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. firefish, 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": [
    "http://misskey/users/ac27zbxd33c70003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/lWq9DNpK9Jg",
  "published": "2025-08-30T19:32:39Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://misskey/users/ac27zbxd33c70003"
    ],
    "id": "http://pasture-one-actor/object/tCah6B9U33s",
    "published": "2025-08-30T19:32:39Z",
    "content": "no description",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    }
  }
}

misskey

{
  "id": "ac283g0ov91q001g",
  "createdAt": "2025-08-30T19:32:39.000Z",
  "userId": "ac28186av91q0003",
  "user": {
    "id": "ac28186av91q0003",
    "name": "Test Actor",
    "username": "actor",
    "host": "pasture-one-actor",
    "avatarUrl": "http://misskey/identicon/actor@pasture-one-actor",
    "avatarBlurhash": null,
    "avatarDecorations": [],
    "isBot": false,
    "isCat": false,
    "instance": {
      "name": null,
      "softwareName": null,
      "softwareVersion": null,
      "iconUrl": null,
      "faviconUrl": null,
      "themeColor": null
    },
    "emojis": {},
    "onlineStatus": "unknown"
  },
  "text": "no description",
  "cw": null,
  "visibility": "public",
  "localOnly": false,
  "reactionAcceptance": null,
  "renoteCount": 0,
  "repliesCount": 0,
  "reactionCount": 0,
  "reactions": {},
  "reactionEmojis": {},
  "emojis": {},
  "fileIds": [
    "ac283gtvv91q001f"
  ],
  "files": [
    {
      "id": "ac283gtvv91q001f",
      "createdAt": "2025-08-30T19:32:40.051Z",
      "name": "100.png",
      "type": "image/png",
      "md5": "284304c052de24eddc2ac67b3676976f",
      "size": 0,
      "isSensitive": false,
      "blurhash": "e2Kk3G^BfQ^BfQ^BjufQjufQfQfQfQfQfQ^BjufQjufQfQfQfQfQfQ",
      "properties": {
        "width": 40,
        "height": 40
      },
      "url": "http://pasture-one-actor/images/100.png",
      "thumbnailUrl": "http://pasture-one-actor/images/100.png",
      "comment": null,
      "folderId": null,
      "folder": null,
      "userId": "ac28186av91q0003",
      "user": null
    }
  ],
  "replyId": null,
  "renoteId": null,
  "uri": "http://pasture-one-actor/object/tCah6B9U33s",
  "clippedCount": 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": [
    "http://misskey/users/ac27zbxd33c70003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/2OQ-sYUbQZM",
  "published": "2025-08-30T19:32:40Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://misskey/users/ac27zbxd33c70003"
    ],
    "id": "http://pasture-one-actor/object/lfaqc9o0D88",
    "published": "2025-08-30T19:32:40Z",
    "content": "text",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}

misskey

{
  "id": "ac283gsgv91q001i",
  "createdAt": "2025-08-30T19:32:40.000Z",
  "userId": "ac28186av91q0003",
  "user": {
    "id": "ac28186av91q0003",
    "name": "Test Actor",
    "username": "actor",
    "host": "pasture-one-actor",
    "avatarUrl": "http://misskey/identicon/actor@pasture-one-actor",
    "avatarBlurhash": null,
    "avatarDecorations": [],
    "isBot": false,
    "isCat": false,
    "instance": {
      "name": null,
      "softwareName": null,
      "softwareVersion": null,
      "iconUrl": null,
      "faviconUrl": null,
      "themeColor": null
    },
    "emojis": {},
    "onlineStatus": "unknown"
  },
  "text": "text",
  "cw": null,
  "visibility": "public",
  "localOnly": false,
  "reactionAcceptance": null,
  "renoteCount": 0,
  "repliesCount": 0,
  "reactionCount": 0,
  "reactions": {},
  "reactionEmojis": {},
  "emojis": {},
  "fileIds": [
    "ac283hbwv91q001h"
  ],
  "files": [
    {
      "id": "ac283hbwv91q001h",
      "createdAt": "2025-08-30T19:32:40.700Z",
      "name": "100.png",
      "type": "image/png",
      "md5": "45bc73254f74646435ed0ffbd41cd2e9",
      "size": 0,
      "isSensitive": false,
      "blurhash": "e3OD%X_2fQ_2fQ_2j[fQj[fQfQfQfQfQfQ_2j[fQj[fQfQfQfQfQfQ",
      "properties": {
        "width": 40,
        "height": 40
      },
      "url": "http://pasture-one-actor/images/100.png",
      "thumbnailUrl": "http://pasture-one-actor/images/100.png",
      "comment": "name",
      "folderId": null,
      "folder": null,
      "userId": "ac28186av91q0003",
      "user": null
    }
  ],
  "replyId": null,
  "renoteId": null,
  "uri": "http://pasture-one-actor/object/lfaqc9o0D88",
  "clippedCount": 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": [
    "http://misskey/users/ac27zbxd33c70003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/l1tGViroiNs",
  "published": "2025-08-30T19:32:41Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://misskey/users/ac27zbxd33c70003"
    ],
    "id": "http://pasture-one-actor/object/b7uwGMGe61I",
    "published": "2025-08-30T19:32:41Z",
    "content": "text",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}

no result

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": [
    "http://misskey/users/ac27zbxd33c70003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/MFKoZfZ5L6I",
  "published": "2025-08-30T19:32:52Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://misskey/users/ac27zbxd33c70003"
    ],
    "id": "http://pasture-one-actor/object/oZCzdLsu9oo",
    "published": "2025-08-30T19:32:52Z",
    "content": "no comment in attachment",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ]
  }
}

no result