Core Types

ActivityStreams 2.0 defines a small set of core types that form the foundation for all ActivityPub objects. Every object in ActivityStreams inherits from these base types.

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	Array	Primary recipients
cc	Array	Secondary recipients
bto	Array	Blind primary recipients
bcc	Array	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	Array	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

• Outbox (newest first)
• Inbox (newest first)
• Followers lists
• Following lists
• Liked posts

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!" }
    }
  ]
}

Type Coercion

Objects can have multiple types:

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

Null Values

Properties with null values should be omitted rather than included:

// 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