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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://mitra/users/admin",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/1NCnauD85kw",
  "published": "2025-10-11T09:47:41Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/object/yrWfUP0dvJ8",
    "published": "2025-10-11T09:47:41Z",
    "content": "no description",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/bab6d2ad-b10c-45b3-9bae-5390e67db3a4.png"
    }
  }
}
mitra
{
  "id": "0199d2ab-3f49-16c4-8b1f-d7693b07f434",
  "uri": "http://pasture-one-actor/object/yrWfUP0dvJ8",
  "url": "http://pasture-one-actor/object/yrWfUP0dvJ8",
  "created_at": "2025-10-11T09:47:41.000Z",
  "edited_at": null,
  "account": {
    "id": "0199d2a8-cc7f-d944-db37-dce1869ca342",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-10-11T09:45:01.823Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 71,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "no description",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0199d2ab-3f44-68f5-bcaa-1c67d65624df",
      "type": "image",
      "url": "http://mitra/media/9d60c900236c43e246b975dffe3ee2aadf904d96de93d5b237b0733a0dcfb2cc.png",
      "preview_url": "http://mitra/media/9d60c900236c43e246b975dffe3ee2aadf904d96de93d5b237b0733a0dcfb2cc.png",
      "description": null
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "0199d2a7-629a-ea91-e871-93a9f6f977b1",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/nKK80u70ERg",
  "published": "2025-10-11T09:47:42Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/object/9FrWVlmj5YE",
    "published": "2025-10-11T09:47:42Z",
    "content": "name, summary and content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/62a231a7-078d-4a93-9431-be42fdf9eb7f.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}
mitra
{
  "id": "0199d2ab-4523-dd2a-3500-6e81c60ffb44",
  "uri": "http://pasture-one-actor/object/9FrWVlmj5YE",
  "url": "http://pasture-one-actor/object/9FrWVlmj5YE",
  "created_at": "2025-10-11T09:47:42.000Z",
  "edited_at": null,
  "account": {
    "id": "0199d2a8-cc7f-d944-db37-dce1869ca342",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-10-11T09:45:01.823Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 72,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "name, summary and content",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0199d2ab-451d-8792-6ad2-3a2087ab371f",
      "type": "image",
      "url": "http://mitra/media/cf0978dd84b083d2e7408a26ca88a3a9d4d445fe15fb938548fcbe4d1f077d70.png",
      "preview_url": "http://mitra/media/cf0978dd84b083d2e7408a26ca88a3a9d4d445fe15fb938548fcbe4d1f077d70.png",
      "description": "name"
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "0199d2a7-629a-ea91-e871-93a9f6f977b1",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/BDdcSlJslLY",
  "published": "2025-10-11T09:47:43Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/object/363MmifxmCU",
    "published": "2025-10-11T09:47:43Z",
    "content": "only name",
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}
mitra
{
  "id": "0199d2ab-4b0c-9c3a-9e58-2ef06fdcbb1e",
  "uri": "http://pasture-one-actor/object/363MmifxmCU",
  "url": "http://pasture-one-actor/object/363MmifxmCU",
  "created_at": "2025-10-11T09:47:43.000Z",
  "edited_at": null,
  "account": {
    "id": "0199d2a8-cc7f-d944-db37-dce1869ca342",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-10-11T09:45:01.823Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 73,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "only name",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0199d2ab-4b07-920c-47be-52a66ef18871",
      "type": "image",
      "url": "http://mitra/media/2d7a61d31393a44edf2a30160cdd70b3582a4f97d5b550f8141dfee1641b4ae7.png",
      "preview_url": "http://mitra/media/2d7a61d31393a44edf2a30160cdd70b3582a4f97d5b550f8141dfee1641b4ae7.png",
      "description": "name"
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "0199d2a7-629a-ea91-e871-93a9f6f977b1",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/jKM2Cu4fWGY",
  "published": "2025-10-11T09:47:45Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/object/W-awXosFyao",
    "published": "2025-10-11T09:47:45Z",
    "content": "only summary",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/9aee3d6f-8a39-4a53-84ed-329f02eb2420.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}
mitra
{
  "id": "0199d2ab-50bd-09f8-9189-e6d72a8a236a",
  "uri": "http://pasture-one-actor/object/W-awXosFyao",
  "url": "http://pasture-one-actor/object/W-awXosFyao",
  "created_at": "2025-10-11T09:47:45.000Z",
  "edited_at": null,
  "account": {
    "id": "0199d2a8-cc7f-d944-db37-dce1869ca342",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-10-11T09:45:01.823Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 74,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "only summary",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0199d2ab-50b9-d1dc-0238-19774dee3457",
      "type": "image",
      "url": "http://mitra/media/a8a1e022e5c13dd7fbfae55ab06a6bc8613a3fb038b20171fabc2498514c6d6f.png",
      "preview_url": "http://mitra/media/a8a1e022e5c13dd7fbfae55ab06a6bc8613a3fb038b20171fabc2498514c6d6f.png",
      "description": "name"
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "0199d2a7-629a-ea91-e871-93a9f6f977b1",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/yp1e2p079s4",
  "published": "2025-10-11T09:47:47Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/object/SDv3O7Euja4",
    "published": "2025-10-11T09:47:47Z",
    "content": "only content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/673fa07d-b7c1-497c-864b-40d0d1d3a480.png",
      "content": "content"
    }
  }
}
mitra
{
  "id": "0199d2ab-56a5-e9b8-3147-61dca19eb51b",
  "uri": "http://pasture-one-actor/object/SDv3O7Euja4",
  "url": "http://pasture-one-actor/object/SDv3O7Euja4",
  "created_at": "2025-10-11T09:47:47.000Z",
  "edited_at": null,
  "account": {
    "id": "0199d2a8-cc7f-d944-db37-dce1869ca342",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-10-11T09:45:01.823Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 75,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "only content",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0199d2ab-56a2-a8a1-ffff-29d727ed6870",
      "type": "image",
      "url": "http://mitra/media/d1f6ca071fbbd3da175024281ba12d822684f5777b0244ac42af11043cf94aab.png",
      "preview_url": "http://mitra/media/d1f6ca071fbbd3da175024281ba12d822684f5777b0244ac42af11043cf94aab.png",
      "description": null
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "0199d2a7-629a-ea91-e871-93a9f6f977b1",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/qAmedxa8A10",
  "published": "2025-10-11T09:47:48Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/object/87vH2_jOUas",
    "published": "2025-10-11T09:47:48Z",
    "content": "no comment in attachment; reused image",
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ]
  }
}
mitra
{
  "id": "0199d2ab-5cbc-b711-07d6-85f67cfe7dc4",
  "uri": "http://pasture-one-actor/object/87vH2_jOUas",
  "url": "http://pasture-one-actor/object/87vH2_jOUas",
  "created_at": "2025-10-11T09:47:48.000Z",
  "edited_at": null,
  "account": {
    "id": "0199d2a8-cc7f-d944-db37-dce1869ca342",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-10-11T09:45:01.823Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
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