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proposals/3961-sliding-sync-typing.md

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+# MSC3961: Sliding Sync Extension: Typing Notifications
+
+This MSC is an extension to [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575)
+which includes support for typing notifications in the `/sync` response.
+
+## Proposal
+
+MSC3575 does not include support for typing notifications. This extension will add support for it.
+
+The proposal is to introduce a new extension called `typing`. It processes the
+core extension arguments `enabled`, `rooms` and `lists`, but defines no 
+arguments of its own.
+```json5
+{
+    "enabled": true, // sticky
+    "lists": ["rooms", "dms"], // sticky
+    "rooms": ["!abcd:example.com"] // sticky
+}
+```
+If `enabled` is `true`, then the sliding sync response MAY include the following response fields in
+the `typing` extension response:
+```json5
+{
+    "rooms": {
+        "!foo:bar": {
+            // m.typing EDU
+        },
+        "!foo2:bar": {
+            // m.typing EDU
+        }
+    }
+}
+```
+
+If a `lists` or `rooms` argument is given to the extension, the `typing` extension
+response SHOULD only include rooms belonging at least one of the sliding windows
+in `lists`, or rooms whose IDs are in `rooms`. See also MSC3575's "Extensions"
+section.
+
+The `m.typing` event in this response is the same event that would appear in the array
+`rooms.join["!foo:bar"].ephemeral.events` under a traditional "v2" `/sync`.
+
+On an initial sync, typing notifications MUST only be sent for rooms returned in the sliding sync response.
+When live streaming, typing notifications MUST be sent as the server receives typing updates. Rooms which
+initially appear (`initial: true`) due to direct subscriptions or rooms moving into the sliding window MUST
+have typing notification data added _if_ there are typing users, else it can be omitted from the typing response.
+
+## Potential issues
+
+No issues identified.
+
+## Alternatives
+
+This extension could be bundled into the core MSC3575, but this would force all clients to receive this
+data. In practice clients can function extremely well without the need for typing notifications, so forcing all
+clients to receive this data feels like the wrong design.
+
+## Security considerations
+
+No additional security considerations beyond what the current `/sync` implementation provides.
+
+## Unstable prefix
+
+No unstable prefix as Sliding Sync is still in review. To enable this extension, just add this to
+your request JSON:
+```json
+{
+    "extensions": {
+        "typing": {
+            "enabled": true
+        }
+    }
+}
+```
+
+## Dependencies
+
+This MSC builds on MSC3575, which at the time of writing has not yet been accepted into the spec.