Telegram allows mentioning other users in case of urgent duckling matters, and quickly navigating to those mentions in order to read them as swiftly as possible.

messageEntityMention#fa04579d offset:int length:int = MessageEntity;
messageEntityMentionName#dc7b1140 offset:int length:int user_id:long = MessageEntity;
inputMessageEntityMentionName#208e68c9 offset:int length:int user_id:InputUser = MessageEntity;

message#85d6cbe2 flags:# out:flags.1?true mentioned:flags.4?true media_unread:flags.5?true silent:flags.13?true post:flags.14?true from_scheduled:flags.18?true legacy:flags.19?true edit_hide:flags.21?true pinned:flags.24?true id:int from_id:flags.8?Peer peer_id:Peer fwd_from:flags.2?MessageFwdHeader via_bot_id:flags.11?long reply_to:flags.3?MessageReplyHeader date:int message:string media:flags.9?MessageMedia reply_markup:flags.6?ReplyMarkup entities:flags.7?Vector<MessageEntity> views:flags.10?int forwards:flags.10?int replies:flags.23?MessageReplies edit_date:flags.15?int post_author:flags.16?string grouped_id:flags.17?long restriction_reason:flags.22?Vector<RestrictionReason> ttl_period:flags.25?int = Message;

channelParticipantsMentions#e04b5ceb flags:# q:flags.0?string top_msg_id:flags.1?int = ChannelParticipantsFilter;


messages.sendMessage#520c3870 flags:# no_webpage:flags.1?true silent:flags.5?true background:flags.6?true clear_draft:flags.7?true peer:InputPeer reply_to_msg_id:flags.0?int message:string random_id:long reply_markup:flags.2?ReplyMarkup entities:flags.3?Vector<MessageEntity> schedule_date:flags.10?int = Updates;

channels.getParticipants#77ced9d0 channel:InputChannel filter:ChannelParticipantsFilter offset:int limit:int hash:long = channels.ChannelParticipants;

Mentions are implemented as message entities, passed to the messages.sendMessage method:

Incoming messages mentioning to the current user will have the mentioned flag set, and will contain one or more messageEntityMention and messageEntityMentionName constructors.

Graphical clients can show a list of mentionable users when the user starts entering an @ in the text bar; for this purpose, the channelParticipantsMentions filter can be used in channels.getParticipants.
This filter can be enhanced by providing an additional query string q (anything the user enters after @); it will also return non-participant users, in case of channel users commenting in post comment sections.

Dialog mentions

dialog#2c171f72 flags:# pinned:flags.2?true unread_mark:flags.3?true peer:Peer top_message:int read_inbox_max_id:int read_outbox_max_id:int unread_count:int unread_mentions_count:int notify_settings:PeerNotifySettings pts:flags.0?int draft:flags.1?DraftMessage folder_id:flags.4?int = Dialog;


messages.getUnreadMentions#46578472 peer:InputPeer offset_id:int add_offset:int limit:int max_id:int min_id:int = messages.Messages;
messages.readMentions#f0189d3 peer:InputPeer = messages.AffectedHistory;

Graphical clients are supposed to show a blue mention indicator next to the message counter of chats in the dialog list.
The dialog constructor contains an unread_mentions_count field to isolate chats with unread mentions; the actual mention counter should be shown inside of the chat itself, above an @ button that can be used, by clicking multiple times, to navigate back (using messages.getUnreadMentions) through the mention history.

When the last unread mention is read, or when long-clicking on the @ button, all mentions for a chat should marked as read using messages.readMentions.