replica-diff

---
name: replica-diff
description: 'Compares an app clone against the original: a feature parity score from the feature matrix (weighted by must, should, could) with the missing list in build order, plus a screenshot layout diff that ignores colour so a rebrand does not count against you. Two standard-library Python tools. Use when the user says "how close is my clone", "compare it to the original", "what''s missing", "parity check", "diff the screens", "is it ready", or after the replica-test skill.'
---

# replica-diff

## Invoking skills

In Cursor, type `/` in chat and choose the skill by name. That attaches it to
one message. To keep a skill active for the whole session, start it as a
Custom Mode: Option+Enter on macOS, or Alt+Enter on Windows. You can also
name the skill in an ordinary message when the request matches its description.

## Tools

The Python tools are shared by the pack. They need Python 3.8 or newer and
the standard library only.

`SKILL_DIR` is the directory that contains this file. After `cursor/install.sh`,
that is `.cursor/skills/<name>/` in a project, or `~/.cursor/skills/<name>/`
for a user install. The tools sit beside the skill folders, in `replica-tools/`.
Set `REPLICA_HOME` to a checkout of this pack to use `tools/` there instead.

```bash
if [ -n "${REPLICA_HOME:-}" ] && [ -d "${REPLICA_HOME}/tools" ]; then
  TOOLS="${REPLICA_HOME}/tools"
elif [ -n "${REPLICA_HOME:-}" ]; then
  TOOLS="${REPLICA_HOME}"
else
  TOOLS="$(cd "$(dirname "$SKILL_DIR")" && pwd)/replica-tools"
fi
```

Two shared tools, both standard-library Python, no installs:

```bash
python3 "$TOOLS/parity.py" replica/features.csv                         # feature parity + missing list
python3 "$TOOLS/imgdiff.py" replica/screens/S07.png replica/clone-screens/S07.png --out diff-S07.png
python3 "$TOOLS/imgdiff.py" a.png b.png --json > replica/diffs/S07.json # for parity.py --visual
python3 "$TOOLS/parity.py" replica/features.csv --visual replica/diffs/*.json --markdown > replica/parity.md
```

## What parity means here

**Parity means it does the same job, not that it looks the same.** The
feature score is the number that matters: can a user do everything they could
do in the original. The layout score checks structure (is the information in
the same places, in the same hierarchy), because that is what makes a switcher
feel at home. It deliberately ignores colour, because replica-brand changes
every colour on purpose. Do not chase pixel parity with the original. Its
exact look is its trade dress, and it changes before launch.

## Step 1: feature parity

Make sure `replica/features.csv` is current: every row's `clone` column is
`yes`, `partial` (with a note), `no`, or `skip` (with a reason). Then:

```bash
python3 "$TOOLS/parity.py" replica/features.csv
```

It weights must 3, should 2, could 1, counts partial as half, leaves out
`skip` rows and rows you added that the original does not have, and prints:
the score, must-haves done of total, each area weakest first, and the missing
list in build order. Must-haves not done means not shippable, and it says so.

## Step 2: layout diff

For each key screen, take two screenshots at the **same viewport** (1440x900
desktop, 390x844 mobile) in the **same state** (same data shape, same tab
open, logged in the same way). The original's come from public pages or the
user's own account, saved in `replica/screens/`. The clone's go in
`replica/clone-screens/`.

```bash
python3 "$TOOLS/imgdiff.py" replica/screens/S07.png replica/clone-screens/S07.png --out replica/diffs/S07.png
```

Layout mode (default) turns both into edge maps, cuts them into a grid, and
compares where things are, scoring only cells with something in them. It
reports the score (matches 90+, close 75+, partly 50+, different), the
regions that differ (in the original's pixel coordinates, biggest first), and
a height difference if the clone's page is much longer or shorter. Retina and
non-retina screenshots compare fine: both are scaled to the same width.

`--mode pixel` is exact comparison. Use it for your own regressions (clone
today against clone last week), not against the original.

## Step 3: behaviour diff

Walk each flow in both apps and compare what the scores cannot see: clicks to
finish the core flow, what happens on errors, what is remembered between
visits, what emails arrive. Fewer clicks than the original is a win. Write
each difference as: flow, original does, clone does, fix or keep.

## Step 4: the report

`replica/parity.md`: overall score, feature score, layout score per screen,
missing features in build order, behaviour differences, and a verdict:

- **not shippable**: any must-have missing, or any open S1 bug
- **shippable**: all must-haves done, feature score 80+, no open S1 or S2
- **better than the original**: shippable, plus fixes from
  replica-entrepreneur. This is the goal. A straight copy has no reason to exist.

Give honest numbers. A clone at 62% is at 62%.

## Output

`replica/parity.md`, the diff images, and the top five things to build next.
Then the replica-build skill for the gaps, or the replica-entrepreneur skill if parity is
there.