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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": [
    "http://hollo/@john",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/xhvxgQKIG8I",
  "published": "2025-11-19T07:21:36Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/hV92LJ7DMao",
    "published": "2025-11-19T07:21:36Z",
    "content": "no description",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/c6fae24f-ea5b-4b21-ab42-86b98265edf8.png"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "019a9afd-8000-7854-958f-d2a672df5bc8",
  "created_at": "2025-11-19T07:21:36.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/hV92LJ7DMao",
  "url": "http://pasture-one-actor/object/hV92LJ7DMao",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "no description",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "019a9afb-3559-79a4-887c-114083a4b131",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-11-19T07:19:05.764Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "019a9afd-842d-7903-bd32-93034ad572e1",
      "type": "image",
      "url": "http://pasture-one-actor/images/c6fae24f-ea5b-4b21-ab42-86b98265edf8.png",
      "preview_url": "http://pasture-one-actor/images/c6fae24f-ea5b-4b21-ab42-86b98265edf8.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": null,
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": null
}

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://hollo/@john",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/c5_F6hDngFY",
  "published": "2025-11-19T07:21:37Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/OsNI8lM0sQk",
    "published": "2025-11-19T07:21:37Z",
    "content": "name, summary and content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/891aba70-9fcc-43e7-a706-db68473afeca.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "019a9afd-83e8-750e-8773-a94d00c5c3b7",
  "created_at": "2025-11-19T07:21:37.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/OsNI8lM0sQk",
  "url": "http://pasture-one-actor/object/OsNI8lM0sQk",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "name, summary and content",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "019a9afb-3559-79a4-887c-114083a4b131",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-11-19T07:19:05.764Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "019a9afd-8695-7cf4-9898-307089cf948e",
      "type": "image",
      "url": "http://pasture-one-actor/images/891aba70-9fcc-43e7-a706-db68473afeca.png",
      "preview_url": "http://pasture-one-actor/images/891aba70-9fcc-43e7-a706-db68473afeca.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": "summary",
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": null
}

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://hollo/@john",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/9Buy69lqkt0",
  "published": "2025-11-19T07:21:38Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/XvrKUQ2ZdAY",
    "published": "2025-11-19T07:21:38Z",
    "content": "only name",
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ],
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "019a9afd-87d0-7e1b-af53-e6e09f693a7a",
  "created_at": "2025-11-19T07:21:38.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/XvrKUQ2ZdAY",
  "url": "http://pasture-one-actor/object/XvrKUQ2ZdAY",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "only name",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "019a9afb-3559-79a4-887c-114083a4b131",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-11-19T07:19:05.764Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "019a9afd-8900-7e80-8471-d3a81e7fe2df",
      "type": "image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": "name",
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": null
}

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://hollo/@john",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/vQpCO59TaPU",
  "published": "2025-11-19T07:21:39Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/7h67ZVUOVbQ",
    "published": "2025-11-19T07:21:39Z",
    "content": "only summary",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/68750ed8-4013-4b1f-bad0-3d1c1d31738d.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "019a9afd-8bb8-7048-b730-8d8699e31d2f",
  "created_at": "2025-11-19T07:21:39.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/7h67ZVUOVbQ",
  "url": "http://pasture-one-actor/object/7h67ZVUOVbQ",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "only summary",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "019a9afb-3559-79a4-887c-114083a4b131",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-11-19T07:19:05.764Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "019a9afd-8db5-7096-b6da-a41a487eb7a7",
      "type": "image",
      "url": "http://pasture-one-actor/images/68750ed8-4013-4b1f-bad0-3d1c1d31738d.png",
      "preview_url": "http://pasture-one-actor/images/68750ed8-4013-4b1f-bad0-3d1c1d31738d.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": "summary",
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": null
}

Example 5

activity
{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "http://hollo/@john",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/PEEiBvtaEeg",
  "published": "2025-11-19T07:21:40Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/A4-f3NSASq8",
    "published": "2025-11-19T07:21:40Z",
    "content": "only content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/74c89e62-78f3-402f-b907-8c944e4a5be1.png",
      "content": "content"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "019a9afd-8fa0-7125-808b-6050f2e2ac8b",
  "created_at": "2025-11-19T07:21:40.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/A4-f3NSASq8",
  "url": "http://pasture-one-actor/object/A4-f3NSASq8",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "only content",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "019a9afb-3559-79a4-887c-114083a4b131",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-11-19T07:19:05.764Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "019a9afd-9047-77c4-be23-eb42db0fb69e",
      "type": "image",
      "url": "http://pasture-one-actor/images/74c89e62-78f3-402f-b907-8c944e4a5be1.png",
      "preview_url": "http://pasture-one-actor/images/74c89e62-78f3-402f-b907-8c944e4a5be1.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": null,
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": null
}

Example 6

activity
{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "http://hollo/@john",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/mXVcmZFlnIA",
  "published": "2025-11-19T07:21:40Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/sFJfhZvN21c",
    "published": "2025-11-19T07:21:40Z",
    "content": "no comment in attachment; reused image",
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "019a9afd-8fa0-7125-808b-60519f5b2c14",
  "created_at": "2025-11-19T07:21:40.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/sFJfhZvN21c",
  "url": "http://pasture-one-actor/object/sFJfhZvN21c",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "no comment in attachment; reused image",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "019a9afb-3559-79a4-887c-114083a4b131",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-11-19T07:19:05.764Z",
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