ActivityStreams 2.0 Overview

ActivityStreams 2.0 is the vocabulary that ActivityPub uses to represent social data. It defines the format for activities, actors, objects, and collections using JSON-LD.

Specification

• Status: W3C Recommendation (May 2017), under maintenance by the Social Web Working Group alongside ActivityPub — no updated version published as of October 2026
• Core: https://www.w3.org/TR/activitystreams-core/
• Vocabulary: https://www.w3.org/TR/activitystreams-vocabulary/

The JSON-LD Context

Every ActivityStreams document includes a context:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Note",
  "content": "Hello, world!"
}

The context maps short property names to full URIs:

• type → @type (and id → @id), JSON-LD keywords rather than vocabulary terms
• content → https://www.w3.org/ns/activitystreams#content

Core Types

Object

The base type for all things:

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

Activity

An action performed by an actor:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Note",
    "content": "Hello!"
  }
}

Actor

An entity that can perform activities:

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

Collection

A group of items:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Collection",
  "totalItems": 2,
  "items": [
    { "type": "Note", "content": "First" },
    { "type": "Note", "content": "Second" }
  ]
}

Type Hierarchy

AS2 has two base types, Object and Link; a Link is not an Object (AS2 Core §4.2). Extension relationships below are from the Activity Vocabulary.

Object
├── Activity
│   ├── Create, Update, Delete, Follow, Like, Announce, Undo, Add, Remove, ...
│   ├── Accept ── TentativeAccept
│   ├── Reject ── TentativeReject
│   ├── Ignore ── Block
│   ├── Offer ── Invite
│   └── IntransitiveActivity
│       ├── Arrive
│       ├── Travel
│       └── Question
├── Person, Organization, Application, Service, Group
│     (actor types — there is no "Actor" class; each extends Object directly)
├── Note, Article, Event, Place, Profile, Relationship, Tombstone
├── Document
│   ├── Audio
│   ├── Image
│   ├── Video
│   └── Page
└── Collection
    ├── OrderedCollection
    └── CollectionPage
        └── OrderedCollectionPage   (also extends OrderedCollection)

Link
└── Mention

Common Properties

Identity

Property	Type	Description
id	URL	Unique identifier
type	String or array	Object type(s)
name	String	Display name
summary	String	Short description (may contain HTML)

Authorship

Property	Type	Description
attributedTo	Object or Link (usually an actor)	Creator
actor	Object or Link (usually an actor)	Activity performer

Content

Property	Type	Description
content	String	Main content (HTML unless mediaType says otherwise)
contentMap	Object	Localized content
mediaType	String	MIME type of content (Object) or of the target (Link)

Timestamps

Property	Type	Description
published	DateTime	Creation time
updated	DateTime	Last modified
startTime	DateTime	Event start
endTime	DateTime	Event end

Addressing

Property	Type	Description
to	Object/Link, one or many	Primary recipients
cc	Object/Link, one or many	Secondary recipients
bto	Object/Link, one or many	Private primary (strip before delivery)
bcc	Object/Link, one or many	Private secondary (strip before delivery)

None of these properties is functional, so a single value and a one-element array mean the same thing; consumers must accept both.

Extending ActivityStreams

Custom Properties

Extensions are added through the @context; the normative AS2 context itself must not be overridden (AS2 Core §5). sensitive below is not part of AS2 — it is a widely used extension that Mastodon declares as as:sensitive (context_helper.rb):

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "sensitive": "as:sensitive",
      "toot": "http://joinmastodon.org/ns#",
      "blurhash": "toot:blurhash"
    }
  ],
  "type": "Note",
  "content": "Hidden content",
  "sensitive": true
}

Custom Types

Define new types the same way. Emoji is Mastodon's extension type (toot:Emoji), not AS2:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Emoji": "toot:Emoji"
    }
  ],
  "type": "Emoji",
  "id": "https://example.com/emojis/awesome",
  "name": ":awesome:",
  "icon": {
    "type": "Image",
    "mediaType": "image/png",
    "url": "https://example.com/emoji/awesome.png"
  }
}

Processing ActivityStreams

Parsing

function parseActivityStreams(json) {
  // Normalize to array context
  let context = json['@context'];
  if (!Array.isArray(context)) {
    context = [context];
  }

  // Get type
  const type = json.type;

  // Handle multiple types
  const types = Array.isArray(type) ? type : [type];

  return {
    context,
    types,
    data: json
  };
}

Type Checking

type may be a string or an array, so check every value. The lists below are the AS2 Vocabulary's activity and actor types; extensions add more (e.g. Mastodon's QuoteRequest), so treat unknown types as "ignore", not "error" (AS2 Core §5).

const ACTIVITY_TYPES = new Set([
  'Activity', 'IntransitiveActivity', 'Accept', 'Add', 'Announce', 'Arrive',
  'Block', 'Create', 'Delete', 'Dislike', 'Flag', 'Follow', 'Ignore', 'Invite',
  'Join', 'Leave', 'Like', 'Listen', 'Move', 'Offer', 'Question', 'Reject',
  'Read', 'Remove', 'TentativeReject', 'TentativeAccept', 'Travel', 'Undo',
  'Update', 'View'
]);
const ACTOR_TYPES = new Set(['Application', 'Group', 'Organization', 'Person', 'Service']);

const typesOf = (obj) => [].concat(obj?.type ?? []);

function isActivity(obj) {
  return typesOf(obj).some((t) => ACTIVITY_TYPES.has(t));
}

function isActor(obj) {
  return typesOf(obj).some((t) => ACTOR_TYPES.has(t));
}

See Also

• Core Types - Base types
• Activity Types - All activities
• Object Types - Content types
• Properties - All properties