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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.

Support Table Preview

attachment Object Activity
no description
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/715dde0e-3ee9-4ccf-80ed-65448a054ff5.png”
}
 Object 🚨 Activity
name, summary and content
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/18033c1e-d691-4c05-b01c-00f9a3b36620.png”,
  “name”: “name”,
  “summary”: “summary”,
  “content”: “content”
}
 Object 🚨 Activity
only name
[
  {
    “type”: “Image”,
    “url”: “http://pasture-one-actor/assets/FediverseLogo.png”,
    “name”: “name”,
    “imageType”: “image/jpeg”
  }
]
 Object Activity
only summary
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/36430b77-9e6a-4f8f-9024-bd568efba065.png”,
  “summary”: “summary”
}
 Object 🚨 Activity
only content
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/9bc29d28-4332-4462-8d85-e3b5e2824be0.png”,
  “content”: “content”
}
 Object 🚨 Activity
no comment in attachment; reused image
[
  {
    “type”: “Image”,
    “url”: “http://pasture-one-actor/assets/FediverseLogo.png”
  }
]
 Object Activity

Objects

Object 1

❌❌❌ does not obey schema ❌❌❌

{'type': 'Image', 'url': 'http://pasture-one-actor/images/715dde0e-3ee9-4ccf-80ed-65448a054ff5.png'} is not of type 'array'

Failed validating 'type' in schema['properties']['attachment']:
    {'description': 'A list of objects that are attached to the original '
                    'object',
     'items': {'additionalProperties': True, 'type': 'object'},
     'type': 'array'}

On instance['attachment']:
    {'type': 'Image',
     'url': 'http://pasture-one-actor/images/715dde0e-3ee9-4ccf-80ed-65448a054ff5.png'}

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "type": "Image",
    "url": "http://pasture-one-actor/images/715dde0e-3ee9-4ccf-80ed-65448a054ff5.png"
  },
  "attributedTo": "http://actor.example",
  "content": "no description",
  "id": "http://actor.example/object/BNI9wVHSVBQ",
  "published": "2025-11-21T12:04:56Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 2

❌❌❌ does not obey schema ❌❌❌

{'type': 'Image', 'url': 'http://pasture-one-actor/images/18033c1e-d691-4c05-b01c-00f9a3b36620.png', 'name': 'name', 'summary': 'summary', 'content': 'content'} is not of type 'array'

Failed validating 'type' in schema['properties']['attachment']:
    {'description': 'A list of objects that are attached to the original '
                    'object',
     'items': {'additionalProperties': True, 'type': 'object'},
     'type': 'array'}

On instance['attachment']:
    {'type': 'Image',
     'url': 'http://pasture-one-actor/images/18033c1e-d691-4c05-b01c-00f9a3b36620.png',
     'name': 'name',
     'summary': 'summary',
     'content': 'content'}

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "content": "content",
    "name": "name",
    "summary": "summary",
    "type": "Image",
    "url": "http://pasture-one-actor/images/18033c1e-d691-4c05-b01c-00f9a3b36620.png"
  },
  "attributedTo": "http://actor.example",
  "content": "name, summary and content",
  "id": "http://actor.example/object/MRfXoBHVG9M",
  "published": "2025-11-21T12:04:56Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 3

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": [
    {
      "imageType": "image/jpeg",
      "name": "name",
      "type": "Image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "attributedTo": "http://actor.example",
  "content": "only name",
  "id": "http://actor.example/object/vGKBnDX33iE",
  "published": "2025-11-21T12:04:56Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 4

❌❌❌ does not obey schema ❌❌❌

{'type': 'Image', 'url': 'http://pasture-one-actor/images/36430b77-9e6a-4f8f-9024-bd568efba065.png', 'summary': 'summary'} is not of type 'array'

Failed validating 'type' in schema['properties']['attachment']:
    {'description': 'A list of objects that are attached to the original '
                    'object',
     'items': {'additionalProperties': True, 'type': 'object'},
     'type': 'array'}

On instance['attachment']:
    {'type': 'Image',
     'url': 'http://pasture-one-actor/images/36430b77-9e6a-4f8f-9024-bd568efba065.png',
     'summary': 'summary'}

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "summary": "summary",
    "type": "Image",
    "url": "http://pasture-one-actor/images/36430b77-9e6a-4f8f-9024-bd568efba065.png"
  },
  "attributedTo": "http://actor.example",
  "content": "only summary",
  "id": "http://actor.example/object/UTiCO8pjzUs",
  "published": "2025-11-21T12:04:56Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 5

❌❌❌ does not obey schema ❌❌❌

{'type': 'Image', 'url': 'http://pasture-one-actor/images/9bc29d28-4332-4462-8d85-e3b5e2824be0.png', 'content': 'content'} is not of type 'array'

Failed validating 'type' in schema['properties']['attachment']:
    {'description': 'A list of objects that are attached to the original '
                    'object',
     'items': {'additionalProperties': True, 'type': 'object'},
     'type': 'array'}

On instance['attachment']:
    {'type': 'Image',
     'url': 'http://pasture-one-actor/images/9bc29d28-4332-4462-8d85-e3b5e2824be0.png',
     'content': 'content'}

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "content": "content",
    "type": "Image",
    "url": "http://pasture-one-actor/images/9bc29d28-4332-4462-8d85-e3b5e2824be0.png"
  },
  "attributedTo": "http://actor.example",
  "content": "only content",
  "id": "http://actor.example/object/Ci3vsxSjjeA",
  "published": "2025-11-21T12:04:56Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 6

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": [
    {
      "type": "Image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "attributedTo": "http://actor.example",
  "content": "no comment in attachment; reused image",
  "id": "http://actor.example/object/l_cwzq5yl5A",
  "published": "2025-11-21T12:04:56Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Activities

Activity 1

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/U36K7ot6YW8",
  "object": {
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/715dde0e-3ee9-4ccf-80ed-65448a054ff5.png"
    },
    "attributedTo": "http://actor.example",
    "content": "no description",
    "id": "http://actor.example/object/tf-NVGsQYOE",
    "published": "2025-11-21T12:04:57Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-11-21T12:04:57Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}

Activity 2

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/jV_seGKBTbA",
  "object": {
    "attachment": {
      "content": "content",
      "name": "name",
      "summary": "summary",
      "type": "Image",
      "url": "http://pasture-one-actor/images/18033c1e-d691-4c05-b01c-00f9a3b36620.png"
    },
    "attributedTo": "http://actor.example",
    "content": "name, summary and content",
    "id": "http://actor.example/object/tbIeD546xt4",
    "published": "2025-11-21T12:04:57Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-11-21T12:04:57Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}

Activity 3

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/0XnzLp0o1nU",
  "object": {
    "attachment": [
      {
        "imageType": "image/jpeg",
        "name": "name",
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "attributedTo": "http://actor.example",
    "content": "only name",
    "id": "http://actor.example/object/sqKlanIK4Bo",
    "published": "2025-11-21T12:04:57Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-11-21T12:04:57Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}

Activity 4

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/JZpKi-Kcw-M",
  "object": {
    "attachment": {
      "summary": "summary",
      "type": "Image",
      "url": "http://pasture-one-actor/images/36430b77-9e6a-4f8f-9024-bd568efba065.png"
    },
    "attributedTo": "http://actor.example",
    "content": "only summary",
    "id": "http://actor.example/object/KHWDodtTcj8",
    "published": "2025-11-21T12:04:57Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-11-21T12:04:57Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}

Activity 5

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/sfm33co3jYY",
  "object": {
    "attachment": {
      "content": "content",
      "type": "Image",
      "url": "http://pasture-one-actor/images/9bc29d28-4332-4462-8d85-e3b5e2824be0.png"
    },
    "attributedTo": "http://actor.example",
    "content": "only content",
    "id": "http://actor.example/object/9RQZcBJKxS4",
    "published": "2025-11-21T12:04:57Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-11-21T12:04:57Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}

Activity 6

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/02wScP-07bU",
  "object": {
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "attributedTo": "http://actor.example",
    "content": "no comment in attachment; reused image",
    "id": "http://actor.example/object/4RPswzJxnA4",
    "published": "2025-11-21T12:04:57Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-11-21T12:04:57Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}