IRCv3.1 Client Capability Negotiation

Copyright © 2004-2012 Kevin L. Mitchell <>

Copyright © 2004-2012 Perry Lorier <>

Copyright © 2004-2012 Lee Hardy <>

Copyright © 2009-2012 William Pitcock <>

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

Updated by: Capability Negotiation 3.2, cap-notify Extension

What IRCv3 Client Capability Negotiation attempts to solve

IRC is an asynchronous protocol, which means that IRC clients may issue additional IRC commands while a command is being processed. Additionally, there is no guarantee of a specific kind of banner being issued upon connection. Some servers also do not complain about unknown commands during registration, which means that a client cannot reliably do passive implementation discovery at registration time.

If a client had to wait for a banner message, it would be incompatible with previous versions of the IRC client protocol.

The solution to these problems is to extend the registration process with actual capability negotiation. If the server supports capability negotiation, the registration process will be suspended until negotiation is completed. If the server does not support capability negotiation, then registration will complete immediately, and the client will not use any IRCv3 capabilities.

The CAP command

The client capability negotiation extension is implemented by the addition of one command with several subcommands. The command added is named CAP. CAP takes a single required subcommand, optionally followed by a single parameter of space-separated capability identifiers. Each capability in the list may be preceded by a capability modifier.

The subcommands for CAP are: LS, LIST, REQ, ACK, NAK, and END.

The LS, LIST, REQ, ACK and NAK subcommands may be followed by a single parameter containing a space-separated list of capabilities. If more than one capability is named, the RFC1459 designated sentinel (:) for a multi-parameter argument must be present. The list of capabilities MUST be parsed from left to right and capabilities SHOULD only be sent once per command. If a capability is sent multiple times, the last one received takes priority.

If a client sends a subcommand which is not in the list above or otherwise issues an invalid command, then numeric 410 (ERR_INVALIDCAPCMD) should be sent. The first parameter after the client identifier (usually nickname) should be the commandname; the second parameter should be a human-readable description of the error.

Replies from the server must contain the client identifier name or asterisk if one is not yet available.


Client: CAP FOO
Server: 410 * FOO :Invalid CAP command

The client MUST be able to use the CAP command anytime, even after registration.

The CAP LS subcommand

The LS subcommand is used to list the capabilities supported by the server. The client should send an LS subcommand with no other arguments to solicit a list of all capabilities.

If a client issues an LS subcommand, registration must be suspended until an END subcommand is received. If no capabilities are available, an empty parameter must be sent.


Client: CAP LS
Server: CAP * LS :multi-prefix sasl

The CAP LIST subcommand

The LIST subcommand is used to list the capabilities associated with the active connection. The client should send a LIST subcommand with no other arguments to solicit a list of active capabilities.

If no capabilities are active, an empty parameter must be sent.


Client: CAP LIST
Server: CAP * LIST :multi-prefix

The CAP REQ subcommand

The REQ subcommand is used to request a change in capabilities associated with the active connection. Its sole parameter must be a list of space-separated capability identifiers. Each capability identifier may be prefixed with a dash (-) to designate that the capability should be disabled.

If a client requests a capability which is already enabled, or tries to disable a capability which is not enabled, the server MUST continue processing the REQ subcommand as though handling this capability was successful.

The capability identifier set must be accepted as a whole, or rejected entirely.

If a client issues a REQ subcommand, registration must be suspended until an END subcommand is received.


Client: CAP REQ :multi-prefix sasl
Server: CAP * ACK :multi-prefix sasl

The CAP ACK subcommand

The ACK subcommand has two uses:

If an ACK reply originating from the server is spread across multiple lines, a client MUST NOT change capabilities until the last ACK of the set is received. Equally, a server MUST NOT change the capabilities of the client until the last ACK of the set has been sent.

In the first usage, acknowledging a REQ subcommand, the ACK subcommand has a single parameter consisting of a space separated list of capability names, which may optionally be preceded with one or more modifiers.

The second usage is when, in the preceding two cases, some capability names have been preceded with the ack modifier. ACK in this case is used to fully enable or disable the capability. Clients MUST NOT issue an ACK subcommand for any capability not marked with the ack modifier in a server-generated ACK subcommand.

The CAP NAK subcommand

The NAK subcommand designates that the requested capability change was rejected. The server MUST NOT make any change to any capabilities if it replies with a NAK subcommand.

The argument of the NAK subcommand MUST consist of at least the first 100 characters of the capability list in the REQ subcommand which triggered the NAK.

The CAP END subcommand

The END subcommand signals to the server that capability negotiation is complete and requests that the server continue with client registration. If the client is already registered, this command MUST be ignored by the server.

Clients that support capabilities but do not wish to enter negotiation SHOULD send CAP END upon connection to the server.

Capability Negotiation Procedure

Clients should take one of the following actions upon connection:

While a client is permitted to not issue any CAP commands upon connection, this may have unintentional side effects (such as forcing a downgrade to RFC1459 client protocol).

Once capability negotiation is completed with the END subcommand, registration should continue as normal.

Rules for naming capabilities

The full capability name MUST be treated as an opaque identifier.

There are two capability namespaces:


Names which contain a slash character (/) designate a vendor-specific capability namespace.

These names are prefixed by a valid DNS domain name.

For example:

In cases where the prefix contains non-ASCII characters, punycode MUST be used, e.g.

Vendor-Specific capabilities should be submitted to the IRCv3 working group for consideration.


Names for which a corresponding document sits in the IRCv3 Extension Registry.

Names in the IRCv3 Extension Registry are reserved for your capability.

The IRCv3 Working Group reserves the right to reuse names which have not been submitted to the registry. If you do not wish to submit your capability then you MUST use a vendor-specific name (see above).

Capability Modifiers

There are three capability modifiers specified by this standard. If a capability modifier is to be used, it MUST directly proceed the capability identifier.

The capability modifiers are:


Previous versions of this specification referred to a CAP CLEAR command, which has been removed because it is not useful. We do not recommend implementing or supporting CAP CLEAR. See issue #134 for more information, including rationale for this clarification.

Previous versions of this spec did not include specific references to re-enabling or re-disabling a capability in the CAP REQ subcommand section. This was clarified in later versions of the specification.

Previous versions of this spec did not specify how to handle CAP LS when a server did not support any capabilities. This was clarified to match CAP LIST, requiring a reply with an empty parameter.

Previous versions of this spec did not specify that the full capability name MUST be treated as an opaque identifier. This was added to better suit real-world usage and to improve client resiliency.