fragment; merge it upward.
• Batch trivial, same-concern, same-owner edits — one "prep" refactor, not one change per call site. (Thousands of call sites = an LSC; see At Scale.)
• Keep stacks shallow — roughly 3–7 deep. Deeper usually means some changes are independent (base on trunk, review in parallel), or the work is really a codemod/LSC.

Over-split litmus: if you can't write a one-line motivation for a change without referencing a later change in the stack, it's too small. Merge it upward.

The Decomposition Process

1. Map the end state          →  what does the final diff actually touch?
2. Find the seams             →  split by thesis; refactor-vs-behavior is the most common seam
3. Order by dependency        →  topological sort: what must land before what (refactors first)
4. Size & scope each change   →  ~200–400 lines; one owner set; small blast radius

1. Map the end state

List every file the finished change touches and why each is touched. Group the "whys" — each distinct why is a candidate change.

2. Find the seams (refactor vs behavior is the most common one)

A seam is a line along which one change honestly becomes two. Look for these, in rough order of how often they pay off:

• Refactor vs behavior — the most common and highest-leverage seam (detailed below).
• Independent feature vs independent feature — two things that don't depend on each other base on trunk and review in parallel; don't stack what isn't dependent.
• Behavior change vs behavior change — two distinct behaviors are two theses, even if they touch the same file.
• Ownership boundary — a change spanning several OWNERS/CODEOWNERS sets splits so each piece has one coherent owner set (see step 4).
• Risky vs safe — isolate the risky, hard-to-review part so the rest lands trivially.

The refactor/behavior seam. A pure refactor changes structure but not behavior: existing tests pass unchanged. A behavior change adds or alters what the code does: it comes with new or modified tests.

Never mix them in one change. A reviewer reading a mixed change can't tell which diff lines are "safe restructuring" and which lines actually change what the system does — so they have to treat all of it as risky. Split, and each half becomes easy:

• The refactor change: "assertions unchanged and still green → behavior is preserved."
• The feature change: a small diff against already-prepared code → the new behavior is the whole story.

Split by thesis, not by layer. "PR1: models, PR2: utils, PR3: tests, PR4: wire it up" is the classic bad split — each piece has no independent value and nothing to prove until the last. Every change carries its own thesis and its proof. (Worked example: reference/why.md.)

3. Order by dependency

Sort the candidate changes so each one only depends on changes before it. Refactors that prepare the ground come first; the feature that uses them comes last.

4. Size & scope each change

Google's eng-practices put numbers on it: ~100 lines is a reasonable CL, ~1000 is usually too large. And distribution matters as much as the count — 200 lines in one file can be fine, but 200 lines spread across 50 files is usually too large. Treat one cohesive thesis of a few hundred lines as the target, but never split a single concern below the point where it can stand alone (see Right-Sizing). Two scale-aware criteria sharpen the split:

• Split along ownership boundaries. Carve changes so each one touches a coherent OWNERS / CODEOWNERS set. A change needing six teams' approval blocks on the slowest of six; three changes of two owners each clear in parallel.
• Mind the blast radius. Keep each change's affected build targets / test impact small (Bazel/Buck reverse-dependency sets). A small change triggers a cheap, fast presubmit; a sprawling one re-runs half the repo.

See git-workflow-and-versioning for sizing heuristics and atomic-commit discipline within each change.

Inspect Local Context (before you plan)

A Split Plan is only as good as what you know about the repo. Gather this first — guessing produces a plausible-but-wrong plan. Run only the commands that apply (skip what isn't present).

1. Current change shape — what you're actually splitting:

   git status --short
   git diff --stat ; git diff --name-only        # unstaged
   git diff --cached --stat                        # staged, if any
   

2. Recent history & base — where the stack roots and what's idiomatic here:

   git log --oneline -n 10
   git branch --show-current
   # Trunk = the remote's default branch — DON'T assume `main` (could be master/develop/trunk,
   # or a non-`origin` remote). Resolve it, then take the merge base = the stack's base:
   trunk=$(git symbolic-ref --quiet --short refs/remotes/origin/HEAD 2>/dev/null) # e.g. origin/main
   trunk=${trunk:-origin/main}                     # fallback; `git remote set-head origin -a` populates it
   git merge-base HEAD "$trunk"
   

3. Review system — run scripts/detect-review-system.sh (or the fallback questions).
4. Ownership & conventions — read if present: CODEOWNERS / OWNERS (split along owner boundaries), .github/pull_request_template.md (match the PR body), CONTRIBUTING.md, CLAUDE.md (house rules, commit/PR conventions).
5. Build/test hints — so each change's Test proof is real, not aspirational. Inspect the manifest(s) — package.json, pnpm-lock.yaml, pom.xml, build.gradle, Cargo.toml, go.mod, pyproject.toml, BUCK / BUILD, etc. — and identify the test command (and, if you can, the affected targets) for each proposed change.

Feed it into the plan: change shape → the proposed stack · merge base → the Base · CODEOWNERS/OWNERS → how you carve and order · manifests → the Test proof column · conventions/template → the description template.

The Split Plan (the artifact to produce)

Before touching git, emit a Split Plan — a predictable artifact the author (or a reviewer) can sign off on before any branches exist. Always produce it in this shape:

## Split Plan

### Final goal
<one sentence: what the whole change accomplishes>

### Concerns currently mixed together
1. <concern> — refactor | behavior
2. ...

### Proposed stack
| # | Title | Type | Depends on | Files | Test proof | Risk |
|---|-------|------|------------|-------|------------|------|
| 1 | refactor: extract … seam | refactor | — | a.ts, b.ts | existing tests unchanged | low |
| 2 | feat: add … | feature | #1 | c.ts | new unit test | med |

### Parallelizable changes
<which changes are independent (base on trunk, review in parallel) vs strictly stacked>

### Land order
<bottom-up order; what must merge before what>

### PR/diff/CL description template
Stack (land in order):
  #1 [1/N] …   ← this change
  #2 [2/N] …
Base: <branch>   Depends on: <#prev or none>   Do not land before: <…>

Produce this first, get agreement on the shape, then execute the mechanics below. The plan is cheap to revise; a half-built stack of branches is not.

Persist it — one on-disk source of truth. Write the plan to STACK_PLAN.md at the repo root, and add STACK_PLAN.md to .gitignore yourself (it's working state — don't leave an uncommitted artifact behind). For a deep stack worked across more than one session this is the anchor: on resume, read STACK_PLAN.md, re-run the per-revision loop to find the first unbuilt/red node, and continue from there — instead of re-deriving the plan from scratch and risking drift. Keep the file current as the plan changes (check off landed nodes).

The Refactor-First Pattern

When you add a feature on top of existing code, the first changes are pure refactors that reshape the existing code so the feature becomes a small diff. The refactors land first, each proving it changed nothing (existing tests pass unchanged); by the time the feature change lands, it's a tiny diff against already-prepared code. When a change doesn't sit on existing code, there's nothing to refactor first — split along the other seams.

(This repo's validation/demo/ carries this further — an expense-report monolith carved into a refactor-first stack of small PRs.)

Dependency Types & Stacking

Independent changes             Dependent (stacked) changes

  trunk                           trunk
   ├── A   (off trunk)             └── 1 (off trunk)
   ├── B   (off trunk)                 └── 2 (off 1)
   └── C   (off trunk)                     └── 3 (off 2)

  Land in any order;              Land bottom-up, in order;
  review in parallel.             each builds on the one below.

• Independent changes — base each on trunk. They don't touch each other; review and land in any order. Prefer this whenever the pieces are truly independent — parallel review is faster.
• Dependent (stacked) changes — each is based on the previous change, because it needs that code to build. Land strictly bottom-up.

Trunk-based + feature-gated is how partial stacks land safely. You don't have to wait for the whole stack to be done before landing the bottom. Land each completed change to trunk with the incomplete feature gated behind a flag (an experiment/config system, or a simple env flag) so trunk stays releasable and you avoid a long-lived branch. See git-workflow-and-versioning.

Detecting Your Review System

Before you stack, know what you are stacking on — the mechanics differ, and only some systems offer commit-per-change. scripts/detect-review-system.sh classifies the current repo and tells you which path below to use:

It exits 0 when commit-per-change is available, 1 for branch-per-PR (github-plain), and 2 for an unknown / non-repo dir. Its full fixture matrix — every row above plus adversarial cases — lives beside it in scripts/detect-review-system.test.sh and runs in CI on every push.

The detector is a heuristic — treat the output as a hint and override it when it's wrong. Known failure modes:

• gerrit fires only on strong signals — .gitreview or the commit-msg hook. A bare Change-Id: in history counts only when there's no GitHub remote, so a github.com repo that imported a Gerrit patch stays github-plain (an adversarial fixture pins this). Residual: a non-GitHub repo with a stray Change-Id and no .gitreview/hook can still read as gerrit — if you don't push to refs/for/*, you're not on Gerrit.
• Graphite needs its config — it looks for .git/.graphite_repo_config; a Graphite user who hasn't gt init-ed degrades to github-plain (a fixture documents this). If you use gt, treat it as github-stacked.
• Any unclassified remote → github-plain. GitLab/Bitbucket (cloud or self-hosted), GitHub Enterprise on a custom domain (github.acme.com, git.acme.com — we can't fingerprint these from the URL), Gitea, etc. all route to github-plain (branch-per-PR), not git-local. git-local (exit 0, commit-per-change) now means no remote configured yet — a true local commit stack. If the detector is wrong about a custom host's exact tool, the mechanics are still branch-per-PR; only the CLI differs (glab/web instead of gh).

If you can't run the script, don't guess — ask the user three questions:

1. Where do code reviews happen — GitHub PRs, Phabricator/Sapling diffs, Gerrit CLs, or just local commits for now?
2. Do you use a stacking tool (Graphite gt, spr, ghstack)?
3. Is there a remote yet, or is this local-only?

Then pick the matching mechanics below.

It works before any remote exists. A git-local repo (commits, no remote) is the normal starting point. Split your local commit stack now — git rebase -i to split/reorder/squash commits, or git reset to uncommit and re-stage in single-thesis pieces — and bind each change to a PR/diff/CL when you push. The decomposition is the same work whether the review tool is chosen yet or not.

Detect and adapt — don't refuse. The result picks the plumbing, not whether to split. The decomposition (one diff one thesis, refactor-first, communicate the dependency) is identical everywhere; only the stacking commands change.

Stacking in Your Review System

Principle: let the tool track the stack; you choose the decomposition. Modern review systems make stacked changes first-class and auto-retarget descendants for you — the manual rebase dance below is only needed when you have no stacking tool.

Strongly prefer a stacking tool (or a native commit-per-change system) over the manual path. The plain-git git rebase --onto fallback is the riskiest thing this skill does — a wrong base silently corrupts the stack, and an agent driving it can't always see the corruption. Use it only as a last resort; rebase one branch at a time, and re-run the per-revision verify loop after each rebase so a bad base fails loudly instead of rotting the stack.

⚠ Safety — history rewrite & force-push. Several mechanics below run git rebase / git reset (which rewrite history) and git push --force-with-lease. Confirm with the user before rewriting history or force-pushing, and never force-push a shared or protected branch (main/master, a release branch, or anyone else's branch). Force-push only your own stack branches. These are hard-to-reverse, outward-facing actions — pause and check first.

Tool specifics drift. The behaviors below (Sapling auto-restack, Graphite auto-retarget on merge, the exact gt / spr / ghstack flags) are current as of writing. If an invocation has changed, the principle still holds — confirm the exact command against the tool's own docs.

Sapling (Meta-style stacked diffs)

Stacks are the default. Each commit is a change; the stack is just your commit stack.

sl commit -m "refactor: extract Report core seam"   # change 1
sl commit -m "refactor: introduce Formatter registry"  # change 2 on top
sl pr submit   # or `sl diff` against Phabricator — submits the whole stack

Amend a lower change (sl amend) and Sapling auto-restacks the descendants — no manual rebase. The UI shows the stack graph; reviewers land bottom-up.

Phabricator (arc)

One commit per change; arc diff turns your commit stack into a diff stack.

arc diff   # creates/updates a Diff per commit; "Depends on Dxxxx" is tracked in the UI

The dependency is recorded structurally — reviewers see "Depends on D123" and the stack view.

Gerrit

One commit = one change, keyed by a Change-Id trailer; consecutive commits form a relation chain.

git commit   # commit-msg hook adds Change-Id: I...
git push origin HEAD:refs/for/main   # each commit becomes a chained Change

Submit bottom-up; Gerrit shows the relation chain and blocks out-of-order submits.

GitHub with a stacking tool

GitHub has no native commit-per-change model. Its reviewable unit is always a branch-to-branch diff (a PR = head branch vs base branch): you can't push a stack of commits and have GitHub turn each commit into its own PR, there is no Change-Id / relation chain, and approval and merge happen at the PR level — never per commit. So on GitHub a stack is always branches underneath; the only question is whether you manage them or a tool does.

A stacking tool (Graphite gt, spr, ghstack) gives you the Sapling/Gerrit-style commit-per-change authoring experience on top of that branch-per-PR substrate: you keep a local stack of plain commits, and the tool generates and retargets one branch + one PR per commit for you.

gt create -m "refactor: extract seam"      # Graphite: each branch is a change in the stack
gt create -m "refactor: registry"
gt submit                                  # opens/updates the whole stack of PRs
# spr / ghstack: you stack plain commits; the tool creates a branch + PR per commit

On merge of the bottom PR, the tool retargets the next onto trunk automatically.

Plain git + GitHub (fallback — last resort, no stacking tool; the riskiest path)

Manage bases and rebases by hand — verify after every rebase (see the warning above):

git switch -c stack/1-seam main
gh pr create --base main --title "[1/4] refactor: extract seam"

git switch -c stack/2-registry stack/1-seam        # branch off the parent change
gh pr create --base stack/1-seam --title "[2/4] refactor: registry (needs #<1>)"

# After #<1> merges into main:
gh pr edit <2> --base main
git rebase --onto main stack/1-seam stack/2-registry
git push --force-with-lease

If a rebase reports a conflict — stop. This is the corruption point. Do not resolve conflicts you don't fully understand, and never git rebase --continue by guessing — a wrong resolution silently rewrites what a "refactor" did (the exact failure the warning above is about):

git rebase --abort     # restores the branch to exactly where it was — nothing is lost
git status             # confirm you're back, clean

Then surface it to the user: report which branch failed to rebase onto which base and the conflicting files, and ask how to proceed (resolve it together, re-order the stack, or switch to a stacking tool). After any successful rebase, re-run the per-revision verify loop for that branch and every branch stacked above it — a wrong base corrupts everything on top, so one green node is not proof.

Communicating the Dependency

Even when the tool tracks the stack structurally, the title should still tell a human skimming a review queue where this change sits and what it needs first.

Title conventions:

• Prefix the stack position: [2/4].
• Name the prerequisite explicitly: (needs #123) / — requires D123.
• Lead with the change type so the refactor/feature split is visible: refactor: / feat:.

[1/4] refactor: extract Report core into a seam
[2/4] refactor: introduce Formatter registry (needs #1)

Description template (put this in every change in the stack so any entry point is self-explanatory):

Stack (land in order):
  1 [1/4] refactor: extract seam        ← this change
  2 [2/4] refactor: Formatter registry
  3 [3/4] feat: CsvFormatter behind flag
  4 [4/4] feat: enable CSV export

Base: trunk
Depends on: (none — bottom of stack)
Do not land before: (none)

Phabricator, Gerrit, Sapling, and Graphite all render the stack graph natively — so the title convention is belt-and-suspenders. Keep it anyway: it's what a reviewer reads first in a list of fifty changes.

At Scale

Splitting is load-bearing in a large org: small-change culture (Google's small-CL guidance), presubmit economics, ownership-aligned splits, feature-gating over long branches, shallow stacks for land queues, and the LSC/codemod escape hatch for repo-wide sweeps (don't hand-split a 3,000-file rename — generate it). Full notes: reference/at-scale.md.

Verify the Stack (per-revision)

Don't assert that each change builds — observe it. The skill doesn't need to understand Bazel vs Cargo vs npm; it runs the project's own build and test command and reads the exit code. That portability is the whole point.

1. Find the build + test command — from the manifest you read in Inspect Local Context, or ask the user. Default to whatever the repo's CI runs. Examples: npm run build && npm test · cargo build && cargo test · go build ./... && go test ./... · mvn -q verify · ./gradlew test · pytest · bazel test //....
2. Walk the stack bottom-up. For each change, in order:

   git checkout <change-ref>     # or `sl goto`, etc.
   <build command>              # exit non-zero on failure
   <test command>
   

   Record green/red for that node.
3. Stop at the first red. A red node means the stack is invalid there — a change references code introduced by a later one, or a "refactor" actually changed behavior. Report which node failed and why; do not present an unverified stack as done. Fix the decomposition, re-verify.

A refactor change has a sharper check: build + test green and the test assertions and expected outputs are unchanged from the parent. Byte-identical test files (git diff <parent> -- <test paths> is empty) is the strongest form of that proof — but it's sufficient, not necessary. A legitimate pure refactor can still touch test files mechanically: a rename updates call sites, a file move updates import paths. So the real invariant is "no asserted value or expected output changed"; when a refactor must edit tests, confirm the diff is purely mechanical (names/paths), with every assertion intact.

When done, restore the user's working state (git checkout - / their branch) — don't leave them on a detached revision. And per the Safety note, confirm before any rebase/force-push.

Verification

Before submitting, confirm every change passes the One-Thing Rule acceptance tests (statable in one sentence, understandable / buildable / testable / revertable alone, not a fragment) — and, at the stack level:

• No change mixes a refactor with a behavior change; refactor changes leave the test assertions / expected outputs unchanged and passing (mechanical edits — renames, import paths — are fine)
• It passes presubmit/CI independently, at its position in the stack
• Title states stack position [n/total] and any prerequisite (needs …)
• Description lists the full stack order and base
• The stack relationship is tracked by the tool (or bases set correctly in the fallback path)
• Each change touches a coherent owner set, not the whole org

Related Skills

• git-workflow-and-versioning — atomic commits, trunk-based development, feature flags, change sizing
• incremental-implementation — building each change in thin, tested slices
• code-review-and-quality — what a reviewable change looks like from the reviewer's side
• planning-and-task-breakdown — deciding the split at plan time, before code exists
• deprecation-and-migration — the Large-Scale-Change / codemod playbook for repo-wide sweeps

Further Reading

• Rebuttals to common excuses ("it's fine as one big change," "I'll split after approval," "atomic commits = atomic review"): reference/rationalizations.md.
• Google's Small CLs + Speed of Code Reviews, and the "one diff, one thesis" framing: reference/further-reading.md.