D2 Slides

d2-slides

Paths below: <skill> is this skill's directory and <skills> the directory the skills are installed in (skills/ in mizchi/explainer, .claude/skills/ in a project that installed them).

A deck is text: the prose is Markdown, every figure is a ```d2 fence, and the build is a pure function of the file. Nothing is dragged, so a slide can be reviewed in a diff, regenerated after a rename, and — because the output is an HTML page — checked. That last part is the point: a slide deck normally has no failing state, so nobody notices the bullet the frame cut in half.

node <skill>/assets/build-deck.mjs deck.md --out built
# ✓ built/index.html: 8 slides, 5 figure(s) laid out by tala, 26 copy lines → built/copy.txt
#   the deck:   built/index.html   → vlmkit check integrity
#   all slides: built/print.html   → vlmkit check copy --manifest, check a11y contrast, print to PDF

Requires d2 with TALA (d2 layout must list tala (bundled) — see d2-diagram for installing it) and nothing else: the builder has no dependencies and the deck loads no network resource.

The deck format

---
title: Nothing tells you the picture is wrong
subtitle: two rounds, measured
date: 2026-09-14
---

# Nothing tells you the picture is wrong      ← a lone `#` heading is the title slide

---

## A D2 diagram is a drawing                  ← `##` is a slide heading

- `d2 validate` reads **syntax**              ← bullets; `code`, **bold**, *italic* work
- there is no `--expect`

```d2                                          ← the figure, laid out by TALA
direction: right
write -> check -> facts
```

<!-- notes: what to say out loud -->          ← speaker notes: printed, never on screen

• Slides are separated by a line of exactly ---; a --- inside a fence is not a separator.
• Bullets plus a figure lays them side by side; a figure alone gets the whole stage; a heading alone is a title slide. > line is a pull quote, and a fence in any other language stays a code block.
• Each slide is a fixed 1280×720 frame, scaled to whatever screen it is shown on. Nothing reflows between a laptop and a projector, which is also why a screenshot of slide 4 is the same picture everywhere.

The loop

1. write deck.md                                     prose in Markdown, every figure a ```d2 fence
2. node …/build-deck.mjs deck.md --out built         fails on the first figure that does not compile,
                                                     naming the slide; warns when a slide's prose
                                                     overflows its frame's character budget
3. vlmkit check integrity built/index.html            a slide that breaks, clips, collides or paints nothing
4. vlmkit check integrity built/print.html --viewports 1280
                                                     every slide at once — this is what catches the
                                                     bullet the frame cut off
5. vlmkit check copy built/print.html --manifest built/copy.txt --allow-invisible unknown
                                                     every line of the deck's own text still on a slide
                                                     (add --forbid stale.txt when you are editing a deck:
                                                     the claim you removed must really be gone)
6. vlmkit check a11y contrast built/print.html        readable from the back of the room
7. node …/d2-facts.mjs --from-svg built/slide-NN.svg --expect slide-NN.facts.json
                                                     what each figure DRAWS, for every figure that has
                                                     facts it must not get wrong — steps 3-6 are blind to it
8. node …/deck-review.mjs built --out review           shoot each slide, hand the PNGs to a reader
   node …/deck-review.mjs built --answers a.json      score what it read back: fidelity, order, SPLITs
9. read built/index.html in a browser, or the PNG of a slide; fix the Markdown; go to 2

Step 7 is not optional decoration, and the four page gates do not cover it. They read the page: a figure that compiles and draws the wrong thing passes all four. The checker takes the slide SVGs the build already writes and needs no d2 (it reads the render), so this costs one command per figure:

node <skills>/d2-diagram/assets/d2-facts.mjs --from-svg built/slide-04.svg \
  --expect slide-04.facts.json     # exits 1 and names the box or edge that is wrong

The same trick pins two figures to each other, which nothing else does: when slides 2 and 3 both claim ledger is the writer, hold both SVGs to one sheet ({"boxes": ["checkout","ledger"], "deps": ["checkout->ledger"], "forbidden": ["checkout->orders"]}). Update one figure and forget the other and that sheet fails on the one you missed. The fact-sheet schema is d2-diagram's; exhaustive: true is what makes an extra box an error rather than a shrug.

Step 8: somebody has to read it

Steps 3-7 are the page and the figures' geometry. None of them reads the slide the way a viewer does, and that gap is not theoretical: in the v1/v2 rounds two defects reached a delivered deck — a sentence rendered as two blocks, and a fragment sitting above its own bullet — with all four gates green, because every fragment was visible text and nothing overflowed.

node <skill>/assets/deck-review.mjs built --out review
#   → review/slide-NN.png, one per slide, plus review-brief.md
#   hand that directory to a vision model or an agent; it answers JSON
node <skill>/assets/deck-review.mjs built --answers review/answers.json
#   read 19/21, invented 6, fidelity 0.70, order 1.00
#   ⚠ SPLIT: "…it calls ledger, which is now the only writer to orders."
#     every word is on the slide, the sentence is not — read back as 2 blocks

The scoring is deterministic, against the slides.json the build writes: each slide's text in reading order (which copy.txt is not — the manifest is a set, and it lists bullets before paragraphs while the page renders paragraphs first) plus the words its figures draw.

Two numbers and one line to read:

• fidelity — sheet lines read / (sheet lines + invented). 1.00 is "the slide, nothing more". Measured in v3: 1.00 on a clean deck from both readers, 0.70 on the same deck built by a builder that split its bullets.
• order — pairs of read lines whose order disagrees with the sheet. It is for a genuinely reordered slide; be aware it did not catch v3's split, because the fragments still appeared in the sheet's relative order. Fidelity and the SPLIT line caught that.
• ⚠ SPLIT — a sheet line no single read line covers, whose words are all present across two or more. That is one sentence rendered as two blocks, and the tool names it whether or not the reader had the word for it: one v3 reader called the same defect "clipped at the right edge" — right slides, wrong mechanism — and the diagnosis came out anyway.

--sheet <other-build>/slides.json scores one render against another build's sheet, which is how a before/after pair is compared against one ground truth.

Honest limits. A reader that transcribes none of the figures' labels is not evidence they are illegible, only that it read prose — the figures: k of n line is a hint, not a verdict. A reading is a measurement and exits 0; what to do about a 0.7 is the author's call. And a reader will occasionally report a mechanism that is not the real one, so read the slide it names before believing the reason it gives.

Done condition

• The build exits 0 with no overflow warning.
• check integrity reports no failures on index.html (all three viewports) and on print.html at 1280. A warn is not a failure and the command still exits 0 — see the figure-internals note below before you spend a round chasing one.
• check copy --manifest reports nothing missing (and nothing forbidden, if you are editing an existing deck).
• check a11y contrast reports no failure, over a count that means every slide was read. The count is content-dependent — a terse deck runs about 7 elements a slide, a dense one about 17 — so the number is not the test. Run it against index.html too: that shows one slide at a time, so it comes back in single digits. Print view ≫ deck view is the check.
• Every figure with facts it must not get wrong passed d2-facts --expect.
• Somebody read it back. You pressed through the deck yourself, or a reader scored it with deck-review — fidelity near 1.00 and no SPLIT. This is the only step that sees a sentence broken into two blocks.

The same loop, in CI

This repo runs its own worked example through that loop on every change to the skill, the example or the three gates it uses, so the claim above is measured rather than asserted. Two halves, because they need different machines:

• tests/d2/d2-slides.test.mjs (in the ordinary suite, no d2, no browser) — the builder's decisions before a figure is drawn: where a slide ends (--- inside a fence is not a separator), which layout a slide gets, the manifest as the rendered text, the overflow warning naming the slide, a figure that does not compile failing the build, and examples/d2-slides/built/ still matching its deck.md. D2 points at a stub that echoes a fixed SVG — the builder already reads that variable.
• .github/workflows/d2-slides.yml (real d2 pinned at v0.9.0, real Chromium) — the four gate runs above on a fresh build, plus a byte comparison against the committed build, plus two negative controls: a manifest line the deck does not say must be reported copy-missing, and a frame shortened to 260px must be reported clipped-content. A suite that only ever sees passes cannot tell a working gate from a gate that always passes.

Regenerate the committed example with npm run deck:example after editing its deck.md — the byte comparison is what keeps a committed build from drifting into a lie.

What the gates found in this skill's own deck

Everything below was a real defect in the template or the example, caught by a gate, fixed, and re-measured. It is why the loop above is in that order.

• A phone showed nothing. check integrity at 375px: "the DOM holds 4 text blocks but almost nothing painted (ink ratio 0.05%)". Centring a 1280px stage as an over-sized grid item leaves the middle off-screen; the stage is centred by transform now.
• Every slide clipped 114px. A percentage height inside a padded fixed box is not the box: the frame is a flex column and the body the flexible child. The gate reported it as clipped-content on slides whose text was well inside the frame.
• Long bullets were cut top and bottom. With align-items: center on the split layout, prose taller than the frame overflows both ways and the copy gate reads those lines as copy a user cannot see. Overflow goes downward now, where it is measurable, and the builder warns above ~430 characters of prose beside a figure.
• The manifest did not match the render. It is generated from the rendered markup, not from the Markdown with its markers stripped: a line containing ```d2 strips to something the page never says.