Clone prototype

Seven phases, in order. Never skip ahead. Tokens before HTML, sampling before tokens. Every colour and every metric in the final HTML must trace back to a measurement, not to a guess that "looks about right". The one thing built out of order is Phase 5's reference row: it needs no measurement, so it goes up first (see Phase 5).

Output lands in mockups/canvases/<slug>/ and the canvas picks it up automatically. The prototype-canvas skill covers how folders become tldraw pages and how layout.json rows work. Name the folder for the source, e.g. notion-ios.

Toolkit: refkit (grid / sample / bands / bbox / scan / hairline / font / shoot / diff / blend / tokens / batch / ink / crops / key / montage), plus artgen for the rare asset that has to be drawn. Both are commands on your PATH; shoot additionally needs Google Chrome.

refkit --help

Not found? The plugin cannot install it. Once per machine:

uv tool install "git+https://github.com/ReScienceLab/super-prototyping#subdirectory=tools"

Before Phase 1, check the project ignores what a run produces, once per project: a run writes third-party captures into ref-*.html and assets/refs/, and committing those is hard to undo. Work in mockups/canvases/<slug>/scratch/. Each rule is checked on its own, and the **/ matters — a pattern with a slash in the middle only matches at the root:

for p in 'ref-*.html' '**/assets/refs/' 'scratch/'; do
  grep -qxF "$p" .gitignore 2>/dev/null || echo "$p" >> .gitignore
done

Then name the board folder once and address everything through it: captures under $B/assets/refs/, everything a run derives under $B/scratch/. A -o without a directory writes into whatever the current directory happens to be, which is the user's project root as often as not:

B=mockups/canvases/<slug>

Worked examples and the folder skeleton ship with the plugin, which is installed outside your project. Address them through the kit root:

KIT="$(sp-canvas root)"
ls "$KIT/mockups/canvases"

A folder this skill names but that listing does not show means the plugin was installed sparsely, which is supported. Work from templates and carry on; nothing here needs an example to be present.

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Phase 0: collect references

Get the highest-resolution capture you can; every later measurement is capped by it.

• The user's own screenshot is usually the authority on which screens and which scroll state. Save each one as its own crop (p1.png … pN.png) in $B/assets/refs/ before doing anything else. Image caches rotate and the attachment will disappear mid-task.
• Mobbin MCP (mcp__mobbin__search_screens, search_flows), when available. Run one search per screen, platform: "ios", and keep task_intent identical across every call in the run. Describe the screen in plain language including its literal on-screen strings; that is what actually matches. Use exclude_screen_ids (a JSON array of quoted UUID strings) to push past near-misses you already rejected. Download with curl -sL <image_url>. Results are ~299 × 678 webp, fine for placing on the canvas but too small to be the only sampling source. Cite results as markdown links to their mobbin_url.
• A native-resolution capture of any screen from the same app (@2x/@3x, e.g. 1179 × 2556) settles ink, scrim and accent values that a downscaled strip cannot. It does not have to be one of the screens you are cloning.

Check the colour space before you sample anything

A capture straight off a device is often untagged Display P3, and every tool in this kit reads raw bytes. Sampled as-is, a P3 capture and an sRGB one of the same screen disagree by 5-10 levels on any saturated colour, and nothing about either looks wrong on its own. Convert to sRGB first, and convert the whole set, so one token cannot end up averaging two spaces.

The test is cheap and it is the only thing that finds this: sample one element that appears in every capture (a brand mark, an accent button, the page ground) and compare across batches. Values that split into two clusters are two colour spaces, not two colours. The claude-ios run had captures 01-07 in sRGB and 08-15 untagged P3; the same brand orange read #E07A54 on one half and #D97757 on the other, and the page ground split with it. One sips/Pillow conversion pass up front collapses both.

Two oranges may still survive the conversion, and then they are real: that run kept #D97757 for the star mark and #CB6442 for the send button, on the same screen. Convert first, then decide what is one token and what is two.

Record the capture scale once, in capture px per design pt, and reuse it everywhere:

scale = screen_inner_width_px / device_pt_width      # e.g. 300 / 393 = 0.7634

Cross-check against height (649 / 852 = 0.7617). If the two disagree by more than ~1%, the crop is wrong. Recrop before sampling.

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Phase 1: grid on the image, then LOOK

Draw the grid onto the pixels and read the result with your eyes. That is the rule that makes this work. Sampling coordinates blind produces numbers with no idea which UI element they belong to, and those numbers end up in the wrong token.

refkit grid "$B/assets/refs/p4.png" \
    -o "$B/scratch/g04.png" --zoom 3 --minor 10 --major 50

Then read g04.png as an image. Cyan every 10 source px, red and labelled every 50. Walk the screen element by element and write down the region each one occupies in source coordinates. Only then sample.

references/measuring.md is the technique: the four kinds of colour region and the command each one needs, how gutters, row heights and radii come off the same grid, and how to measure the type face instead of guessing it. Read it before you sample anything.

Deliverable of this phase

A table with an evidence column, one row per token. Anything without evidence does not become a token.

token	value	evidence
--n-font	-apple-system, "SF Pro"…	refkit font on the page title, 0.93 (2nd 0.87)
--n-text	#2C2C2C	H1 core ink @3x, 3 screens
--n-hairline	#E9E8E7	1pt coverage solve, settings dividers
--n-border	#EFEEEC	card outline solve

Write the machine half of that table as you measure: probes.json in the canvas folder, committed with the boards, one entry per measurement, {"id", "img", "cmd", "box"}. Phase 4 replays it against your renders, so every number that justified a token is re-checked after the token changes. Without it the evidence drifts: one finished run shipped three rows still citing an alpha of .174 after the token had become .10, and refkit tokens cannot catch that; it checks that tokens exist, not that their evidence still agrees with them.

Two things batch will not tell you. It compares the first colour a probe prints, and sample prints its flat-fill census first, so an ink probe needs --only ink or it silently compares two backgrounds and reports a perfect Δ 0. And a key starting with _ in a probe is ignored, which is where the sanity note below lives.

Every probe box carries a one-line sanity note proving the window holds the element and only it. The window is wrong far more often than the measurement is: a probe at cy=681 for a button row that sits at 564, or a box that catches the neighbouring label instead of the glyph, returns a confident, plausible number either way.

Re-sample rather than argue. Where the strip and a native capture disagree, the native capture wins.

---

Phase 2: design system before any screen

Write one :root block and inline it, byte-identically, into every artboard. Artboards render in <iframe srcDoc sandbox="">, so there is no shared stylesheet. A single generator script is what keeps them in sync.

Cover, in this order, with a short prefix per app (--n- for Notion):

• font: the real platform stack, never a webfont, and measured with refkit font in Phase 1 rather than assumed
• colour: backgrounds → fills → hairline/border/track → text ramp → accents
• radii, one per component class (field, card, sheet, tile, pill)
• type: composite font: shorthands, not separate size/weight vars, e.g. --n-t-row: 400 17px/22px var(--n-font)
• spacing and geometry constants: gutters, row height, tap target, status bar, sheet top inset

$KIT/mockups/canvases/luma-ios/ is a complete run to copy from: 19 boards (a token board, two evidence boards, 8 screens, 8 references), a four-row layout.json, a committed gen.py, and per-screen mean deltas of 3.47 to 4.50 levels against the captures.

$KIT/mockups/canvases/duolingo-ios/ is the second complete run, and the one to read when the screens are mostly illustration: 58 tokens, 8 screens, 128 pieces of art, and per-screen mean deltas of 1.32 to 2.93, the best screenshot-sourced numbers in the repo. Every picture on it is a crop of the capture at a measured box, which is most of why. Its README.md carries the experiment that settled crop against generate, the stand-in face whose cap ratio is not SF Pro's, and two defects that produced no error message. Board 00e-art-gen is the generation result, kept as evidence beside the crops it loses to.

Start from the skeleton rather than a finished board: mkdir -p mockups/canvases && cp -r "$KIT/mockups/canvases/templates" "$B". Its gen.py builds the :root block and the evidence table from one TOKENS list, so a value cannot drift from the evidence behind it and a token cannot ship without one. Change NAME and the prefix, then replace every placeholder row with something you measured.

Build the token board as the first generated artboard of the folder (the reference row is already up). It is the contract. When a screen looks wrong later, this is what you check it against.

Once the screens exist, refkit tokens mockups/canvases/<slug> enforces the two invariants this phase rests on: that every board inlines the same :root, and that nothing references a token that does not exist, in CSS or in the evidence table. Run it before you call the board done; a --x-scrim-3 in an evidence row that never existed as a token is invisible to the eye and obvious to the linter.

When the evidence table outgrows the 478 × 980 box, split it onto its own 00b-evidence board rather than trimming it. The evidence is the deliverable.

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Phase 3: one generator, N artboards

Write one script that emits every .html file, and commit it with the boards it produces: mockups/canvases/<slug>/gen.py, plus its asset JSON, resolving paths relative to __file__ so python3 mockups/canvases/<slug>/gen.py regenerates the folder in place ($KIT/mockups/canvases/templates/gen.py is the skeleton, $KIT/mockups/canvases/luma-ios/gen.py a finished one, and $KIT/mockups/canvases/duolingo-ios/gen.py a finished one that also cuts and places its own artwork from a crops.json). Do not hand-edit the artboards afterwards; edit the generator and re-run. That is what keeps eight files consistent through a dozen correction passes, and it only outlives the session if the generator is in the repo; a scratch-dir generator dies with the session and leaves the boards unmaintainable under this skill's own rules. A board with no committed generator is unfinished.

TOKENS = ":root{...}"          # from Phase 2, one source of truth
BASE   = ".phone{width:393px;height:852px;...}"
SB     = '<div class="sb">...</div>'          # status bar, 54px
def page(title, extra_css, body): ...          # TOKENS + BASE + extra_css + body
def write(name, html): ...

Hard constraints from the canvas renderer (also in prototype-canvas's references/layout.md):

• Fully self-contained. The iframe is sandbox="". No external CSS, JS, fonts or images. Every image is a data: URI; icons are inline SVG. Keep each icon as assets/icons/<name>.svg and inline it through a helper in gen.py: the canvas's inspector names an inline <svg> from that folder by its geometry, and hands it back as a vector asset. A glyph written as a literal in gen.py gets no name.
• Artboard box is 478 × 980. Overflow clips silently. Do not check this by eye; refkit shoot ... --check-overflow asks the layout engine and exits non-zero with the exact px, so a clipped board fails in Phase 3 rather than turning up in Phase 4. It also flags elements clipped inside their own containers; mark a deliberate clip (a fade, a masked hero) with data-clip-ok.
• Phone frame is 393 × 852 pt at 1pt = 1px: 54px status bar, 125 × 36 Dynamic Island, 139 × 5 home indicator.
• No board background on a screen artboard. Give body no background at all, so the phone floats on the canvas ground and its drop shadow lands on whatever the board is placed over. A cream or grey field behind the phone paints one opaque rectangle per artboard, and a row of those reads as jarring next to its neighbours. The exceptions are the full-bleed sheets, the token board and the evidence boards, which are their background and keep it.
• Avoid SF Symbols private-use glyphs; they render as tofu without SF Pro installed. Inline the SVG, or embed a rasterized symbol as a data: URI.
• Icon size is not a calibration loop. Set each SVG's viewBox to the measured ink box, in pt, and give the span the same numbers; scale is then 1:1 with nothing to converge. The default preserveAspectRatio (xMidYMid meet) means that when the span's aspect ratio disagrees with the viewBox's, only one axis binds, and a size loop silently converges on the wrong axis and stops improving.

Copy is part of the replica. Transcribe the reference's strings exactly, including where each line wraps. A title that breaks after "iPhone and" instead of "iPhone and AirPods" is a real defect. Force it with explicit widths, <br>,   or <wbr> rather than hoping the browser agrees.

When a substituted face fights a measured width, the wrap wins. A stand-in that sets wider pushes a string onto a second line inside a container the capture shows holding one, and two measured rules collide. Widen the container and record why; never shrink the type to make a measured width hold. Claude's user bubble measures 302.6 and ships at 316.

Artwork: crop it, do not draw it and do not generate it

A screen that is mostly illustration is not mostly work. Every picture on the capture is cropped out of the capture at its own measured box, keyed by id in a crops.json the generator reads, cut to assets/art/<id>.png and placed back by an art() helper at the same numbers. The asset then cannot drift from where it was measured, and its pixels are the reference's own.

The rule and the arithmetic behind it: a crop scores 0 by construction, and the same crop redrawn by gpt-image-2 scores 38.53, the same character with a different head-to-body ratio and the props moved. So generate only pixels the capture does not contain, name those assets in the folder README, and give each one a probe like any other measurement.

When you do have to generate, the thing that decides the result is the input's layout, not the prompt: pack the assets into a grid, each in its own cell at the size and position it must come back at, and the model upscales in place instead of composing. That took this repo's set from 18.41 to 3.96 mean delta, with every cell returning at scale 0.99-1.00. Density is free (77 assets in one call beat 6), but the asset's native size in the capture is not: under about 128px, colour does not survive the redraw, so small icons stay CSS or inline SVG. artgen runs the whole loop, including keying the cells back out, solving the fit and scoring each asset against the crop it came from.

references/assets.md has the decision table, the crops.json/cut()/art() shape, and why assets/art/ is committed while assets/refs/ is not; references/generating.md is the generation procedure end to end, with the key-colour, alpha-ramp and fit-sign traps that each cost a run.

Model the line box once, then place by ink

A screen carrying real prose is a dozen placements of the same few numbers, so measure the block model once with bands and let the generator solve the rest: the line box per level, the margin between each pair of block types, and the offset from a line box's top to the ink inside it. Then write the builder to take an ink top and solve back to the box, so every call site is a number read straight off the grid rather than one derived by hand.

K = {"sans": 0.2708, "serif": 0.2240}      # SF Pro, Georgia: cap top within the line
def ct(ink, fs, lh, face="sans"):          # the box top that puts ink where you want it
    return ink - ((lh - fs) / 2 + K[face] * fs)

Claude's answer column is one such model: h1 29/36.3, h2 25/31, body 17.8/25.5, margins of 8.4 generic, 5.7 into a heading, 10.3 out of one, 8.2 between list items. Measured once, it placed five long screens to within 0.5pt. The alternative is a hand-tuned top per paragraph, which drifts the moment any string above it changes.

Never let content you invented carry a measured element into position. Where a composer, a fade or a scroll edge hides part of a block, the hidden span is not on the capture and cannot be transcribed. Filling it with plausible prose so that the flow pushes the visible remainder down makes that remainder's position an invention, and it will read as measurement to everyone after you. Place what you can see as its own absolutely-positioned block on its own measured ink top, and treat the gap as a line count or as nothing at all. Say so in the folder README.

layout.json already holds the reference row (Phase 5 builds it first). Add the Foundations row and the numbered-screens row as the boards land.

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Phase 4: verify by rendering, not by reading

Render at the capture's own scale and cut the screen out of the frame, so your render and the reference share one pixel grid, then replay Phase 1's probes against the renders before anything else:

refkit shoot "$B"/[01]*.html \
    -o "$B/scratch/mine" --scale 3 --crop-phone --check-overflow
refkit batch probes.json --against "$B/scratch/mine"
refkit diff "$B/scratch/mine/07-models-sheet.png" "$B/assets/refs/cp7.png" \
    --pt 3 -o "$B/scratch/d07.png" --regions regions.json

--scale takes an integer, and your capture's scale is not one. A 2.2417 px/pt capture against a 3× render is a shape mismatch and diff refuses it. Shoot at the nearest integer and downscale each render to the capture's own pixel size (Pillow, LANCZOS) before diffing. Put the whole chain, regenerate to shoot to downscale to diff all N, in one scratch script on the first iteration; you will run it thirty times.

batch re-runs every measurement that justified a token, reference against render, in one table. crops probes.json --against mine -o crops/ writes a paired NEAREST-upscaled crop per probe. Size is a numbers problem, shape is a picture problem: the crops are what find "too small, too small, and rotated 10 degrees" while the numbers read fine.

--crop-phone removes the crop step from every iteration; it also masks the 52pt corners, so a cropped screen composites onto any ground without the four black wedges of bezel a rectangular crop keeps. A bare capture has square corners full of real content, so diff and blend score only where both images have ink and say what fraction they dropped; without that the four wedges quietly add a level or two to every number you publish. diff writes the side-by-side and prints the numbers behind it. With --regions (inline {"name": [x0,y0,x1,y1]}, or a file you write once and reuse for the run) it tables mine-vs-ref per region with a Δ column; with no regions it ranks the bands where the two disagree most, which is how you find a defect you have not thought to look for yet.

Then read the side-by-side image, in this order:

1. Nothing clipped; --check-overflow has already answered this.
2. Line wraps match the reference, string for string.
3. Structure: what sits inside the card vs. outside it; which insets differ between header and body.
4. Colour: the diff table. Trust it over your eye. A defect described as "~4 levels dark in one band" turned out, measured, to be a whole backdrop desaturated (mean chroma 2.0 vs 5.9) at matching luminance.

Re-render after every correction pass. A correction you have not re-rendered is not a correction.

Going deeper

references/comparing.md covers subtraction (the blend that finds "right colour, wrong place"), the offset probe that separates placement error from colour error, fitting a material you cannot sample, when a run is actually done, and how to fan out the looking without fanning out the editing.

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Phase 5: park the reference under the mockup

Build these boards at the start of the run, not the end. The captures need no measurement and exist at t = 0; embedding them puts the source of truth on the canvas within two minutes and gives the user something real to look at while the replica is measured. The section sits here because the row is only finished when every mockup stands over its capture.

The replica is only auditable next to its source. Embed each capture as a base64 data: URI in its own ref-NN-<slug>.html, listed as a third row in layout.json in the same order as the mockup row. The canvas lays rows out at index * (w + gap) from x = 0, so item N of row 3 lands directly under item N of row 2.

{ "title": "Source of truth: captures",
  "numbered": true,
  "files": [{ "file": "ref-01-splash", "label": "Splash" }] }

Keep the source's attribution watermark in the image. Caption each one with its screen id, and state in your report where a reference is a near-match rather than the exact frame (a toast, a different scroll position, one row label off). Never let a near-match pass as exact.

Three mismatches are by design; name them in the report as expected, not as defects: Mobbin composites out the Dynamic Island while the frame spec draws it; --crop-phone masks the 52pt corners, so crops show rounded corners where raw captures are square; and hero, map and avatar bitmaps are crops of the capture itself.

Open the board as soon as its first HTML file lands. The dev server watches the boards directory and picks up a folder created after it booted, so there is nothing to restart. If ?canvas=<slug> opens the wrong page anyway — right URL, no error — the folder holds no .html file yet, and an empty folder is not a board.

open "http://127.0.0.1:<port>/?canvas=<slug>"

#<file> after that opens one board in the inspector, with the camera on it: ?canvas=<slug>#03-home is the link to give for one screen.

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Phase 6: the folder documents itself

A board nobody can audit in six months is not finished, and the canvas shows pixels rather than reasoning. Three files, all of them small:

mockups/canvases/<slug>/README.md. Copy the shape from $KIT/mockups/canvases/apple-settings/README.md.

references/documenting.md lists what it has to carry past a list of screens: the delta table, every substitution and its consequence, the defects that are the source's rather than yours, and which assets were generated rather than cropped.