IRCv3.2 Message Tags

Copyright (c) 2012 Alexey Sokolov
Copyright (c) 2012 St├ęphan Kochen
Copyright (c) 2012 Kyle Fuller

Additional optional tags are added to the start of each message. The message pseudo-BNF, as defined in RFC 1459, section 2.3.1 is extended to look as follows:

<message> ::= ['@' <tags> <SPACE>] [':' <prefix> <SPACE> ] <command> <params> <crlf>
<tags>    ::= <tag> [';' <tag>]*
<tag>     ::= <key> ['=' <value>]
<key>     ::= [ <vendor> '/' ] <sequence of letters, digits, hyphens (`-`)>
<value>   ::= <sequence of any characters except NUL, BELL, CR, LF, semicolon (`;`) and SPACE>
<vendor>  ::= <host>

Every message tag is enabled by a capability. The same capability can be used for several tags, if these tags are intended to be used together.

Every tag can have own rules when it can be used: from client to server only, from server to client only, or in both directions.

Server MUST NOT add a tag to messages, if client didn't request the capability which enables the tag using CAP REQ. Server MUST NOT add a tag to messages before replying to client's CAP REQ with CAP ACK. If client requested a capability, which enables one or more message tags, client MUST be able to parse the tags syntax.

Client MUST NOT add a tag to messages before server replies to client's CAP REQ with CAP ACK. If server accepted the capability request with CAP ACK, server MUST be able to parse the tags syntax.

Client and server MAY parse the tags even without any capabilities enabled. They SHOULD ignore tags of capabilities which are not enabled.

This ensures back-compatibility with standard IRC protocol, because no capabilities are enabled by default, and therefore messages can't have any tags unless client explicitly enables such capabilities.

Size limit for the message tags MUST be 512 bytes, including @ and the trailing space characters, leaving 510 bytes for tags themselves. So the total size of a message becomes limited by 1024 bytes: 512 bytes for message tags, and 512 bytes for the rest as specified by RFC 1459.

If a message has multiple tags, order of the tags MUST NOT matter. The same tag MUST NOT appear on the same message two or more times.

Rules for naming message tags

There are two tag namespaces:


Message with 0 tags:

:nick! PRIVMSG me :Hello

Message with 3 tags:

home > specification > message-tags-3.2 Valid HTML 4.01 Strict! Valid CSS!