Image Description

The Image type is defined in ActivityStreams Vocabulary.

In this support table, we only consider how the image description, commonly called AltText is handled. Image descriptions are important from an accessibility standpoint, see WCAG 2.2. Text Alternatives.

It seems that certain implementations, e.g. misskey, store the image description on a per image URL basis and not for every instance of an image reference.

Examples

Example 1

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://akkoma/users/witch"
  ],
  "id": "http://pasture-one-actor/activity/Y-ahNeC4l5o",
  "published": "2025-09-09T10:09:35Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://akkoma/users/witch"
    ],
    "id": "http://pasture-one-actor/object/VQYRtHqdLoI",
    "published": "2025-09-09T10:09:35Z",
    "content": "no description",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    }
  }
}

akkoma

{
  "reblogged": false,
  "pleroma": {
    "content": {
      "text/plain": "no description"
    },
    "context": "http://pasture-one-actor/object/VQYRtHqdLoI",
    "conversation_id": 742349304,
    "direct_conversation_id": null,
    "emoji_reactions": [],
    "expires_at": null,
    "in_reply_to_account_acct": null,
    "local": false,
    "parent_visible": false,
    "pinned_at": null,
    "spoiler_text": {
      "text/plain": ""
    },
    "thread_muted": false
  },
  "in_reply_to_account_id": null,
  "uri": "http://pasture-one-actor/object/VQYRtHqdLoI",
  "edited_at": null,
  "quote": null,
  "favourited": false,
  "text": null,
  "pinned": false,
  "emojis": [],
  "id": "Ay1hNIKyoZNyLEZAsS",
  "mentions": [
    {
      "acct": "witch",
      "id": "Ay1h3YE0L8LMp8AbVQ",
      "url": "http://akkoma/users/witch",
      "username": "witch"
    }
  ],
  "content": "no description",
  "akkoma": {
    "source": null
  },
  "spoiler_text": "",
  "media_attachments": [
    {
      "blurhash": null,
      "description": null,
      "id": "983810756",
      "pleroma": {
        "mime_type": "application/octet-stream"
      },
      "preview_url": "http://pasture-one-actor/images/100.png",
      "remote_url": "http://pasture-one-actor/images/100.png",
      "text_url": "http://pasture-one-actor/images/100.png",
      "type": "unknown",
      "url": "http://pasture-one-actor/images/100.png"
    }
  ],
  "url": "http://pasture-one-actor/object/VQYRtHqdLoI",
  "reblogs_count": 0,
  "visibility": "public",
  "bookmarked": false,
  "card": null,
  "favourites_count": 0,
  "in_reply_to_id": null,
  "quote_id": null,
  "poll": null,
  "emoji_reactions": [],
  "application": null,
  "tags": [],
  "sensitive": false,
  "account": {
    "acct": "actor@pasture-one-actor",
    "akkoma": {
      "instance": {
        "favicon": null,
        "name": "pasture-one-actor",
        "nodeinfo": {}
      },
      "permit_followback": false,
      "status_ttl_days": null
    },
    "avatar": "http://akkoma/images/avi.png",
    "avatar_static": "http://akkoma/images/avi.png",
    "bot": false,
    "created_at": "2025-09-09T10:07:35.000Z",
    "display_name": "Test Actor",
    "emojis": [],
    "fields": [],
    "followers_count": 0,
    "following_count": 0,
    "fqn": "actor@pasture-one-actor",
    "header": "http://akkoma/images/banner.png",
    "header_static": "http://akkoma/images/banner.png",
    "id": "Ay1hCFX0uGzqSBvc24",
    "last_status_at": "2025-09-09",
    "locked": false,
    "note": "",
    "pleroma": {
      "also_known_as": [],
      "ap_id": "http://pasture-one-actor/actor",
      "background_image": null,
      "favicon": null,
      "hide_favorites": true,
      "hide_followers": false,
      "hide_followers_count": false,
      "hide_follows": false,
      "hide_follows_count": false,
      "is_admin": false,
      "is_confirmed": true,
      "is_moderator": false,
      "is_suggested": false,
      "relationship": {},
      "skip_thread_containment": false,
      "tags": []
    },
    "source": {
      "fields": [],
      "note": "",
      "pleroma": {
        "actor_type": "Person",
        "discoverable": false
      },
      "sensitive": false
    },
    "statuses_count": 68,
    "url": "http://pasture-one-actor/actor",
    "username": "actor"
  },
  "reblog": null,
  "language": null,
  "created_at": "2025-09-09T10:09:35.000Z",
  "muted": false,
  "replies_count": 0
}

Example 2

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://akkoma/users/witch"
  ],
  "id": "http://pasture-one-actor/activity/CeZ4rvTXuko",
  "published": "2025-09-09T10:09:35Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://akkoma/users/witch"
    ],
    "id": "http://pasture-one-actor/object/z1hCI_lsHY0",
    "published": "2025-09-09T10:09:35Z",
    "content": "name and summary",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}

akkoma

{
  "reblogged": false,
  "pleroma": {
    "content": {
      "text/plain": "name and summary"
    },
    "context": "http://pasture-one-actor/object/z1hCI_lsHY0",
    "conversation_id": 1561370409,
    "direct_conversation_id": null,
    "emoji_reactions": [],
    "expires_at": null,
    "in_reply_to_account_acct": null,
    "local": false,
    "parent_visible": false,
    "pinned_at": null,
    "spoiler_text": {
      "text/plain": ""
    },
    "thread_muted": false
  },
  "in_reply_to_account_id": null,
  "uri": "http://pasture-one-actor/object/z1hCI_lsHY0",
  "edited_at": null,
  "quote": null,
  "favourited": false,
  "text": null,
  "pinned": false,
  "emojis": [],
  "id": "Ay1hNMl0Nka236m0zw",
  "mentions": [
    {
      "acct": "witch",
      "id": "Ay1h3YE0L8LMp8AbVQ",
      "url": "http://akkoma/users/witch",
      "username": "witch"
    }
  ],
  "content": "name and summary",
  "akkoma": {
    "source": null
  },
  "spoiler_text": "",
  "media_attachments": [
    {
      "blurhash": null,
      "description": "name",
      "id": "983810756",
      "pleroma": {
        "mime_type": "application/octet-stream"
      },
      "preview_url": "http://pasture-one-actor/images/100.png",
      "remote_url": "http://pasture-one-actor/images/100.png",
      "text_url": "http://pasture-one-actor/images/100.png",
      "type": "unknown",
      "url": "http://pasture-one-actor/images/100.png"
    }
  ],
  "url": "http://pasture-one-actor/object/z1hCI_lsHY0",
  "reblogs_count": 0,
  "visibility": "public",
  "bookmarked": false,
  "card": null,
  "favourites_count": 0,
  "in_reply_to_id": null,
  "quote_id": null,
  "poll": null,
  "emoji_reactions": [],
  "application": null,
  "tags": [],
  "sensitive": false,
  "account": {
    "acct": "actor@pasture-one-actor",
    "akkoma": {
      "instance": {
        "favicon": null,
        "name": "pasture-one-actor",
        "nodeinfo": {}
      },
      "permit_followback": false,
      "status_ttl_days": null
    },
    "avatar": "http://akkoma/images/avi.png",
    "avatar_static": "http://akkoma/images/avi.png",
    "bot": false,
    "created_at": "2025-09-09T10:07:35.000Z",
    "display_name": "Test Actor",
    "emojis": [],
    "fields": [],
    "followers_count": 0,
    "following_count": 0,
    "fqn": "actor@pasture-one-actor",
    "header": "http://akkoma/images/banner.png",
    "header_static": "http://akkoma/images/banner.png",
    "id": "Ay1hCFX0uGzqSBvc24",
    "last_status_at": "2025-09-09",
    "locked": false,
    "note": "",
    "pleroma": {
      "also_known_as": [],
      "ap_id": "http://pasture-one-actor/actor",
      "background_image": null,
      "favicon": null,
      "hide_favorites": true,
      "hide_followers": false,
      "hide_followers_count": false,
      "hide_follows": false,
      "hide_follows_count": false,
      "is_admin": false,
      "is_confirmed": true,
      "is_moderator": false,
      "is_suggested": false,
      "relationship": {},
      "skip_thread_containment": false,
      "tags": []
    },
    "source": {
      "fields": [],
      "note": "",
      "pleroma": {
        "actor_type": "Person",
        "discoverable": false
      },
      "sensitive": false
    },
    "statuses_count": 69,
    "url": "http://pasture-one-actor/actor",
    "username": "actor"
  },
  "reblog": null,
  "language": null,
  "created_at": "2025-09-09T10:09:35.000Z",
  "muted": false,
  "replies_count": 0
}

Example 3

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://akkoma/users/witch"
  ],
  "id": "http://pasture-one-actor/activity/hdHHP9ZVat8",
  "published": "2025-09-09T10:09:36Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://akkoma/users/witch"
    ],
    "id": "http://pasture-one-actor/object/RluzeAwwZr8",
    "published": "2025-09-09T10:09:36Z",
    "content": "only name",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}

akkoma

{
  "reblogged": false,
  "pleroma": {
    "content": {
      "text/plain": "only name"
    },
    "context": "http://pasture-one-actor/object/RluzeAwwZr8",
    "conversation_id": 607261361,
    "direct_conversation_id": null,
    "emoji_reactions": [],
    "expires_at": null,
    "in_reply_to_account_acct": null,
    "local": false,
    "parent_visible": false,
    "pinned_at": null,
    "spoiler_text": {
      "text/plain": ""
    },
    "thread_muted": false
  },
  "in_reply_to_account_id": null,
  "uri": "http://pasture-one-actor/object/RluzeAwwZr8",
  "edited_at": null,
  "quote": null,
  "favourited": false,
  "text": null,
  "pinned": false,
  "emojis": [],
  "id": "Ay1hNQePuBge7pioHg",
  "mentions": [
    {
      "acct": "witch",
      "id": "Ay1h3YE0L8LMp8AbVQ",
      "url": "http://akkoma/users/witch",
      "username": "witch"
    }
  ],
  "content": "only name",
  "akkoma": {
    "source": null
  },
  "spoiler_text": "",
  "media_attachments": [
    {
      "blurhash": null,
      "description": "name",
      "id": "-751667840",
      "pleroma": {
        "mime_type": "application/octet-stream"
      },
      "preview_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "text_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "type": "unknown",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "url": "http://pasture-one-actor/object/RluzeAwwZr8",
  "reblogs_count": 0,
  "visibility": "public",
  "bookmarked": false,
  "card": null,
  "favourites_count": 0,
  "in_reply_to_id": null,
  "quote_id": null,
  "poll": null,
  "emoji_reactions": [],
  "application": null,
  "tags": [],
  "sensitive": false,
  "account": {
    "acct": "actor@pasture-one-actor",
    "akkoma": {
      "instance": {
        "favicon": null,
        "name": "pasture-one-actor",
        "nodeinfo": {}
      },
      "permit_followback": false,
      "status_ttl_days": null
    },
    "avatar": "http://akkoma/images/avi.png",
    "avatar_static": "http://akkoma/images/avi.png",
    "bot": false,
    "created_at": "2025-09-09T10:07:35.000Z",
    "display_name": "Test Actor",
    "emojis": [],
    "fields": [],
    "followers_count": 0,
    "following_count": 0,
    "fqn": "actor@pasture-one-actor",
    "header": "http://akkoma/images/banner.png",
    "header_static": "http://akkoma/images/banner.png",
    "id": "Ay1hCFX0uGzqSBvc24",
    "last_status_at": "2025-09-09",
    "locked": false,
    "note": "",
    "pleroma": {
      "also_known_as": [],
      "ap_id": "http://pasture-one-actor/actor",
      "background_image": null,
      "favicon": null,
      "hide_favorites": true,
      "hide_followers": false,
      "hide_followers_count": false,
      "hide_follows": false,
      "hide_follows_count": false,
      "is_admin": false,
      "is_confirmed": true,
      "is_moderator": false,
      "is_suggested": false,
      "relationship": {},
      "skip_thread_containment": false,
      "tags": []
    },
    "source": {
      "fields": [],
      "note": "",
      "pleroma": {
        "actor_type": "Person",
        "discoverable": false
      },
      "sensitive": false
    },
    "statuses_count": 70,
    "url": "http://pasture-one-actor/actor",
    "username": "actor"
  },
  "reblog": null,
  "language": null,
  "created_at": "2025-09-09T10:09:36.000Z",
  "muted": false,
  "replies_count": 0
}

Example 4

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://akkoma/users/witch"
  ],
  "id": "http://pasture-one-actor/activity/yFwdosTRkDo",
  "published": "2025-09-09T10:09:37Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://akkoma/users/witch"
    ],
    "id": "http://pasture-one-actor/object/LgUagM_Idlc",
    "published": "2025-09-09T10:09:37Z",
    "content": "no comment in attachment",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ]
  }
}

akkoma

{
  "reblogged": false,
  "pleroma": {
    "content": {
      "text/plain": "no comment in attachment"
    },
    "context": "http://pasture-one-actor/object/LgUagM_Idlc",
    "conversation_id": 73919593,
    "direct_conversation_id": null,
    "emoji_reactions": [],
    "expires_at": null,
    "in_reply_to_account_acct": null,
    "local": false,
    "parent_visible": false,
    "pinned_at": null,
    "spoiler_text": {
      "text/plain": ""
    },
    "thread_muted": false
  },
  "in_reply_to_account_id": null,
  "uri": "http://pasture-one-actor/object/LgUagM_Idlc",
  "edited_at": null,
  "quote": null,
  "favourited": false,
  "text": null,
  "pinned": false,
  "emojis": [],
  "id": "Ay1hNUt69Xo7GWcaiu",
  "mentions": [
    {
      "acct": "witch",
      "id": "Ay1h3YE0L8LMp8AbVQ",
      "url": "http://akkoma/users/witch",
      "username": "witch"
    }
  ],
  "content": "no comment in attachment",
  "akkoma": {
    "source": null
  },
  "spoiler_text": "",
  "media_attachments": [
    {
      "blurhash": null,
      "description": null,
      "id": "-751667840",
      "pleroma": {
        "mime_type": "application/octet-stream"
      },
      "preview_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "text_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "type": "unknown",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "url": "http://pasture-one-actor/object/LgUagM_Idlc",
  "reblogs_count": 0,
  "visibility": "public",
  "bookmarked": false,
  "card": null,
  "favourites_count": 0,
  "in_reply_to_id": null,
  "quote_id": null,
  "poll": null,
  "emoji_reactions": [],
  "application": null,
  "tags": [],
  "sensitive": false,
  "account": {
    "acct": "actor@pasture-one-actor",
    "akkoma": {
      "instance": {
        "favicon": null,
        "name": "pasture-one-actor",
        "nodeinfo": {}
      },
      "permit_followback": false,
      "status_ttl_days": null
    },
    "avatar": "http://akkoma/images/avi.png",
    "avatar_static": "http://akkoma/images/avi.png",
    "bot": false,
    "created_at": "2025-09-09T10:07:35.000Z",
    "display_name": "Test Actor",
    "emojis": [],
    "fields": [],
    "followers_count": 0,
    "following_count": 0,
    "fqn": "actor@pasture-one-actor",
    "header": "http://akkoma/images/banner.png",
    "header_static": "http://akkoma/images/banner.png",
    "id": "Ay1hCFX0uGzqSBvc24",
    "last_status_at": "2025-09-09",
    "locked": false,
    "note": "",
    "pleroma": {
      "also_known_as": [],
      "ap_id": "http://pasture-one-actor/actor",
      "background_image": null,
      "favicon": null,
      "hide_favorites": true,
      "hide_followers": false,
      "hide_followers_count": false,
      "hide_follows": false,
      "hide_follows_count": false,
      "is_admin": false,
      "is_confirmed": true,
      "is_moderator": false,
      "is_suggested": false,
      "relationship": {},
      "skip_thread_containment": false,
      "tags": []
    },
    "source": {
      "fields": [],
      "note": "",
      "pleroma": {
        "actor_type": "Person",
        "discoverable": false
      },
      "sensitive": false
    },
    "statuses_count": 71,
    "url": "http://pasture-one-actor/actor",
    "username": "actor"
  },
  "reblog": null,
  "language": null,
  "created_at": "2025-09-09T10:09:37.000Z",
  "muted": false,
  "replies_count": 0
}