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+# seo/json-ld: schema.org vocabulary validation via a schema-dts-derived catalog
+
+Date: 2026-08-09
+Status: Design (spike deliverable for issue #421 — answers its four questions; implementation is a
+separate, later PR)
+Origin: 2026-08-09 v1.0 rule-validity review follow-up; suggested by the maintainer during #417
+review.
+
+## The gap (restated)
+
+A typo'd type — `"@type": "Artcle"` — passes every rule in the json-ld family today.
+`seo/json-ld-validity` gates parse + `@context` + `@type` _presence_; `seo/json-ld-required-props`
+looks types up in its curated Google-requirements table (an unknown type simply has no row); nothing
+checks that a declared name exists in the schema.org ontology at all. The ontology has a maintained,
+typed distribution — [google/schema-dts](https://github.com/google/schema-dts) — from which a
+catalog can be generated and kept current by a normal Renovate bump.
+
+Verified facts about schema-dts (2.0.0, inspected via unpkg):
+
+- Types only: one `dist/schema.d.ts` (~992 KB) and a 77-byte runtime stub. Nothing importable at
+  runtime — the catalog must be extracted at build time.
+- Every schema.org type is an `export type` alias (`export type Airplane = AirplaneLeaf;`,
+  `export type Accommodation = AccommodationLeaf | Apartment | … | string;`) — roughly 1,000
+  exported aliases. Property containers (`*Base`) and `@type` discriminators (`*Leaf`) are
+  interfaces, not aliases, so the alias namespace is almost exactly the vocabulary. Four generic
+  helpers (`WithContext`, `Graph`, `SchemaValue`, `IdReference`) are the exceptions to filter.
+- schema.org-level deprecations/supersessions are carried as `@deprecated` JSDoc on the export
+  (e.g. Attorney → "LegalService is more inclusive and less ambiguous").
+
+## Decision summary
+
+**Phase 1 (recommended): `@type` existence only, as a new arm of `seo/json-ld-validity`, backed by
+a generated name catalog committed in `@svelte-vitals/core` (~800–1,000 names, ~20 KB) with a
+drift test.** Property-name and property-domain validation are rejected (not deferred for size
+reasons alone — they have a legitimate-use false-positive problem; see below). Ontology
+supersession warnings are deferred as a possible phase 2.
+
+## The four spike questions
+
+### 1. Scope: type existence / property existence / property-domain
+
+**Type existence only.**
+
+- `@type` typos are the highest-value class: a typo'd type silently voids the whole node's
+  rich-result eligibility, and `json-ld-required-props` goes blind at the same moment (no row
+  found). One catalog lookup catches it.
+- **Property-name existence is rejected, not just deferred**: schema.org's data model explicitly
+  tolerates extension properties and mixed vocabularies — publishers legitimately attach custom or
+  third-party-vocabulary properties to schema.org nodes, and Google's parser ignores unknown
+  properties rather than penalizing them. A property-existence check therefore flags legitimate
+  markup with no eligibility consequence — a false-positive class we would own forever. (The one
+  false-positive-free variant — checking only properties that are _almost_ a known name, i.e.
+  did-you-mean on properties — needs the full property list anyway and still guesses about intent;
+  not worth the bytes.)
+- **Property-domain validation** ("is this property valid _on this type_") inherits the same
+  false-positive problem plus the largest catalog (per-type property maps, 100 KB+ in core for a
+  check Google doesn't perform). Rejected.
+
+### 2. Home: new rule vs. extending `seo/json-ld-validity`
+
+**Extend `seo/json-ld-validity`.** It already owns "malformed at the JSON-LD level" (unparseable
+JSON, missing `@context`, missing `@type`), and an unknown `@type` is the same family: the document
+is structurally broken as schema.org data, independent of any specific consumer's requirements.
+This also keeps the severity story coherent — the rule is registered `warning`, and an unknown type
+is exactly as consequential as a missing one (the node drops out of eligibility either way). No
+severity split: the new arm fires at the rule's registered severity, so a global
+`rules: { 'seo/json-ld-validity': … }` override behaves unsurprisingly.
+
+A second registered rule would need its own docs pages, inventory weight, and severity
+narrative for what is one concept; rejected.
+
+Message shape (message text is not part of `findingKey`, so wording stays adjustable):
+
+```
+Unknown @type 'Artcle' — not a schema.org type. Did you mean 'Article'?
+```
+
+The did-you-mean suggestion is a case-insensitive exact match against the catalog (cheap, no
+edit-distance dependency); omitted when nothing matches. Multi-typed nodes
+(`"@type": ["Product", "Vehicle"]`) validate each name; one finding per unknown name.
+
+### 3. Case handling and non-schema.org contexts
+
+- **Exact, case-sensitive match.** schema.org names are case-sensitive PascalCase; `"article"` is
+  not a valid type and Google's parser treats it as unknown. The case-insensitive pass exists only
+  to power the suggestion, never to accept.
+- **Non-schema.org `@context` exempts the document.** Custom vocabularies are legal JSON-LD. The
+  arm runs only when the node's governing `@context` is schema.org — accepted spellings:
+  `http(s)://schema.org` with optional trailing slash (the existing `@context` handling in
+  `json-ld-validity` already recognizes these; reuse it). If `@context` is an array or object that
+  mentions any non-schema.org vocabulary, the document is exempt — term remapping means a name we
+  don't recognize may be perfectly valid. Conservative by construction: silence, never a false
+  positive, on exotic contexts.
+
+### 4. Catalog size vs. core bundle budget
+
+- Names only: ~800–1,000 entries × ~13 chars ≈ **20 KB source** as a
+  `ReadonlySet<string>` literal in a generated module. Acceptable for `@svelte-vitals/core` (the
+  curated tables in `jsonld-engine.ts` set the precedent; this is one order larger but still
+  trivial next to the parser).
+- The property-domain map (the only expensive option) is rejected under Q1, so no budget question
+  remains.
+
+## Catalog + generator design
+
+Follows the repo's committed-generated-file pattern (`packages/cli/src/docs/generated.ts` +
+`docs-embed.test.mjs`; `packages/cli/src/ci/action-pin.generated.ts`):
+
+- **Dev dependency**: `schema-dts` added to the workspace catalog (`pnpm-workspace.yaml`) and
+  `packages/core`'s devDependencies as `catalog:`. Renovate bumps it like any other dep.
+- **Generator**: `packages/core/scripts/gen-schema-vocab.mjs`, run manually via
+  `pnpm --filter @svelte-vitals/core run gen:schema-vocab`. Core's no-`node:`/no-I/O rule covers
+  `src/` only; dev-time scripts are exempt (same as its test suite). The script:
+  1. Locates `schema-dts/dist/schema.d.ts` via `import.meta.resolve`.
+  2. Parses it with the TypeScript compiler API (already a workspace devDependency) and collects
+     exported **type alias** names — interfaces (`*Base`, `*Leaf`) are excluded by node kind, and
+     the four generic helpers (`WithContext`, `Graph`, `SchemaValue`, `IdReference`) by an explicit
+     list, not by pattern (a hypothetical future vocabulary type ending in "Leaf" must not be
+     silently dropped; the alias-vs-interface distinction does the structural work).
+  3. Sanity-asserts the count lands in 700–1,500 — outside that band the upstream shape changed and
+     the script fails loudly instead of committing a gutted catalog.
+  4. Emits `packages/core/src/rules/seo/schema-vocabulary.generated.ts`: a header naming the
+     schema-dts version it was generated from, plus
+     `export const SCHEMA_ORG_TYPES: ReadonlySet<string>`.
+- **Drift test**: `packages/core/test/schema-vocabulary.test.mjs` re-runs the extraction against
+  the installed schema-dts and fails if the committed module differs — so a Renovate bump of
+  schema-dts fails CI until the catalog is regenerated and committed, which is the mechanism that
+  keeps the catalog current (mirrors `docs-embed.test.mjs`).
+
+## Deferred (phase 2 candidates, each needing its own decision)
+
+- **Supersession warnings**: the generator can also capture `@deprecated` exports into a
+  `SUPERSEDED_TYPES` map (name → replacement parsed from the JSDoc). This overlaps conceptually
+  with `seo/json-ld-deprecated-type` — whose curated list is about _Google dropping rich-result
+  support_ (HowTo, FAQPage), a different statement than _schema.org superseded this term_. Shipping
+  both without confusing users needs message design; deferred rather than bundled into phase 1.
+- **Vite provider parity**: nothing here is provider-specific — the arm lives in the shared engine,
+  so both providers get it for free. Listed only to note there is no extra work.
+
+## Implementation-plan requirements (for the eventual phase-1 PR)
+
+- New penalized findings can appear on existing projects (any typo'd `@type` that was silently
+  passing), so under `--fail-on warning` a previously green run can turn red. The changeset must
+  declare this movement explicitly, and the review runs the full fixture × `--fail-on` × pre/post
+  truth table (established practice since #428).
+- The arm adds a per-node set lookup to the existing parse pass — no new I/O, no io-budget effect.
+- Docs (en + ja): new "Unknown type" section on the `seo/json-ld-validity` rule pages, including
+  the exemption rules for non-schema.org contexts; regenerate rule indexes if the pages' headings
+  change.
+- `@svelte-vitals/core` changeset: minor (new detection surface).
+
+## Rejected alternatives (record, so this isn't re-litigated)
+
+- **Runtime dependency on schema-dts**: impossible — it ships no runtime values.
+- **Fetching the schema.org vocabulary JSON at generation time**: adds a network step and a second
+  source of truth; schema-dts already tracks releases and Renovate handles currency.
+- **Hand-curating a "common types" subset**: the full name list is only ~20 KB; curation would
+  reintroduce exactly the staleness this design eliminates.
+- **Property-name / property-domain validation**: see Q1 — legitimate-use false positives
+  (extension properties are valid schema.org usage), plus the only expensive catalog. This is the
+  standing rejection; revisit only with evidence that unknown-property typos materially hurt
+  eligibility.