Verified against ActivityPub REC §3, §6.4, AS2 Vocabulary, Mastodon v4.7.3 source (tag_manager, note_serializer, context_helper), Lemmy 0.19.20 page.json fixture on 2026-10-15.

Objects

Objects in ActivityPub represent content — posts, images, videos, events, and more. They're the "things" that activities act upon.

Base Object Properties

All objects can have these properties:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Note",
  "id": "https://example.com/notes/1",
  "attributedTo": "https://example.com/users/alice",
  "content": "Hello, Fediverse!",
  "published": "2024-01-15T10:30:00Z",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://example.com/users/alice/followers"]
}

Core Properties

Property	Type	Description
`type`	String	Object type (Note, Article, etc.)
`id`	URL	Unique, publicly dereferenceable identifier (HTTPS); MUST be present except on transient objects (AP §3.1)
`attributedTo`	URL/Actor	Creator of the object
`content`	String	Main content (HTML allowed)
`published`	DateTime	Creation timestamp
`updated`	DateTime	Last modification timestamp

Addressing

Property	Description
`to`	Primary recipients
`cc`	Secondary recipients
`bto`	Private primary recipients
`bcc`	Private secondary recipients

Content Properties

Property	Description
`summary`	Content warning or preview
`contentMap`	Localized content `{"en": "Hello"}`
`source`	The source the `content` was rendered from, e.g. Markdown (AP §3.3)
`mediaType`	MIME type of content

Common Object Types

Note

Short-form content (like tweets/toots):

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Note",
  "id": "https://example.com/notes/1",
  "attributedTo": "https://example.com/users/alice",
  "content": "<p>Hello, Fediverse! 👋</p>",
  "published": "2024-01-15T10:30:00Z",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://example.com/users/alice/followers"],
  "sensitive": false,
  "tag": [],
  "attachment": []
}

Article

Long-form content (blog posts):

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Article",
  "id": "https://blog.example/posts/1",
  "attributedTo": "https://blog.example/users/writer",
  "name": "My First Blog Post",
  "content": "<p>This is the full content...</p>",
  "summary": "A brief introduction to my blog",
  "published": "2024-01-15T10:30:00Z",
  "url": "https://blog.example/posts/my-first-blog-post"
}

Key difference: Article typically has a name (title), Note does not.

Image

Photo posts:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Image",
  "id": "https://example.com/photos/1",
  "attributedTo": "https://example.com/users/photographer",
  "name": "Sunset at the beach",
  "url": {
    "type": "Link",
    "href": "https://example.com/media/1.jpg",
    "mediaType": "image/jpeg",
    "width": 1920,
    "height": 1080
  }
}

:::note Photo posts in practice Most fediverse software — including Pixelfed — federates a photo post as a Note with Image attachments, not as a top-level Image (Pixelfed ActivityPub docs). A bare Image object is valid AS2, but many servers won't display it. :::

Video

Video content (a generic AS2 example — PeerTube's real format is covered in the PeerTube guide):

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Video",
  "id": "https://video.example/videos/1",
  "attributedTo": "https://video.example/accounts/creator",
  "name": "Tutorial: Building with ActivityPub",
  "duration": "PT15M30S",
  "url": [
    {
      "type": "Link",
      "href": "https://video.example/videos/1.mp4",
      "mediaType": "video/mp4",
      "height": 1080
    },
    {
      "type": "Link",
      "href": "https://video.example/videos/1.webm",
      "mediaType": "video/webm"
    }
  ]
}

Page

Lemmy posts are Page objects addressed to a community. Lemmy puts the shared link in attachment as a Link, not in url, and sets audience to the community (based on Lemmy's page.json fixture):

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Page",
  "id": "https://lemmy.example/post/1",
  "attributedTo": "https://lemmy.example/u/alice",
  "to": [
    "https://lemmy.example/c/technology",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "audience": "https://lemmy.example/c/technology",
  "name": "Interesting Article About ActivityPub",
  "content": "<p>This is my commentary on the article...</p>",
  "mediaType": "text/html",
  "attachment": [
    { "type": "Link", "href": "https://example.com/article" }
  ]
}

See the Lemmy guide for details.

Event

Calendar events:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Event",
  "id": "https://events.example/events/1",
  "name": "Fediverse Developer Meetup",
  "startTime": "2024-02-15T18:00:00Z",
  "endTime": "2024-02-15T21:00:00Z",
  "location": {
    "type": "Place",
    "name": "Community Center"
  }
}

Replies

Objects can be replies to other objects:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Note",
  "id": "https://example.com/notes/2",
  "attributedTo": "https://example.com/users/bob",
  "content": "Great point!",
  "inReplyTo": "https://other.example/notes/1",
  "to": [
    "https://other.example/users/alice",
    "https://www.w3.org/ns/activitystreams#Public"
  ]
}

Reply Collection

Posts can link to their replies:

{
  "type": "Note",
  "id": "https://example.com/notes/1",
  "replies": {
    "type": "Collection",
    "id": "https://example.com/notes/1/replies",
    "first": "https://example.com/notes/1/replies?page=1"
  }
}

Media Attachments

Objects can have media attachments:

{
  "type": "Note",
  "content": "Check out this photo!",
  "attachment": [
    {
      "type": "Document",
      "mediaType": "image/jpeg",
      "url": "https://example.com/media/photo.jpg",
      "name": "A beautiful sunset",
      "blurhash": "LEHV6nWB2yk8pyo0adR*.7kCMdnj",
      "width": 1920,
      "height": 1080
    }
  ]
}

Blurhash

Mastodon uses blurhash for image placeholders (toot:blurhash in its context):

{
  "type": "Document",
  "url": "https://example.com/image.jpg",
  "blurhash": "LEHV6nWB2yk8pyo0adR*.7kCMdnj"
}

Content Warnings

Use sensitive and summary for content warnings:

{
  "type": "Note",
  "sensitive": true,
  "summary": "Spoilers for the new movie",
  "content": "<p>The ending was amazing when...</p>"
}

Visibility

AP has no visibility levels; they are conventions built from addressing. These are Mastodon's mappings (tag_manager.rb); Mastodon also adds mentioned accounts to cc (or to to for direct messages):

Public

{
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://example.com/users/alice/followers"]
}

Unlisted (Public but not in timelines)

{
  "to": ["https://example.com/users/alice/followers"],
  "cc": ["https://www.w3.org/ns/activitystreams#Public"]
}

Followers-Only

{
  "to": ["https://example.com/users/alice/followers"],
  "cc": []
}

Direct Message

{
  "to": ["https://other.example/users/bob"],
  "cc": [],
  "tag": [
    { "type": "Mention", "href": "https://other.example/users/bob", "name": "@bob@other.example" }
  ]
}

Include a Mention tag for each recipient — see Direct Messages.

Polls (Question)

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Question",
  "id": "https://example.com/questions/1",
  "attributedTo": "https://example.com/users/alice",
  "content": "What's your favorite programming language?",
  "endTime": "2024-01-22T10:30:00Z",
  "oneOf": [
    {
      "type": "Note",
      "name": "JavaScript",
      "replies": {
        "type": "Collection",
        "totalItems": 42
      }
    },
    {
      "type": "Note",
      "name": "Python",
      "replies": {
        "type": "Collection",
        "totalItems": 38
      }
    }
  ]
}

Use oneOf for single-choice, anyOf for multiple-choice.

Location

Adding location to objects:

{
  "type": "Note",
  "content": "Beautiful day at the park!",
  "location": {
    "type": "Place",
    "name": "Central Park",
    "latitude": 40.785091,
    "longitude": -73.968285
  }
}

Object Fetching

When you receive a reference to an object, you may need to fetch it. Clients MUST send the ld+json profile media type (AP §3.2); sending both types is the safe choice:

GET /notes/1 HTTP/1.1
Host: example.com
Accept: application/ld+json; profile="https://www.w3.org/ns/activitystreams", application/activity+json

Only trust the result if its id is on the origin you fetched it from — see Origin model. Servers with authorized fetch require the GET to be signed — see Authorized Fetch.

Tombstones

Deleted objects become Tombstones:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Tombstone",
  "id": "https://example.com/notes/1",
  "formerType": "Note",
  "deleted": "2024-01-16T12:00:00Z"
}

If a deleted object is requested, the server SHOULD respond with 410 Gone when it serves a Tombstone, or 404 Not Found otherwise (AP §6.4). Mastodon returns 404 for deleted posts — see Deletion.

Next Steps

Activities - Actions on objects
Collections - Groups of objects
Posts and Replies - Implementation guide
Threads, Replies and Backfill - The context and replies properties in practice

Objects

Base Object Properties

Core Properties

Addressing

Content Properties

Common Object Types

Note

Article

Image

Video

Page

Event

Replies

Reply Collection

Media Attachments

Blurhash

Tags

Hashtags

Mentions

Custom Emoji

Content Warnings

Visibility

Public

Unlisted (Public but not in timelines)

Followers-Only

Direct Message

Polls (Question)

Location

Object Fetching

Tombstones

Next Steps

Base Object Properties​

Core Properties​

Addressing​

Content Properties​

Common Object Types​

Note​

Article​

Image​

Video​

Page​

Event​

Replies​

Reply Collection​

Media Attachments​

Blurhash​

Tags​

Hashtags​

Mentions​

Custom Emoji​

Content Warnings​

Visibility​

Public​

Unlisted (Public but not in timelines)​

Followers-Only​

Direct Message​

Polls (Question)​

Location​

Object Fetching​

Tombstones​

Next Steps​

Base Object Properties

Core Properties

Addressing

Content Properties

Common Object Types

Note

Article

Image

Video

Page

Event

Replies

Reply Collection

Media Attachments

Blurhash

Tags

Hashtags

Mentions

Custom Emoji

Content Warnings

Visibility

Public

Unlisted (Public but not in timelines)

Followers-Only

Direct Message

Polls (Question)

Location

Object Fetching

Tombstones

Next Steps