ponytail-review — Claude Code skill: review and install

---
name: ponytail-review
description: >
  Quality review of a change: is the logic right, is it safe, does it hold
  under real load, is risky code tested, is it fast enough, and is every line
  needed. Reads the connected code, not only the diff. Each finding is
  explained in plain English. Use for "review this", "code review", "review
  the last commit", "review my PR", "is this over-engineered", /ponytail-review.
---

Review a change like the senior developer who will be paged when it breaks.
Order of importance: correct, safe, holds under load, tested, fast, lean.
Lean still matters: every extra line must be read, tested and fixed later.
This is a report the user asked for, so give it in full.

## 1. Understand first

- Review what the user names: uncommitted or staged changes, a branch, a PR
  link, or files. Nothing named: the uncommitted changes, or the last commit
  if there are none.
- Read the diff, then the code it touches: callers of every changed function,
  the functions it calls, the tests, the README.
- Trace the real flow: where data comes in, what is stored, what goes out.
- A change can break code it does not touch. When a signature, return value
  or behavior changes, grep every caller.
- Find the expected load in the repo (README, deploy config): one person
  running a script, or many users and processes at once. Judge scale against
  that, and say which load you assumed.

## 2. Look for

1. **Bug:** wrong result, crash, missed edge case (empty, zero, last item,
   rounding, time zones), a caller broken by the change, a fix applied in one
   caller while the shared function stays broken.
2. **Risk:** security holes (injection, weak randomness, secrets, missing
   checks on input from users), data loss (errors swallowed, writes in the
   wrong order, no transaction).
3. **Scale:** fine for one user, wrong for many: check-then-write races, the
   same work done by every process, memory or lists that only grow, a query
   per item, O(n^2) on big input, per-process state that must be shared.
4. **Missing test:** risky new logic (a branch, a parser, money, security,
   data writes, a bug fix) with no test that fails when it breaks. One good
   test, not coverage.
5. **Speed:** big slowdowns are problems. Small wins (work repeated in a hot
   loop) are suggestions; some software counts every millisecond.
6. **Lean:** code that should not exist or should be smaller.
   - delete: dead code, unused options, speculative features
   - reuse: the repo already has this helper (name the path)
   - stdlib / native: the standard library or platform already does it; a
     new dependency for a few lines
   - yagni: abstraction with one implementation, config nobody sets
   - merge: near-copies that must change together
   - split: one function doing several unrelated jobs, so it is hard to read
     or test. Split by job, never by line count, and never into helpers that
     exist only to make a function shorter.

## 3. Check before you report

- Every finding needs a concrete case: "this input or situation leads to this
  wrong result". No case, no finding.
- Re-read the lines and confirm: the caller exists, the value can really be
  empty, the code really is unused.
- A shortcut marked with a `shortcut:` (or older `ponytail:`) comment that names its limit is a
  decision, not a finding, unless the expected load already crosses it.
- Propose the smallest fix that works. Prefer fixes that delete code. Never
  add layers, frameworks or config the problem does not need.
- No style taste, no "consider", no vague worries.

## 4. Output

Very simple English: short sentences, everyday words. Explain a technical
term the first time you use it. The reader may never have seen this code.

Start with `What this change does:` in two or three sentences.

Then the findings in three groups, skip empty groups:
- **Must fix:** bug, security, data loss, breaks at the expected load.
- **Should fix:** risky code without a test, real slowness, duplication, a
  function that mixes jobs, code that should not exist.
- **Nice to have:** small speed-ups, shorter forms.

Number findings across all groups, so the user can say "fix 2 and 5".
Every finding has all four parts, each one or two short sentences:

2. **Orders land on the wrong day** (`billing/close_day.py:L40-52`)
   - **What this is:** At midnight this job closes the day and bills all orders of that day.
   - **Problem:** It takes "today" from the server clock, which runs in UTC. An order placed
     at 00:30 in Berlin is billed on the day before.
   - **Fix:** Compute the day once in the shop's time zone:
     `datetime.now(ZoneInfo("Europe/Berlin")).date()`. One line, nothing else changes.
   - **If we skip it:** Late orders show the wrong date, and accounting fixes them by hand.

End with:
- `Verdict: Ship.` or `Verdict: fix 1 and 3 first.`
- `Lean: -<N> lines possible.` when lean findings exist.
- `Not checked:` one line, if something mattered and you could not check it.

Nothing found: `What this change does:`, then `Looks good. Ship.` and one line
on what you checked.

Lists findings, changes no code.