MSC4461: Storing per-message profiles for users

proposals/4461-per-message-profile-storage.md

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+# MSC4461: Storing per-message profiles for users
+[MSC4144] introduces per-message profiles, which allow users and bots to
+override their profile info for a single message. The original use case was
+primarily bots, but the feature is useful for humans too. However, humans don't
+like re-entering their profile info every time they send a message. Therefore,
+a method for storing reusable profiles is needed.
+
+[MSC4144]: https://github.com/matrix-org/matrix-spec-proposals/pull/4144
+
+## Proposal
+A new `m.per_message_profiles` account data event is introduced. The event
+content is a map from a user-defined profile "shortcode" to the per-message
+profile data that gets sent in events. All fields inside the profile object
+are defined by [MSC4144].
+
+The shortcode is an arbitrary user-defined string, which can be used as a key
+when selecting a profile to use. Arbitrary unicode is allowed, but spaces
+SHOULD NOT be used, as clients that use a command-based UI might split
+parameters on spaces.
+
+```json
+{
+  "type": "m.per_message_profiles",
+  "content": {
+    "meow": {
+      "id": "cat",
+      "displayname": "Cat 🐈️"
+    },
+    "mrrp": {
+      "id": "black_cat",
+      "displayname": "🐈‍⬛",
+      "avatar_url": "mxc://maunium.net/hgXsKqlmRfpKvCZdUoWDkFQo"
+    }
+  }
+}
+```
+
+A client could then have a `/profile` command to pick a profile when sending
+a message, such that `/profile mrrp hello` turns into
+
+```
+{
+  "type": "m.room.message",
+  "content": {
+    "msgtype": "m.text",
+    "body": "🐈‍⬛: hello",
+    "format": "org.matrix.custom.html",
+    "formatted_body": "<p><strong data-mx-profile-fallback>🐈‍⬛: </strong>hello</p>",
+    "m.per_message_profile": {
+      "id": "black_cat",
+      "displayname": "🐈‍⬛",
+      "avatar_url": "mxc://maunium.net/hgXsKqlmRfpKvCZdUoWDkFQo",
+      "has_fallback": true
+    }
+  }
+}
+```
+
+## Potential issues
+Users with lots of profiles can end up with a large account data event, but
+it's unlikely to be larger than existing big account data like push rules or
+`m.direct`.
+
+Since it's a single account data event, editing can hit race conditions. It's
+likely not a problem, as it's only edited directly by humans (as opposed to
+something like `m.direct`, which clients may update in the background).
+
+## Alternatives
+There are various other places where profiles could be stored, like
+[rooms](https://github.com/matrix-org/matrix-spec-proposals/pull/4201)
+or multiple account data events. This proposal uses a single account
+data event for simplicity.
+
+## Security considerations
+This proposal only adds a way to store reusable profile data.
+Security considerations with per-message profiles are covered in [MSC4144]
+
+## Unstable prefix
+`fi.mau.msc4461.per_message_profiles` can be used instead of
+`m.per_message_profiles` in global account data.
+
+## Dependencies
+This MSC builds on [MSC4144], which at the time of writing has not yet been
+accepted into the spec.