Actor Types

Actors are the entities that perform activities in ActivityPub. They represent users, organizations, bots, and services that can create content and interact with others.

Actor Types Overview

Person

Represents an individual human user.

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/v1"
  ],
  "type": "Person",
  "id": "https://example.com/users/alice",
  "preferredUsername": "alice",
  "name": "Alice",
  "summary": "<p>Software developer and fediverse enthusiast</p>",
  "inbox": "https://example.com/users/alice/inbox",
  "outbox": "https://example.com/users/alice/outbox",
  "followers": "https://example.com/users/alice/followers",
  "following": "https://example.com/users/alice/following",
  "liked": "https://example.com/users/alice/liked",
  "icon": {
    "type": "Image",
    "mediaType": "image/png",
    "url": "https://example.com/avatars/alice.png"
  },
  "image": {
    "type": "Image",
    "mediaType": "image/jpeg",
    "url": "https://example.com/headers/alice.jpg"
  },
  "publicKey": {
    "id": "https://example.com/users/alice#main-key",
    "owner": "https://example.com/users/alice",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"
  },
  "endpoints": {
    "sharedInbox": "https://example.com/inbox"
  }
}

Used by: All social platforms for user accounts

Group

Represents a collection of actors, often used for communities.

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/v1"
  ],
  "type": "Group",
  "id": "https://lemmy.example/c/technology",
  "preferredUsername": "technology",
  "name": "Technology",
  "summary": "<p>A community for tech discussion</p>",
  "inbox": "https://lemmy.example/c/technology/inbox",
  "outbox": "https://lemmy.example/c/technology/outbox",
  "followers": "https://lemmy.example/c/technology/followers",
  "attributedTo": "https://lemmy.example/c/technology/moderators",
  "publicKey": {
    "id": "https://lemmy.example/c/technology#main-key",
    "owner": "https://lemmy.example/c/technology",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"
  }
}

Used by:

• Lemmy (communities)
• PeerTube (channels)
• Groups in various platforms

Organization

Represents a company, institution, or organization.

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/v1"
  ],
  "type": "Organization",
  "id": "https://example.com/orgs/acme",
  "preferredUsername": "acme",
  "name": "ACME Corporation",
  "summary": "<p>Building better products</p>",
  "inbox": "https://example.com/orgs/acme/inbox",
  "outbox": "https://example.com/orgs/acme/outbox",
  "url": "https://acme.example.com",
  "publicKey": {
    "id": "https://example.com/orgs/acme#main-key",
    "owner": "https://example.com/orgs/acme",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"
  }
}

Used by: Business accounts, official accounts

Application

Represents a software application. AS2 doesn't say what it's for; in practice it is used for bots and instance actors. Mastodon treats both Application and Service actors as bots (AUTOMATED_ACTOR_TYPES in account.rb).

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/v1"
  ],
  "type": "Application",
  "id": "https://example.com/bots/newsbot",
  "preferredUsername": "newsbot",
  "name": "News Bot",
  "summary": "<p>Automated news aggregator</p>",
  "inbox": "https://example.com/bots/newsbot/inbox",
  "outbox": "https://example.com/bots/newsbot/outbox",
  "publicKey": {
    "id": "https://example.com/bots/newsbot#main-key",
    "owner": "https://example.com/bots/newsbot",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"
  }
}

Used by: Bots, automated accounts

Service

Represents a service or daemon process.

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/v1"
  ],
  "type": "Service",
  "id": "https://relay.example/actor",
  "preferredUsername": "relay",
  "name": "Fediverse Relay",
  "summary": "<p>ActivityPub relay service</p>",
  "inbox": "https://relay.example/inbox",
  "outbox": "https://relay.example/outbox",
  "publicKey": {
    "id": "https://relay.example/actor#main-key",
    "owner": "https://relay.example/actor",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"
  }
}

Used by: Relay servers, system accounts

Required Actor Properties

ActivityPub requires id and type on every object (AP §3.1) and inbox and outbox on every actor (AP §4.1):

Property	Description
id	Unique, dereferenceable URL
type	One of the actor types
inbox	URL to receive activities
outbox	URL listing published activities

followers and following are SHOULDs in the spec. Mastodon additionally rejects actors without preferredUsername (see Troubleshooting).

Recommended Actor Properties

Property	Description
preferredUsername	The @handle part
name	Display name
summary	Bio/description (HTML)
url	Profile page URL
icon	Avatar image
image	Header/banner image
publicKey	RSA key for HTTP Signatures (Ed25519 and other keys go in assertionMethod — see Keys and Identity)
endpoints.sharedInbox	Shared inbox URL
followers	Followers collection URL
following	Following collection URL

Mastodon-Specific Properties

{
  "type": "Person",
  "discoverable": true,
  "manuallyApprovesFollowers": false,
  "featured": "https://example.com/users/alice/collections/featured",
  "featuredTags": "https://example.com/users/alice/collections/tags",
  "alsoKnownAs": [],
  "attachment": [
    {
      "type": "PropertyValue",
      "name": "Website",
      "value": "<a href=\"https://alice.example\">alice.example</a>"
    }
  ]
}

Property	Description
discoverable	Show in directory/search
manuallyApprovesFollowers	Locked account
featured	Pinned posts collection
featuredTags	Featured hashtags
movedTo	Migration target
alsoKnownAs	Aliases for migration
attachment	Profile fields
indexable	Opt in to full-text search of the account's public posts
memorial	Account has been memorialized
attributionDomains	Domains allowed to credit this account as author of linked articles

These are extensions, not AS2 core, and they come from different namespaces. Per Mastodon's context_helper.rb:

• as: (proposed AS2 extensions): manuallyApprovesFollowers, movedTo. (alsoKnownAs is also defined in the published AS2 context.)
• toot: (http://joinmastodon.org/ns#): discoverable, indexable, memorial, featured, featuredTags, attributionDomains, suspended
• schema: (http://schema.org#): PropertyValue and its value, used in attachment for profile fields

Profile Fields

Mastodon uses attachment with PropertyValue type for profile fields:

{
  "attachment": [
    {
      "type": "PropertyValue",
      "name": "Website",
      "value": "<a href=\"https://example.com\" rel=\"me\">example.com</a>"
    },
    {
      "type": "PropertyValue",
      "name": "Location",
      "value": "San Francisco"
    },
    {
      "type": "PropertyValue",
      "name": "Pronouns",
      "value": "she/her"
    }
  ]
}

Mastodon marks a link field as verified when the linked page links back to the profile with rel="me". The verification happens on the server that displays the profile; nothing about it federates.

Public Key Structure

Required for HTTP Signatures:

{
  "publicKey": {
    "id": "https://example.com/users/alice#main-key",
    "owner": "https://example.com/users/alice",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...\n-----END PUBLIC KEY-----"
  }
}

Property	Description
id	Key identifier — usually the actor URL plus #main-key, but it may be a separate URL (GoToSocial uses …/main-key)
owner	The actor who owns this key
publicKeyPem	PEM-encoded RSA public key

Instance Actor

Servers often have a special instance-level actor:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/v1"
  ],
  "type": "Application",
  "id": "https://example.com/actor",
  "preferredUsername": "example.com",
  "inbox": "https://example.com/actor/inbox",
  "outbox": "https://example.com/actor/outbox",
  "endpoints": {
    "sharedInbox": "https://example.com/inbox"
  },
  "publicKey": {
    "id": "https://example.com/actor#main-key",
    "owner": "https://example.com/actor",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"
  }
}

This mirrors what mastodon.social serves at /actor (an Application whose preferredUsername is the domain).

Used for:

• Sending Flag (report) activities
• Following relays
• Signing fetches when authorized fetch is on — see Authorized Fetch

Actor Type Comparison

Type	Use Case	Has Followers	Creates Content
Person	Users	Yes	Yes
Group	Communities	Yes	Redistributes
Organization	Companies	Yes	Yes
Application	Bots, instance actors	Yes	Yes
Service	Bots, relays, system actors	Sometimes	Sometimes

Content Negotiation

Actors must be served with correct content type based on Accept header:

app.get('/users/:username', (req, res) => {
  const accept = req.headers.accept || '';

  // The same URL serves HTML and JSON, so tell caches to key on Accept
  res.vary('Accept');

  if (accept.includes('application/activity+json') ||
      accept.includes('application/ld+json')) {
    res.type('application/activity+json');
    return res.json(getActor(req.params.username));
  }

  // Return HTML profile page
  res.render('profile', { username: req.params.username });
});

Next Steps

• Link Types - Link and Mention types
• Properties - All properties reference
• Building an Actor - Implementation guide