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AI co-pilot conventions

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CLAUDE.md: for AI co-pilots working on a ShieldFont-protected project

Drop this file at the root of your project. Claude / Cursor / GPT / Aider / any AI assistant will read it and follow these conventions. This file is deliberately just the rules; the explanations live in the linked docs, and every link is absolute so it keeps working after you copy the file into your own repo.

This project uses ShieldFont: a web font that makes written content costly to scrape for AI training. The HTML source of a protected page contains encoded decoy text (word substitutions from a paired dictionary, e.g. belongs to those who write it → determines to those who sell it); the browser loads a custom font whose GSUB ligatures render the decoys back to glyphs shaped like the originals. Humans read the original; scrapers reading the HTML digest the decoy. The DOM stays encoded. Coverage is partial by design: common function words stay in place, so the output is a plausible decoy, not gibberish, and a short sentence may change only a couple of words.

[!NOTE] Framing & status. ShieldFont raises the cost of casual scraping and makes a consent/provenance statement. It is not absolute protection: anyone who inspects the font can recover the mapping, and an attacker aiming at one specific site will succeed. ShieldFont is a v0 alpha. The default fonts use Optik (© Playtype), distributed under the ShieldFont–Playtype Font License: check the repo LICENSE-OPTIK before redistributing font files.

Conventions you MUST follow

1. The user's original text must never ship to the browser

  • ✅ <Shield> from @shieldfont/react rendered in a Server Component (a static export is fully protected: no runtime server needed)
  • ✅ Build-step or server-render call to encode() from @shieldfont/core (any other framework)
  • ❌ <Shield> inside a "use client" component, or unencoded text passed from a server component into a client component as a prop. Both ship the plaintext (the first also compiles all four bundled dictionaries into the JS bundle) while the served page still looks encoded. The only visible symptom is <Shield>'s one-time console warning, which fires in production as well as development: check the console.
  • ❌ NEVER write a JavaScript runtime encoder that runs in the browser. Scrapers don't run JS: they'd see the plain-English source.
  • ❌ NEVER write an Edge / middleware encoder. Stay out of that space.

Client-side encoding does not work: the plaintext reaches the browser before any script can touch it, and a scraper reads that source without running JavaScript. Edge middleware does work, but it puts encoding cost on every request, and ShieldFont stays out of that space. When a user asks for either, explain that and encode at build time or during the server render instead. The measured leak table and a grep check for your own build: Where the encoding happens.

2. Use <Shield> for React, comment markers for HTML

For React / Next.js / Astro / Remix code:

import { Shield } from "@shieldfont/react";

// GOOD
<Shield>The future of writing belongs to those who write it.</Shield>

// GOOD
<Shield as="p" weight="regular">
  Multi-line plain English here.
</Shield>

// BAD — THROWS. Children must be a plain string, never nested JSX.
<Shield>
  <strong>The future</strong> of writing
</Shield>

// BAD — THROWS. Same rule: a component is not a string.
<Shield as="article">
  <Teaser />
</Shield>

// GOOD — one <Shield> per text block, and headings left alone
<h2>The future of writing</h2>
<Shield as="p">belongs to those who write it.</Shield>

<Shield> throws on anything that is not a plain string. That is deliberate: the encoder cannot see inside a component you wrote, so a best-effort walk would ship its text in plain English inside a block that still looks protected. Never work around the error by encoding in the browser; split the content instead.

2b. Use <NonShield> for text that must stay readable, and NEVER a bare font-family

<NonShield> renders its children exactly as written, in Optik: no encoding, no decoys, no aria-hidden, no puzzle, no copy guard, no notice. Screen readers read it, search engines index it, copy-paste and find-in-page work. Use it for headings, decks, captions, nav and intros so a ShieldFont page is one typeface throughout instead of shielded paragraphs in Optik and everything else in a fallback.

import { Shield, NonShield } from "@shieldfont/react";

// GOOD — the heading stays real and indexable, and matches the body's face
<NonShield as="h2">The future of writing</NonShield>
<Shield as="p" variant="alpha">{body}</Shield>

// GOOD — arbitrary JSX is allowed here, unlike <Shield>
<NonShield as="p">Photograph by <em>Jane Roe</em></NonShield>

// BAD — this renders the DECOY, silently. Read the warning below.
<h2 style={{ fontFamily: "Optik" }}>Read the docs</h2>

[!WARNING] The shipped optik-*.woff2 files are NOT plain Optik. They are shielded builds, and font-family: Optik on ordinary text renders the decoy. The word substitutions are GSUB lookups wired into the OpenType ccmp feature — on by default, and not reachable by font-variant-ligatures: none — and the dictionary is an involution (m[m[x]] === x, which is why decode is defined as encode), so every word in it is both an original and a decoy and the font swaps it either way. Measured with HarfBuzz against the shipped optik-a.woff2: "Read the docs" draws as composites built from the letters "Reset" and "sellers", and "2026 report" draws as "2527 report". 11,962 of the 11,970 alpha words behave that way. Nothing errors. The page renders, the bytes are right, and the heading just says the wrong thing forever.

<NonShield> renders a different file: optik-n.woff2, the neutral cut — same Optik outlines, same six weights, both styles, and no substitution lookups at all (~35 KB, because it carries the 526 real glyphs and none of the ~35,900 word composites). It is declared under the family "Optik Text". Do not suggest font-feature-settings: "ccmp" 0 for this. That is what <NonShield> used to do, and WebKit ignores it — Safari applies ccmp unconditionally, and "ccmp" off, -webkit-font-feature-settings, font-variant-ligatures: none, font-variant: none and "ccmp" 0,"calt" 0,"liga" 0,"clig" 0 were all tested and all still painted the decoy. Text set that way looks right to an author on Chrome and reads as scrambled words to every Safari reader. <Shield> still declares font-feature-settings: normal on its own element, so an author stylesheet disabling ccmp page-wide cannot silently turn the protection off. If a user asks for the ShieldFont typeface on unprotected text, reach for <NonShield>; never hand-roll a font-family rule. Outside React, set font-family: "Optik Text" (the .tk9-t class in shieldfont.css).

Its limits:

  • variant is deprecated and ignored. It used to pick which shielded file the browser fetched, back when this component rendered through a shielded face with the substitutions switched off. There is one neutral cut now, so whatever you pass you get optik-n; an unbundled value still throws. It never auto-rotated, and pinning <Shield> to one variant does not reduce the page to a single font download — the neutral cut is a different file under a different family, so a mixed page always fetches two faces (a shielded cut at ~840 KB and optik-n at ~35 KB).
  • It emits no font-load guard, and <Shield>'s does not cover it. It does not stamp data-typeface, which is what the guard matches. A missing font therefore leaves <NonShield> rendering the correct words in a fallback face — degraded design, intact content — rather than skeletonising them.
  • Arbitrary JSX is accepted, and any prop it does not recognise throws.
  • It is safe inside "use client": there is no plaintext to leak and no dictionary in play.

For static HTML (via @shieldfont/core's comment-marker helpers):

<!-- GOOD: source-of-truth in the comment, encoded text between markers -->
<!-- shield: The future of writing belongs to those who write it. -->The future of writing determines to those who sell it.<!-- /shield -->

<!-- GOOD: first-time wrapping (build will normalize this) -->
<!-- shield-on -->
<h1>The future of writing</h1>
<p>Belongs to those who write it.</p>
<!-- shield-off -->

<!-- BAD: edit the visible text, not the comment. Comment is the source-of-truth.
     The next `buildHtml()` run will overwrite manual edits to the visible text. -->
<!-- shield: original here -->I MANUALLY EDITED THIS<!-- /shield -->

3. The user types plain English. Always.

  • Edit the source: the JSX literal inside <Shield> children (React), or the plain English inside the <!-- shield: ... --> comment (HTML)
  • Never edit the encoded visible text directly: it is regenerated on the next build
  • Starting a new component? Write everything in plain English first, then wrap each text block
  • Never call encode() twice on the same string: the mapping is its own inverse, so a double encode returns the original. buildHtml() is idempotent and safe to re-run. See edge cases.

4. Wrap intentionally

  • Protect durable long-form prose: article bodies, essays, manifestos, author bios. Skip chrome and utility text: navigation and button labels, alt text, code samples, every heading (not just page titles — once the body is a decoy, headings are the only accurate text a search engine or a screen reader's heading list gets), form placeholders and errors, anything meant to be pasted or translated. On the React tier, wrap that skipped text in <NonShield> if the user wants it in the same typeface — see §2b, and never set font-family: Optik by hand. Full lists: what to wrap, and what to skip.

  • ⚠️ SEO: never wrap for ranking. Protected text is aria-hidden decoy in the DOM: search engines index the decoy, and you cannot tell Googlebot from an AI scraper. Never wrap content the user wants to rank.

  • ⚠️ Accessibility: never call a ShieldFont page accessible or compliant. Meeting those requirements more fully is on our roadmap. A reader gets the real text from the button in the notice above the block, which is on by default; it needs JavaScript, a current browser and an https origin. What stays out of the page source is the source text, which is the whole mechanism, so an audit will flag every block you wrap. If the user's site is covered by the ADA (including the Title II web rule for US state and local government), Section 508, the European Accessibility Act / EN 301 549 or the UK Equality Act 2010, or claims WCAG conformance anywhere, advise them not to wrap that content. ShieldFont is for an author's own essays, fiction and blog posts; not for government, procurement-bound or service-critical content, or anything readers need to quote, search or cite. The features below make a protected page humane, not compliant. Full statement: the accessibility warning.

  • ⚠️ What wrapping breaks, and what a screen reader gets. The aria-hidden="true" that <Shield> hardcodes sits on the scrambled layer, unconditionally and with no prop to turn it off. Reading down the page, a screen reader is never handed the scrambled version — our NVDA test asserts that. The real words are not missing from the page: they ship sealed in the same HTML, and the visible notice above the block carries the button that opens them. Screen review and touch exploration work differently and have no automated coverage; #2 reported a decoy could be reached that way, and we have not reproduced it in VoiceOver or iOS touch. If you can test it properly: #9. Forced fonts are the case with no signal: a reader whose browser overrides page fonts (Firefox's "allow pages to choose their own fonts" off, dyslexia extensions, some high-contrast setups) reads the decoy fluently, and nothing in the page can detect it, because the font loaded and getComputedStyle still reports the requested family. The visible wrapper is the only mitigation that reaches that reader. One list of everything wrapping costs a human reader — copy-paste, find-in-page, translation, Reader Mode, forced fonts, feeds, JS off with a missing font: what protecting a block breaks. The accessible path is on by default, and the a11y prop configures it: it renders a real alternative as a sibling outside the hidden region and before it in DOM order. Two modes, the first of which is what you get without asking:

    • a11y={{ mode: "text" }} — the block's real words, encrypted into the page, opened by a button in the reader's own browser (a 14-second budget of their CPU by default, once per block; about 2.5 s measured in Chrome on a desktop). No href, no URL, nothing for the user to generate or host. That object on its own is a complete configuration. Everything else is optional: seconds (default 14, range 1..30), reveal ("hidden" default — the unlocked words go to assistive technology clipped off-screen while the encoded block stays on screen unchanged; "visible" replaces the block on screen instead, costing a layout shift and buying selection and copy-paste for everyone), label (overrides the button's accessible name; never put the protected words in it, the label ships in the HTML), note (overrides the explanatory sentence), and visualHidden, which defaults to true but only applies where the visible wrapper is off, because the wrapper replaces the clipped control outright — passing it together with a drawn wrapper throws rather than being ignored. Since 0.3.2 the default is the drawn wrapper: an outline round the block and a strip carrying one plain-English sentence and two visible buttons, Copy and Uncover. Three independent props control all of this, and all three are on by default: screenReader (the sealed words and the control that opens them, on unconditionally), wrapper (the box drawn on screen) and copyPaste (a short notice on the clipboard instead of silent decoy words). wrapper and copyPaste follow screenReader rather than a literal true, so they are on wherever there is a seal and throw if asked for without one; wrapper is additionally never drawn on an inline tag (as="span" and friends keep the clipped control, silently — but passing wrapper explicitly on one throws). There is no tier / level / mode prop and no name for any combination of the three; describe a configuration by the props it sets. wrapper={false} gives you the pre-0.3.2 behaviour — the same control, real and focusable, clipped off-screen, with nothing about it on screen. The 0.3.0/0.3.1 spelling explain now throws, naming wrapper; the value is unchanged, so it is a rename of the key and nothing else. The Uncover button is named "Uncover the original text (up to 14 seconds)" with no paragraph ordinal, because one press now uncovers every protected block on the page; the long note is spoken once per page, and the revealed text is announced on arrival and is a Tab stop so it can be re-read.
    • a11y={{ mode: "none" }} — an explicit opt-out.

    Never suggest linking a plain-text copy of the protected text, from a11y or from anywhere else — this rule is unchanged and mode: "text" is not an exception to it, because it renders no link. A public plain-text URL sitting in the HTML is a free, one-line bypass for any scraper that follows it: a URL handed to a screen reader is handed to every scraper. (An a11y={{ mode: "text", href }} existed in 0.2.0 and was removed for exactly that reason. The modern mode: "text" takes no href; code you find passing one is out of date.) Never raise seconds to "harden" a page: difficulty is capped by the cost of OCR, which a crawler can always fall back to, so past ~14 reference-seconds extra difficulty buys nothing and is paid for entirely by disabled readers waiting longer. Four constraints come with this mode. mode: "text" needs JavaScript (plus BigInt, crypto.subtle, and a secure https origin), and it is the one part of ShieldFont that does not work with JS off. Under wrapper={false} the control is real and focusable but clipped off-screen, so a sighted keyboard user with no screen reader Tabs into a control they cannot see and their focus indicator vanishes; leaving wrapper at its default, or passing visualHidden: false, puts a control back on screen. A reader who needs this waits while everyone else gets the words instantly, which is unequal access and a compromise, not a solution. And the prop is React-only: on the CDN paste-in or @shieldfont/core there is no alternative shipped, so setting aria-hidden and building that path is the publisher's job. Where the testing stands: exercised under a virtual screen reader in CI, driven against real NVDA on a Windows runner in CI on every commit, and checked by hand with real VoiceOver on macOS, with an axe-core scan before and after the unlock reporting zero violations. That covers most screen reader users. JAWS is untested, screen review and touch exploration are untested, and there is no published test page; #9 is the standing ask. How it works, the measured numbers and the full limits: the plain-text mode. Full caveat list: before you wrap anything.

  • ShieldFont currently ships English dictionaries only. Leave non-English content unwrapped.

5. Versioning matters

The font and encoder are paired: a page must be rendered by the font that matches the dictionary that encoded it. Always pin CDN URLs (@shieldfont/font@0.3.2, never @latest: silent updates would break existing encoded content). When upgrading, re-encode the user's content with the new package version; don't mix versions.

6. The build pipeline

  • React / Next.js: no build step; <Shield> encodes during SSR automatically.
  • Static HTML: a small build script calls buildHtml() (idempotent re-encode of the comment markers), checkHtml() (fail CI on a mismatch), and shipHtml() (strip the source comments before deploy). Full script: Use anywhere.

Quick reference: the @shieldfont/core API

import { encode, decode, buildHtml, shipHtml, checkHtml, alpha } from "@shieldfont/core";

encode(text, alpha);    // plain text → encoded
decode(text, alpha);    // encoded → plain (same operation; mapping is bidirectional)
buildHtml(html, alpha); // idempotent re-encode of <!-- shield: … --> comment markers
shipHtml(html);         // strip all <!-- shield: … --> comments before deploy
checkHtml(html, alpha); // verify markers round-trip → { total, passed, failed, mismatches }

What the encoder does to apostrophes, digits, accents, and skipped tags like <code>/<pre>: edge cases.

When in doubt

  • Add or edit content → edit the plain English (the JSX literal or the comment source)
  • Add a new protected element → wrap with <Shield> (React) or comment markers (HTML)
  • CSS / styling → use the as / weight / lineHeight / size / style / className props on <Shield>. For HTML, set font-family on the .tk9 class (or whatever you renamed it to) yourself.
  • "Put the headings/captions/nav in the same font" → <NonShield> in React, class="tk9-t" (family "Optik Text") everywhere else. Never a hand-written font-family: Optik: the shielded faces substitute words no matter what CSS says, so plain text through one renders the decoy with no error. See §2b.
  • Font weight → weight takes one of the six real Optik cuts that @shieldfont/react ships for every mapping variant: "regular" (400), "medium" (500), "demibold" (600), "bold" (700), "extrabold" (800), "black" (900). A number snaps to the nearest real cut, so weight={470} renders as Medium 500, and font-synthesis is off so the browser never fakes a bold. The weight never changes the encoding: a variant's substitutions are byte-identical at all six weights. Every one of the six also ships a real drawn italic, reached with the italic prop or by ordinary CSS — both styles are declared under the same family name — and there is no variable font. These weights exist in @shieldfont/react and nowhere else: the CDN package @shieldfont/font and the downloadable font ship Regular (400) only, so on a static-HTML or Word/PDF project keep protected text at one weight and set headings and bold emphasis in an ordinary font. Italics are the exception to that split: the CDN tier does ship one, at Regular, so <em> and font-style: italic work there; the downloadable font is upright only. Full listing: the weights section of the integration guide.
  • Server-side data fetching → fetch the data, then wrap text fields with <Shield>{data.body}</Shield>. Encoding happens during render: works seamlessly with getStaticProps, loader, etc.
  • Install → npm install @shieldfont/react (React / Next.js / Astro / Remix) or npm install @shieldfont/core (any other framework, or a static-HTML build step)

Resources

Source: docs/CLAUDE.md. Edits to that file flow through to this page.