IRCv3.3 Message Tags

This specification is a work-in-progress and may have major incompatible changes without warning.

This specification may change at any time and we do not recommend implementing it in a production environment.

Copyright © 2016 Kiyoshi Aman <>

Copyright © 2016 James Wheare <>

Unlimited redistribution and modification of this document is allowed provided that the above copyright notice and this permission notice remains intact.

Notes for implementing work-in-progress version

This is a work-in-progress specification.

Software implementing this work-in-progress specification MUST NOT use the unprefixed message-tags capability name. Instead, implementations SHOULD use the draft/message-tags-0.2 capability name to be interoperable with other software implementing a compatible work-in-progress version.

The final version of the specification will use an unprefixed capability name.


This specification adds a new capability for general message tags support and a prefix for expressing client-only tags. It also defines a new command for tag-only messages, and increases the byte limit for tags.


Previously, clients were required to negotiate a capability with servers for each supported tag. This made tags inappropriate for client-only features. By adding a new base capability, this specification allows clients to indicate support for receiving any well-formed tag, whether or not it is recognised or used. This also frees servers from having to filter message tags for each individual client response.

The client-only tag prefix allows servers to safely relay untrusted client tags, keeping them distinct from server-initiated tags that carry verified meaning.

To allow for tagged data to be sent to channels and users without any accompanying message text, a new command for tag-only messages is needed.

With the scope of tags expanded for use as general purpose message metadata, the number and size of tags attached to a message will potentially increase. As a result, an increased limit is implied by negotiating the new capability.



This specification adds the draft/message-tags-0.2 capability. Clients requesting this capability indicate that they are capable of parsing all well-formed tags, even if they don’t handle them specifically.

Servers advertising this capability indicate that they are capable of parsing any tag received from a client, with or without the client-only prefix.


Client-only tags are client-initiated tags that servers MUST attach as-is to any relevant message relayed to other clients. A client-only tag is prefixed with a plus sign (+) and otherwise conforms to the format specified in IRCv3.2 tags.

Client-only tags MUST be relayed on PRIVMSG and NOTICE messages, and MAY be relayed on other messages.

Any server-initiated tags attached to messages MUST be included before client-only tags to prevent them from being pushed outside of the byte limit.

The expected client behaviour of individual client-only tags SHOULD be defined in separate specifications, in the same way as server-initiated tags.

This means client-only tags that aren’t specified in the IRCv3 extension registry MUST use a vendor prefix and SHOULD be submitted to the IRCv3 working group for consideration if they are appropriate for more widespread adoption.

The updated pseudo-BNF for keys is as follows:

<key> ::= [ '+' ] [ <vendor> '/' ] <sequence of letters, digits, hyphens ('-')>

Individual tag keys MUST only be used a maximum of once per message. Clients receiving messages with more than one occurrence of a tag key SHOULD discard all but the final occurrence.

The TAGMSG tag-only message

   Command: TAGMSG
Parameters: <msgtarget>

A new message command TAGMSG is defined for sending messages with tags but no text content. This message MUST be delivered to targets in the same way as PRIVMSG and NOTICE messages. This means for example, honouring channel membership, modes, echo-message, STATUSMSG prefixes, etc.

Servers MAY apply moderation to this command using existing or newly specified modes or configuration.

Servers MUST NOT deliver TAGMSG to clients that haven’t negotiated the message tags capability.

Servers SHOULD reject any TAGMSG command sent without tags. In this case, they MUST use the ERR_NEEDMOREPARAMS (461) error numeric.

See PRIVMSG in RFC2812 for more details on replies and examples.

Clients that receive a TAGMSG command MUST NOT display them in the message history by default. Display guidelines are defined in the specifications of tags attached to the message.

Size limit

The size limit for message tags is increased from 512 to 4607 bytes, including the leading '@' (0x40) and trailing space ' ' (0x20) characters. The size limit for the rest of the message is unchanged.

This limit is separated between server-initiated and client-initiated tags. This prevents servers from overflowing the overall limit by adding tags to a client message sent within the allowed limit.

In the following description, tag data describes the bytes between the leading and trailing tag separators mentioned above.

Clients MUST NOT send messages with tag data exceeding 4094 bytes, this includes tags with or without the client-only prefix.

Servers MUST NOT add tag data to messages that exceeds 510 bytes.

<server_max>    (512)  :: '@' <tag_data  510> ' '
<client_max>   (4096)  :: '@' <tag_data 4094> ' '
<combined_max> (4607)  :: '@' <tag_data  510> ';' <tag_data 4094> ' '

Servers MUST reply with the ERR_INPUTTOOLONG (417) error numeric if a client sends a message with more tag data than the allowed limit. Servers MUST NOT truncate tags to allow lines that exceed this limit.

      ":Input line was too long"

Security considerations

Client-only tags should be treated as untrusted data. They can contain any value and are not validated by servers in any way.

Some specifications may involve servers accepting client-initiated tags without the client-only prefix. Such tag values SHOULD be validated by the server before being sent with any response to a client, unless they are specifically specified to be untrusted data.

Client implementation considerations

There is no guarantee that other clients will support or display the client-only tags they receive, so these tags should only be used to enhance messages with non-critical information. Messages should still make sense to clients with no support for tags.

Moderation considerations

This section is non-normative.

Moderation tools available to channel and network operators typically operate on message text and sender information only. To avoid abusive content being sent in client-only tags, servers, services and management bot implementations may wish to enable moderation features that act on tag content as well.


This section is non-normative. The tags used in these examples may or may not have a specified meaning elsewhere.

A message sent by a client with the example-tag tag:

C: @example-tag=example-value PRIVMSG #channel :Message

A message sent by a client with the +example-client-tag client-only tag:

C: @+example-client-tag=example-value PRIVMSG #channel :Message

In this example

The server sends these messages:

S: :nick! PRIVMSG #channel :
S: @+icon= :url_bot! PRIVMSG #channel A News Story

An example of a vendor-prefixed client-only tag:

C: NOTICE #channel :A vendor-prefixed client-only tagged message

A client-only tag +example with a value containing valid raw and escaped characters:

In this example, plus signs, colons, equals signs and commas are transmitted raw in tag values; while semicolons, spaces and backslashes are escaped. Escaping rules are unchanged from IRCv3.2 tags.

C: @+example=raw+:=,escaped\:\s\\ NOTICE #channel :Message

A TAGMSG sent by a client with the +example-client-tag client-only tag:

C: @+example-client-tag=example-value TAGMSG #channel

A TAGMSG sent by a client to channel ops with a client-only tag and STATUSMSG prefix:

C: @+example-client-tag=example-value TAGMSG @#channel

A TAGMSG sent to a channel with labeled-response and client-only tags. The message is returned by the server with a msgid tag via labeled echo-message to the originating client and as normal to other clients in the channel.

C1-S: @label=123;+example-client-tag=example-value TAGMSG #channel
S-C1: @label=123;msgid=abc;+example-client-tag=example-value :nick! TAGMSG #channel
S-C2: @msgid=abc;+example-client-tag=example-value :nick! TAGMSG #channel

A TAGMSG sent by a client without any tags and rejected by the server with an ERR_NEEDMOREPARAMS (461) error numeric.

C: TAGMSG #channel
S: 461 nick TAGMSG :Not enough parameters

A TAGMSG sent by a client with tags that exceed the size limit and rejected by the server with an ERR_INPUTTOOLONG (417) error numeric. [...] is used to represent tags omitted for readability.

C: @+tag1;+tag2;+tag[...];+tag5000 TAGMSG #channel
S: 417 nick :Input line was too long