If a client implements message reactions, it MUST specify the 'urn:xmpp:reactions:0' feature in its service discovery information features as specified in &xep0030; and the Entity Capabilities profile specified in &xep0115;.

In some cases, only a limited number and/or subset of emojis are allowed to react to a message. This is particularly useful in the context of gateways to other chat networks that are not as permissive about which and how many emojis can be used for message reactions. MUC components may also allow room admins to restrict the subset and/or number of emojis per message.

To allow clients to adapt their UIs, entities that have such restrictions in place SHOULD advertise it with &xep0128; in a &xep0004; using the 'urn:xmpp:reactions:0:restrictions' namespace as FORM_TYPE.

When a user chooses to react to a message with a certain emoji, the client sends a <message> stanza containing a <reactions> element. The chosen emoji is included in a <reaction> element within the <reactions> element. The message is referred to by including its id or in MUCs its stanza-id as defined in &xep0359; in the 'id' attribute of the reactions element.

If the user chooses to remove reactions from or add reactions to a message they have already reacted to, the client sends a <message> with all <reaction> elements that are (still or newly) applicable to that message.

In order to remove all reactions from a message, an empty <reactions> element is sent.

Entities implementing reaction restrictions MUST reply to invalid reactions payloads with a message type="error" containing a 'not-acceptable' error. The error payload SHOULD contain a human-readable text message explaining the reaction rules.

When receiving this error message, the reaction-sending entity MUST revert the "message reactions" to its previous state in its user interface.

In direct conversations, a reaction MUST only be accepted if the senders bare JID matches the bare JID of any of the two involved parties.

In MUCs and MUC PMs, the recipient SHOULD ensure that the real bare JID of the sending occupant did not already send a reaction to that message to accept it as a new reaction, e.g. by keeping track of leave/join presences since the message was send. This implies that in semi-anonymous MUCs it MAY be impossible to attach reactions to a message received from the history. A reaction MAY still be a valid reaction update (as per the next paragraph) if it was not accepted as a new reaction.

A reaction MUST only be considered an update if it orignates from the same sender as a previous reaction message. In direct conversations, this means the bare JID MUST match the original bare JID. In MUCs and MUC PMs the senders full JID MAY not match the original full JID, but the recipient MUST ensure that the real bare JID of the sending occupant is the same as the real bare JID of the previous reaction message, e.g. by keeping track of leave/join presences.

If a message containing a <reactions> element arrives delayed, which means it carries a <delay/> element, as defined in &xep0203; it SHOULD only be accepted, if no newer reaction from the same sender was already accepted.

For messages of type 'groupchat', the stanza's 'id' attribute MUST NOT be used for reactions. Instead, in group chat situations, the ID assigned to the stanza by the group chat itself must be used. This is discovered in a <stanza-id> element with a 'by' attribute that matches the bare JID of the group chat, as defined in &xep0359;.

This implies that group chat messages without a &xep0359; stanza-id cannot be reacted to.

For other message types the sender should use the 'id' from a &xep0359; <origin-id> if present, or the value of the 'id' attribute on the <message> otherwise.

The ®ISTRAR; includes 'urn:xmpp:reactions:0' in its registry of protocol namespaces (see &NAMESPACES;).

• urn:xmpp:reactions:0