add-vocab

name	add-vocab
description	This skill should be used when adding new ActivityPub/JSON-LD vocabulary types to the @fedify/vocab package, defining new YAML vocabulary schema files, or updating existing vocabulary definitions in Fedify. Applies when implementing FEPs, extending ActivityStreams vocabulary, or adding third-party vocab types such as Mastodon extensions, Litepub types, or other fediverse vocabularies.
version	1.0.0

Adding vocabulary to @fedify/vocab

To add a new vocabulary type to the @fedify/vocab package, create a YAML definition file in packages/vocab/src/, then run code generation.

Human review requirement

Vocabulary definitions must always be reviewed carefully by a human before being merged. Errors in vocabulary definitions are difficult to fix after release because they break wire compatibility with existing software in the fediverse. Specifically, verify:

• The type URI and property URIs match the spec exactly (a single character difference causes silent interoperability failures)
• The defaultContext is complete and all terms compact correctly (see “Ensuring Complete Compaction Coverage” below)
• The entity flag is correct—getting this wrong changes the entire async/sync interface contract
• The functional flag is correct—marking a multi-valued property as functional silently drops values
• Every property range entry is accurate—wrong range types produce incorrect TypeScript types
• The spec document (FEP or W3C spec) has been read in full, not just skimmed

Do not rely solely on automated checks (mise run check)—they verify only TypeScript compilation, not semantic correctness of the vocabulary.

Workflow

1. Read the spec document (FEP, W3C spec, or informal documentation) carefully
2. Identify the type's URI, JSON-LD context, and properties
3. Check whether the JSON-LD context URL is already preloaded in packages/vocab-runtime/src/contexts.ts
4. Create the YAML file at packages/vocab/src/<typename-lowercase>.yaml
5. If a new context URL is needed, add it to packages/vocab-runtime/src/contexts.ts
6. Run mise run codegen to generate TypeScript classes
7. Run mise run check to verify everything compiles
8. Ask the user to review the YAML definition and generated code carefully before committing

The generated TypeScript class is automatically exported from @fedify/vocab via packages/vocab/src/vocab.ts (generated) and packages/vocab/src/mod.ts.

YAML file format

Every YAML file must begin with the schema reference:

$schema: ../../vocab-tools/schema.yaml

Top-level fields

Field	Required	Description
name	yes	TypeScript class name (PascalCase, e.g. Note)
uri	yes	Fully qualified RDF type URI
entity	yes	true for entity types (async property accessors); false for value types (sync accessors). Must be consistent across the inheritance chain.
description	yes	JSDoc string. May use {@link ClassName} for cross-references.
properties	yes	Array of property definitions (can be empty [])
compactName	no	Short name in compact JSON-LD (e.g. "Note"). Omit if the type has no compact representation.
extends	no	URI of the parent type. Omit for root types.
typeless	no	If true, @type is omitted when serializing to JSON-LD. Used for anonymous structures like Endpoints or Source.
defaultContext	no	JSON-LD context used by toJsonLd(). See below.

Entity vs. value type (entity flag): entity: true : Property accessors are async and can fetch remote objects.

entity: false : Property accessors are synchronous; used for embedded values. value objects (e.g. Endpoints, Source, Hashtag)

defaultContext format

The defaultContext field specifies the JSON-LD @context written when toJsonLd() is called. It can be:

A single context URL:

defaultContext: "https://www.w3.org/ns/activitystreams"

An array of URLs and/or embedded context objects:

defaultContext:
  - "https://www.w3.org/ns/activitystreams"
  - "https://w3id.org/security/data-integrity/v1"
  - toot: "http://joinmastodon.org/ns#"
    Emoji: "toot:Emoji"
    sensitive: "as:sensitive"
    featured:
      "@id": "toot:featured"
      "@type": "@id"

Embedded context entries are YAML mappings where:

• String value "prefix:term" or "https://..." defines a simple term alias
• Object value with "@id" and optionally "@type": "@id" defines a term that should be treated as an IRI (linked resource)

Ensuring complete compaction coverage

The defaultContext must cover every term that appears in the JSON-LD document produced by toJsonLd(), including:

The type's own compactName : If the type has a compactName, the context must map that name to the type's URI.

All own property compactNames : Every property defined directly on this type must have its compactName (or full URI fallback) resolvable via the context.

Inherited properties : Properties from parent types are usually covered by the parent's context URL (e.g., https://www.w3.org/ns/activitystreams covers all core ActivityStreams properties). Verify that the parent's context URL is included.

Properties of embedded types : When a property's value is an object type that is serialized inline (not just referenced by URL), the context must also cover all of that embedded type's properties. This is the most commonly missed case.

Common embedded types and the context URLs that cover them:

| Embedded type                       | Context URL to
include                        | | —————————————————- |
——————————————————————- | | `DataIntegrityProof` (from `proof`) |
`https://w3id.org/security/data-integrity/v1` | | `Key` (from
`publicKey`)            | `https://w3id.org/security/v1`                | |
`Multikey` (from `assertionMethod`) |
`https://w3id.org/security/multikey/v1`       | | `DidService` (from
`service`)       | `https://www.w3.org/ns/did/v1`                | |
`PropertyValue` (from `attachment`) | schema.org terms in embedded
context          |

Redundant property compactNames : If a property has redundantProperties, all their compactNames must also be defined in the context.

Practical rule: look at an existing type with similar embedded relationships as a reference. For example, Note and Article include "https://w3id.org/security/data-integrity/v1" because they embed DataIntegrityProof objects via the proof property. Person additionally includes security and DID contexts because it embeds Key, Multikey, and DidService objects inline.

Omitting a required context causes silent compaction failure: the property appears in expanded form ("https://example.com/ns#term": [...]) rather than compact form ("term": ...) in the output.

Property definitions

Each entry in properties is one of two kinds:

Non-functional property (multiple values)

Generates get<PluralName>() async iterable and optionally a singular accessor.

- pluralName: attachments # accessor: getAttachments() / attachments
  singularName: attachment # used if singularAccessor: true
  singularAccessor: true # also generate getAttachment() / attachment
  compactName: attachment # JSON-LD compact key
  uri: "https://www.w3.org/ns/activitystreams#attachment"
  description: |
    Identifies a resource attached or related to an object.
  range:
    - "https://www.w3.org/ns/activitystreams#Object"
    - "https://www.w3.org/ns/activitystreams#Link"

Required: pluralName, singularName, uri, description, range

Optional: singularAccessor (default false), compactName, subpropertyOf, container ("graph" or "list"), embedContext, untyped

Functional property (exactly one value)

Generates a single get<SingularName>() / <singularName> accessor.

- singularName: published
  functional: true
  compactName: published
  uri: "https://www.w3.org/ns/activitystreams#published"
  description: The date and time at which the object was published.
  range:
    - "http://www.w3.org/2001/XMLSchema#dateTime"

Required: singularName, functional: true, uri, description, range

Optional: compactName, subpropertyOf, redundantProperties, untyped, embedContext

Redundant properties (functional only)

When a property has equivalent URIs from multiple vocabularies, use redundantProperties to write all aliases on serialization and try them in order on deserialization:

- singularName: quoteUrl
  functional: true
  compactName: quoteUrl
  uri: "https://www.w3.org/ns/activitystreams#quoteUrl"
  redundantProperties:
    - compactName: _misskey_quote
      uri: "https://misskey-hub.net/ns#_misskey_quote"
    - compactName: quoteUri
      uri: "http://fedibird.com/ns#quoteUri"
  description: The URI of the quoted ActivityStreams object.
  range:
    - "fedify:url"

The embedContext field

Use embedContext when a nested object should carry its own @context (e.g., proof graphs in Data Integrity):

embedContext:
  compactName: proof # key under which the context is embedded
  inherit: true # use the same context as the enclosing document

The untyped field

When untyped: true, the serialized value will not have a @type field. Requires exactly one type in range. Used for embedded anonymous structures:

- singularName: source
  functional: true
  compactName: source
  uri: "https://www.w3.org/ns/activitystreams#source"
  description: The source from which the content markup was derived.
  untyped: true
  range:
    - "https://www.w3.org/ns/activitystreams#Source"

Range type reference

XSD scalar types → TypeScript types

Range URI	TypeScript type
http://www.w3.org/2001/XMLSchema#string	string
http://www.w3.org/2001/XMLSchema#boolean	boolean
http://www.w3.org/2001/XMLSchema#integer	number
http://www.w3.org/2001/XMLSchema#nonNegativeInteger	number
http://www.w3.org/2001/XMLSchema#float	number
http://www.w3.org/2001/XMLSchema#anyURI	URL (stored as @id)
http://www.w3.org/2001/XMLSchema#dateTime	Temporal.Instant
http://www.w3.org/2001/XMLSchema#duration	Temporal.Duration
http://www.w3.org/1999/02/22-rdf-syntax-ns#langString	LanguageString

Security scalar types

Range URI	TypeScript type
https://w3id.org/security#cryptosuiteString	"eddsa-jcs-2022"
https://w3id.org/security#multibase	Uint8Array

Fedify internal types

Range URI	TypeScript type	Notes
fedify:langTag	Intl.Locale	BCP 47 language tag as plain string
fedify:url	URL	URL stored as @value (not @id)
fedify:publicKey	CryptoKey	PEM SPKI-encoded public key
fedify:multibaseKey	CryptoKey	Multibase-encoded key (Ed25519)
fedify:proofPurpose	"assertionMethod" | "authentication" | ...	Proof purpose string
fedify:units	"cm" | "feet" | "inches" | "km" | "m" | "miles"	Place units

Vocabulary types

Any uri from another YAML vocabulary file can be used as a range. The TypeScript type will be the corresponding generated class (e.g., "https://www.w3.org/ns/activitystreams#Object" → Object).

Adding preloaded JSON-LD contexts

When defaultContext references a URL not already in packages/vocab-runtime/src/contexts.ts, add it to preloadedContexts.

Check existing keys in that file first by searching for the URL. If missing, fetch the actual context document from its canonical URL and add an entry:

"https://example.com/ns/v1": {
  "@context": {
    // ... paste actual context content here ...
  },
},

The keys of preloadedContexts must match the URL strings used in YAML defaultContext fields. This enables offline JSON-LD processing.

Complete minimal example

$schema: ../../vocab-tools/schema.yaml
name: Move
compactName: Move
uri: "https://www.w3.org/ns/activitystreams#Move"
extends: "https://www.w3.org/ns/activitystreams#Activity"
entity: true
description: |
  Indicates that the actor has moved object from origin to target.
  If the origin or target are not specified, either can be determined by
  context.
defaultContext:
  - "https://www.w3.org/ns/activitystreams"
  - "https://w3id.org/security/data-integrity/v1"
properties: []

Complete extended example (new vocabulary type)

When implementing a new type from a FEP (e.g. a new Interaction type with custom properties from a hypothetical https://example.com/ns# namespace):

$schema: ../../vocab-tools/schema.yaml
name: Interaction
compactName: Interaction
uri: "https://example.com/ns#Interaction"
extends: "https://www.w3.org/ns/activitystreams#Activity"
entity: true
description: |
  Represents a generic interaction with an object.
  See [FEP-xxxx](https://w3id.org/fep/xxxx).
defaultContext:
  - "https://www.w3.org/ns/activitystreams"
  - "https://w3id.org/security/data-integrity/v1"
  - example: "https://example.com/ns#"
    Interaction: "example:Interaction"
    interactionCount:
      "@id": "example:interactionCount"
      "@type": "http://www.w3.org/2001/XMLSchema#nonNegativeInteger"

properties:
  - singularName: interactionCount
    functional: true
    compactName: interactionCount
    uri: "https://example.com/ns#interactionCount"
    description: The number of interactions recorded.
    range:
      - "http://www.w3.org/2001/XMLSchema#nonNegativeInteger"

  - pluralName: participants
    singularName: participant
    singularAccessor: true
    compactName: participant
    uri: "https://example.com/ns#participant"
    description: Actors who participated in this interaction.
    range:
      - "https://www.w3.org/ns/activitystreams#Person"
      - "https://www.w3.org/ns/activitystreams#Group"
      - "https://www.w3.org/ns/activitystreams#Organization"
      - "https://www.w3.org/ns/activitystreams#Service"
      - "https://www.w3.org/ns/activitystreams#Application"