Core Types

ActivityStreams 2.0 defines a small set of core types that form the foundation for all ActivityPub objects. There are two base types — Object and Link — and every other type extends one of them (AS2 Core §4). "Actor types" in the diagram is a grouping, not a class: Person, Group, Application, Service and Organization each extend Object directly.

Type Hierarchy

Object

The base type for all ActivityStreams objects.

Definition

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Object",
  "id": "https://example.com/objects/1",
  "name": "A Simple Object"
}

Common Properties

Property	Type	Description
@context	String/Array	JSON-LD context
type	String/Array	The object type(s)
id	String (URI)	Globally unique identifier
name	String	Human-readable name
content	String	Text content (often HTML)
summary	String	Brief description
published	DateTime	When created
updated	DateTime	When last modified
attributedTo	Object/Link	Creator/author
to	Object/Link, one or many	Primary recipients
cc	Object/Link, one or many	Secondary recipients
bto	Object/Link, one or many	Blind primary recipients
bcc	Object/Link, one or many	Blind secondary recipients

Example

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Object",
  "id": "https://example.com/objects/unique-id",
  "name": "My Object",
  "content": "This is the content of the object",
  "published": "2024-01-15T10:00:00Z",
  "updated": "2024-01-15T12:00:00Z",
  "attributedTo": "https://example.com/users/alice"
}

Link

Represents a reference to another resource. Links are not Objects—they're a separate base type.

Definition

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Link",
  "href": "https://example.com/resource",
  "mediaType": "text/html"
}

Link Properties

Property	Type	Description
href	String (URI)	The target resource URL
rel	String/Array	Link relation type
mediaType	String	MIME type of the target
name	String	Human-readable name
hreflang	String	Language of the target
height	Integer	Height in pixels
width	Integer	Width in pixels
preview	Object/Link	Preview of the linked resource

Example

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Link",
  "href": "https://example.com/article",
  "mediaType": "text/html",
  "name": "An Interesting Article",
  "hreflang": "en"
}

Activity

Represents an action. Activities are the primary way to describe what's happening in ActivityPub.

Definition

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Activity",
  "actor": "https://example.com/users/alice",
  "object": "https://example.com/notes/1"
}

Activity Properties

Property	Type	Description
actor	Object/Link	Who performed the activity
object	Object/Link	What the activity is about
target	Object/Link	Indirect object (e.g., Add to target)
result	Object/Link	Result of the activity
origin	Object/Link	Where the activity originated
instrument	Object/Link	Tool used to perform the activity

Example

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "id": "https://example.com/activities/1",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Note",
    "content": "Hello, world!"
  },
  "published": "2024-01-15T10:00:00Z"
}

IntransitiveActivity

An Activity without an object property. The action is self-contained.

Examples

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Arrive",
  "actor": "https://example.com/users/alice",
  "location": {
    "type": "Place",
    "name": "San Francisco"
  }
}

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Travel",
  "actor": "https://example.com/users/alice",
  "origin": {
    "type": "Place",
    "name": "New York"
  },
  "target": {
    "type": "Place",
    "name": "Los Angeles"
  }
}

Collection

Represents an ordered or unordered set of Objects or Links.

Definition

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Collection",
  "totalItems": 3,
  "items": [
    "https://example.com/objects/1",
    "https://example.com/objects/2",
    "https://example.com/objects/3"
  ]
}

Collection Properties

Property	Type	Description
totalItems	Integer	Total number of items
items	Object/Link, one or many	The items (unordered)
current	CollectionPage/Link	Current page
first	CollectionPage/Link	First page
last	CollectionPage/Link	Last page

Example

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Collection",
  "id": "https://example.com/users/alice/followers",
  "totalItems": 150,
  "first": "https://example.com/users/alice/followers?page=1"
}

OrderedCollection

A Collection where the order of items is significant.

Definition

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "OrderedCollection",
  "totalItems": 3,
  "orderedItems": [
    "https://example.com/objects/3",
    "https://example.com/objects/2",
    "https://example.com/objects/1"
  ]
}

Used For

ActivityPub requires inbox and outbox to be OrderedCollections, presented in reverse chronological order (newest first). followers, following, liked, likes and shares MUST be either an OrderedCollection or a Collection (AP §5).

Example

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "OrderedCollection",
  "id": "https://example.com/users/alice/outbox",
  "totalItems": 1250,
  "first": "https://example.com/users/alice/outbox?page=true",
  "last": "https://example.com/users/alice/outbox?min_id=0&page=true"
}

CollectionPage

A single page of a Collection.

Definition

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "CollectionPage",
  "partOf": "https://example.com/collection",
  "items": [...]
}

CollectionPage Properties

Property	Type	Description
partOf	Collection/Link	The parent collection
next	CollectionPage/Link	Next page
prev	CollectionPage/Link	Previous page

Example

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "CollectionPage",
  "id": "https://example.com/users/alice/followers?page=1",
  "partOf": "https://example.com/users/alice/followers",
  "next": "https://example.com/users/alice/followers?page=2",
  "items": [
    "https://example.com/users/bob",
    "https://example.com/users/carol"
  ]
}

OrderedCollectionPage

A single page of an OrderedCollection.

Definition

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "OrderedCollectionPage",
  "partOf": "https://example.com/collection",
  "orderedItems": [...],
  "startIndex": 0
}

Additional Property

Property	Type	Description
startIndex	Integer	Index of the first item

Example

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "OrderedCollectionPage",
  "id": "https://example.com/users/alice/outbox?page=true",
  "partOf": "https://example.com/users/alice/outbox",
  "next": "https://example.com/users/alice/outbox?max_id=123&page=true",
  "prev": "https://example.com/users/alice/outbox?min_id=456&page=true",
  "orderedItems": [
    {
      "type": "Create",
      "object": { "type": "Note", "content": "Hello!" }
    }
  ]
}

Multiple Types

type may hold more than one value, so consumers must handle an array. A common use is adding an extension type alongside an AS2 type that non-aware consumers still understand:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": ["Person", "http://schema.org/Person"],
  "id": "https://example.com/users/alice",
  "name": "Alice"
}

Null Values

AS2 Core treats an omitted property and a null value as equivalent — "no value has been assigned" — and an empty array MUST be represented by omitting the property or setting it to null (AS2 Core §2). Omitting is the simpler choice and what most implementations emit:

// Correct
{
  "type": "Note",
  "content": "Hello"
}

// Avoid
{
  "type": "Note",
  "content": "Hello",
  "summary": null
}

Next Steps

• Activity Types - All activity types
• Object Types - Content object types
• Actor Types - Actor types
• Properties - All properties reference