Spec for MSC4356: Recently used emoji (#2291)

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

changelogs/client_server/newsfragments/2291.feature

@@ -0,0 +1 @@
+Add `m.recent_emoji` account data event to track recently used emoji as per [MSC4356](https://github.com/matrix-org/matrix-spec-proposals/pull/4356).

content/client-server-api/_index.md

@@ -3898,6 +3898,7 @@ that profile.
 | [Guest Access](#guest-access)                              | Optional  | Optional | Optional | Optional | Optional |
 | [Moderation Policy Lists](#moderation-policy-lists)        | Optional  | Optional | Optional | Optional | Optional |
 | [OpenID](#openid)                                          | Optional  | Optional | Optional | Optional | Optional |
+| [Recently used emoji](#recently-used-emoji)                | Optional  | Optional | Optional | Optional | Optional |
 | [Reference Relations](#reference-relations)                | Optional  | Optional | Optional | Optional | Optional |
 | [Reporting Content](#reporting-content)                    | Optional  | Optional | Optional | Optional | Optional |
 | [Rich replies](#rich-replies)                              | Optional  | Optional | Optional | Optional | Optional |
@@ -3999,5 +4000,6 @@ systems.
 {{% cs-module name="Spaces" filename="spaces" %}}
 {{% cs-module name="Event replacements" filename="event_replacements" %}}
 {{% cs-module name="Event annotations and reactions" filename="event_annotations" %}}
+{{% cs-module name="Recently used emoji" filename="recent_emoji" %}}
 {{% cs-module name="Threading" filename="threading" %}}
 {{% cs-module name="Reference relations" filename="reference_relations" %}}

content/client-server-api/modules/recent_emoji.md

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+### Recently used emoji
+
+{{% added-in v="1.18" %}}
+
+This module enables clients to track a user's cumulated emoji usage across different
+devices. The data is stored in the [`m.recent_emoji`](#mrecent_emoji)
+global [account data](#client-config) and can, among other things, be used to
+generate recommendations in emoji pickers.
+
+#### Events
+
+{{% event event="m.recent_emoji" %}}
+
+#### Client behaviour
+
+What exactly constitutes trackable emoji usage is left as an implementation detail
+for clients. It is RECOMMENDED to include sending emoji in both messages and
+annotations.
+
+When an emoji is used, the sending client moves (or adds) it to the beginning of
+the `recent_emoji` array and increments (or initializes) its counter. This keeps
+the array ordered by last usage time which facilitates evaluating the data. How
+exactly the client evaluates and uses the collected data is deliberately left
+unspecified.
+
+To prevent excessive growth of the event as new emoji are being used, clients
+SHOULD limit the length of the `recent_emoji` array by dropping elements from
+its end. A RECOMMENDED maximum length is 100 emoji.
+
+To enable future extension, clients MUST tolerate and preserve array elements
+within `recent_emoji` regardless of whether they understand or support the
+contained `emoji` value. This means ignoring entries with unrecognised values
+of `emoji` when deciding what to display to the user while retaining them when
+modifying the array (unless the modification is for truncation).
+
+To prevent undefined behavior, clients SHOULD remove array elements that
+don't conform to the event schema such as elements with negative counters.
+
+
+

data/api/client-server/definitions/recent_emoji.yaml

@@ -0,0 +1,29 @@
+# Copyright 2026 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+title: Recent Emoji
+type: object
+properties:
+  emoji:
+    type: string
+    description: The Unicode emoji as string.
+    example: 🚀
+  total:
+    type: number
+    description: |-
+      The number of times the emoji has been used.
+      MUST be non-negative and smaller than 2^53.
+required:
+  - emoji
+  - total

data/event-schemas/examples/m.recent_emoji.yaml

@@ -0,0 +1,16 @@
+{
+  "$ref": "core/event.json",
+  "type": "m.recent_emoji",
+  "content": {
+    "recent_emoji": [{
+      "emoji": "🤔",
+      "total": 19
+    }, {
+      "emoji": "👍",
+      "total": 7
+    }, {
+      "emoji": "😅",
+      "total": 84
+    }]
+  }
+}

data/event-schemas/schema/m.recent_emoji.yaml

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+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "object",
+  "title": "Recent Emoji Event",
+  "description": "Lets clients maintain a list of recently used emoji.",
+  "allOf": [{
+    "$ref": "core-event-schema/event.yaml"
+  }],
+  "properties": {
+    "type": {
+      "type": "string",
+      "enum": ["m.recent_emoji"]
+    },
+    "content": {
+      "type": "object",
+      "properties": {
+        "recent_emoji": {
+          "description": "The list of recently used emoji. Elements in the list are ordered descendingly by last usage time.",
+          "type": "array",
+          "items": {
+            "$ref": "../../api/client-server/definitions/recent_emoji.yaml"
+          },
+        }
+      },
+      "required": ["recent_emoji"]
+    }
+  },
+  "required": ["type", "content"]
+}