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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://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/yhwChEqEkOE",
  "published": "2025-11-21T13:27:14Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/object/RaubgH3Z4sQ",
    "published": "2025-11-21T13:27:14Z",
    "content": "no description",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/4dcf8d70-161b-4aef-979c-02d252a7f5d6.png"
    }
  }
}
mastodon
{
  "id": "115587916431728122",
  "created_at": "2025-11-21T13:27:14.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/RaubgH3Z4sQ",
  "url": "http://pasture-one-actor/object/RaubgH3Z4sQ",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "quotes_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "no description",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "115587904635725355",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-11-21T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://localhost:2970/system/avatars/original/missing.png",
    "avatar_static": "http://localhost:2970/system/avatars/original/missing.png",
    "header": "http://localhost:2970/system/headers/original/missing.png",
    "header_static": "http://localhost:2970/system/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 71,
    "last_status_at": "2025-11-21",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115587916426307147",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/426/307/147/original/12dc9e40f1e87c0d.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/426/307/147/small/12dc9e40f1e87c0d.png",
      "remote_url": "http://pasture-one-actor/images/4dcf8d70-161b-4aef-979c-02d252a7f5d6.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        },
        "small": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        }
      },
      "description": null,
      "blurhash": "U2RDmW~jfQ~j~jj@fQj@fQfQfQfQ~jj@fQj@"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null,
  "quote_approval": {
    "automatic": [],
    "manual": [],
    "current_user": "denied"
  }
}

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://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/Qaj4VfpxSVA",
  "published": "2025-11-21T13:27:15Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/object/ODJpxbFMAuA",
    "published": "2025-11-21T13:27:15Z",
    "content": "name, summary and content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/0ebdd7bf-aad9-484a-89a2-060e4107d007.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}
mastodon
{
  "id": "115587916489022463",
  "created_at": "2025-11-21T13:27:15.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/ODJpxbFMAuA",
  "url": "http://pasture-one-actor/object/ODJpxbFMAuA",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "quotes_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "name, summary and content",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "115587904635725355",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-11-21T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://localhost:2970/system/avatars/original/missing.png",
    "avatar_static": "http://localhost:2970/system/avatars/original/missing.png",
    "header": "http://localhost:2970/system/headers/original/missing.png",
    "header_static": "http://localhost:2970/system/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 72,
    "last_status_at": "2025-11-21",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115587916483703675",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/483/703/675/original/716c32717ec8a163.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/483/703/675/small/716c32717ec8a163.png",
      "remote_url": "http://pasture-one-actor/images/0ebdd7bf-aad9-484a-89a2-060e4107d007.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        },
        "small": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        }
      },
      "description": "summary",
      "blurhash": "U2Bu=UucfQucucfjfQfjfQfQfQfQucfjfQfj"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null,
  "quote_approval": {
    "automatic": [],
    "manual": [],
    "current_user": "denied"
  }
}

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://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/FuoE-dzD0FE",
  "published": "2025-11-21T13:27:16Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/object/M4To1nqoSz4",
    "published": "2025-11-21T13:27:16Z",
    "content": "only name",
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}
mastodon
{
  "id": "115587916554379859",
  "created_at": "2025-11-21T13:27:16.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/M4To1nqoSz4",
  "url": "http://pasture-one-actor/object/M4To1nqoSz4",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "quotes_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "only name",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "115587904635725355",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-11-21T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://localhost:2970/system/avatars/original/missing.png",
    "avatar_static": "http://localhost:2970/system/avatars/original/missing.png",
    "header": "http://localhost:2970/system/headers/original/missing.png",
    "header_static": "http://localhost:2970/system/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 73,
    "last_status_at": "2025-11-21",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115587916540940399",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/540/940/399/original/190490dd67042dc6.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/540/940/399/small/190490dd67042dc6.png",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 1548,
          "height": 1548,
          "size": "1548x1548",
          "aspect": 1.0
        },
        "small": {
          "width": 480,
          "height": 480,
          "size": "480x480",
          "aspect": 1.0
        }
      },
      "description": "name",
      "blurhash": "UOEVB3.}4.T=}fjEEVw[4=K[RUw~9,O?V?bV"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null,
  "quote_approval": {
    "automatic": [],
    "manual": [],
    "current_user": "denied"
  }
}

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://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/JbbM7pwJ4VU",
  "published": "2025-11-21T13:27:17Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/object/9Sdi5xNrPRQ",
    "published": "2025-11-21T13:27:17Z",
    "content": "only summary",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/5c040f6b-b8e4-4d46-b12c-51b8fc1eaf0e.png",
      "summary": "summary"
    }
  }
}
mastodon
{
  "id": "115587916605088352",
  "created_at": "2025-11-21T13:27:17.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/9Sdi5xNrPRQ",
  "url": "http://pasture-one-actor/object/9Sdi5xNrPRQ",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "quotes_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "only summary",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "115587904635725355",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-11-21T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://localhost:2970/system/avatars/original/missing.png",
    "avatar_static": "http://localhost:2970/system/avatars/original/missing.png",
    "header": "http://localhost:2970/system/headers/original/missing.png",
    "header_static": "http://localhost:2970/system/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 74,
    "last_status_at": "2025-11-21",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115587916599428508",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/599/428/508/original/3d822fa52014e874.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/599/428/508/small/3d822fa52014e874.png",
      "remote_url": "http://pasture-one-actor/images/5c040f6b-b8e4-4d46-b12c-51b8fc1eaf0e.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        },
        "small": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        }
      },
      "description": "summary",
      "blurhash": "U1LqiW^{fQ^{^{j@fQj@fQfQfQfQ^{j@fQj@"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null,
  "quote_approval": {
    "automatic": [],
    "manual": [],
    "current_user": "denied"
  }
}

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://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/mnTEDWHQKeA",
  "published": "2025-11-21T13:27:18Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/object/b-H_hbueWBs",
    "published": "2025-11-21T13:27:18Z",
    "content": "only content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/33db188f-5b71-4ae0-b68b-91bed7c1eb77.png",
      "content": "content"
    }
  }
}
mastodon
{
  "id": "115587916664955302",
  "created_at": "2025-11-21T13:27:18.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/b-H_hbueWBs",
  "url": "http://pasture-one-actor/object/b-H_hbueWBs",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "quotes_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "only content",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "115587904635725355",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-11-21T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://localhost:2970/system/avatars/original/missing.png",
    "avatar_static": "http://localhost:2970/system/avatars/original/missing.png",
    "header": "http://localhost:2970/system/headers/original/missing.png",
    "header_static": "http://localhost:2970/system/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 75,
    "last_status_at": "2025-11-21",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115587916658547534",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/658/547/534/original/de0a923fb43058a4.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/115/587/916/658/547/534/small/de0a923fb43058a4.png",
      "remote_url": "http://pasture-one-actor/images/33db188f-5b71-4ae0-b68b-91bed7c1eb77.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        },
        "small": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        }
      },
      "description": null,
      "blurhash": "U07{UEyEfQyEyEfkfQfkfQfQfQfQyEfkfQfk"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null,
  "quote_approval": {
    "automatic": [],
    "manual": [],
    "current_user": "denied"
  }
}

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://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/oF0o7zizD8U",
  "published": "2025-11-21T13:27:19Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/object/FHw89whQQrE",
    "published": "2025-11-21T13:27:19Z",
    "content": "no comment in attachment; reused image",
    "attachment": [
      {
        "type": "Image",
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