Add @+channel-context

Fixes: ircv3#497

client-tags/channel-context.md

@@ -0,0 +1,60 @@
+---
+title: "`channel-context` client tag"
+layout: spec
+work-in-progress: true
+copyrights:
+  -
+    name: "delthas"
+    period: 2022
+    email: "delthas@dille.cc"
+---
+
+## 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 `+channel-context` tag name. Instead, implementations SHOULD use the
+`+draft/channel-context` tag name to be interoperable with other software
+implementing a compatible work-in-progress version.
+
+The final version of the specification will use an unprefixed tag name.
+
+## Introduction
+
+This specification defines a client-only message tag to indicate the channel a private PRIVMSG or NOTICE should be displayed in.
+
+## Motivation
+
+Responding to PRIVMSG messages in channels with NOTICE messages in private is a widely-used IRC practice for bots, in order to prevent flooding a channel with messages only useful for a single user. Clients have developed heuristics for matching incoming private NOTICE to their channel context, but these heuristics are error-prone and difficult to implement for past messages with CHATHISTORY.
+
+This tag enables clients to systematically know in which channel a private PRIVMSG or NOTICE should be displayed, because the sender can now communicate that information in the message.
+
+## Architecture
+
+### Dependencies
+
+Clients wishing to use this tag MUST negotiate the [`message-tags`](../extensions/message-tags.html) capability with the server.
+
+### Format
+
+The channel-context tag is sent by a client with the client-only prefix `+`. The value MUST be a valid channel name.
+
+    +draft/channel-context=<channel>
+
+This tag MUST be attached to PRIVMSG and NOTICE messages to users only. Clients MUST ignore this tag if it is not attached to a private PRIVMSG or NOTICE message or if its value is not a valid channel name.
+
+## Client implementation considerations
+
+This section is non-normative.
+
+When clients receive a private PRIVMSG or NOTICE message from a user with this tag, they can display that message in the buffer specified in the tag, as if it was a channel message.
+
+If possible, the client can check whether both the sender and itself were present in the specified channel at the time of the message, before adding the message to its buffer, in order to prevent the sender from showing messages in buffers they have not joined.
+
+## Examples
+
+In this example, user `me` requests documentation from bots in channel `#chan`, then gets the response in private, but in the context of that channel, using `channel-context`.
+
+    C: PRIVMSG #chan :!help
+    S: @+draft/channel-context=#chan :bot!bot@bot NOTICE me :I'm helping.