matrix-spec/content/client-server-api/modules/mentions.md

### User and room mentions

This module allows users to mention other users and rooms within a room message.
This is achieved by including metadata in the `m.mentions` content property of
the event. The `m.mentions` property consists of an object with two fields:

* `user_ids`: A list of Matrix IDs of mentioned users.
* `room`: A boolean set to `true` to mention the room. (`room` should otherwise
  not be included on the event.)

Although it is possible to silently mention users, it is recommended to include a
[Matrix URI](/appendices/#uris) in the HTML body of  an [m.room.message](#mroommessage)
event. This applies only to [m.room.message](#mroommessage) events where the `msgtype` is
`m.text`, `m.emote`, or `m.notice`. The `format` for the event must be
`org.matrix.custom.html` and therefore requires a `formatted_body`.

To make a mention, reference the entity being mentioned in the
`m.mentions` property, like so:

```json
{
    "body": "Hello Alice!",
    "msgtype": "m.text",
    "format": "org.matrix.custom.html",
    "formatted_body": "Hello <a href='https://matrix.to/#/@alice:example.org'>Alice</a>!",
    "m.mentions": {
        "user_ids": ["@alice:example.org"]
    }
}
```

Similarly, the entire room can be mentioned:

```json
{
    "body": "Hello @room!",
    "msgtype": "m.text",
    "format": "org.matrix.custom.html",
    "formatted_body": "Hello @room!",
    "m.mentions": {
        "room": true
    }
}
```

Additionally, see [the `.m.rule.is_user_mention` and `.m.rule.is_room_mention` push rules](#predefined-rules).

#### Client behaviour

In addition to using the appropriate `Matrix URI` for the mention,
clients should use the following guidelines when making mentions in
events to be sent:

-   When mentioning users, use the user's potentially ambiguous display
    name for the anchor's text. If the user does not have a display
    name, use the user's ID.
-   When mentioning rooms, use the canonical alias for the room. If the
    room does not have a canonical alias, prefer one of the aliases
    listed on the room. If no alias can be found, fall back to the room
    ID. In all cases, use the alias/room ID being linked to as the
    anchor's text.

The text component of the anchor should be used in the event's `body`
where the mention would normally be represented, as shown in the example
above.

Clients should display mentions differently from other elements. For
example, this may be done by changing the background color of the
mention to indicate that it is different from a normal link.

If the current user is mentioned in a message, the client should show that
mention differently from other mentions, such as by using a red
background color to signify to the user that they were mentioned.

When clicked, the mention should navigate the user to the appropriate
user or room information.

TODO Behavior with replies.

TODO Behavior with edits.

TODO Always include `m.mentions` property.

{{% boxes/note %}}
Similar to legacy [matrix.to URLs](/appendices/#matrixto-navigation),
groups used to be representable by mentions. They follow a similar format
to room mentions, though using the group ID in both the link and anchor
text.
{{% /boxes/note %}}