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+# MSC3960: Sliding Sync Extension: Receipts
+
+This MSC is an extension to [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575)
+which includes support for room receipts in the `/sync` response.
+
+## Proposal
+
+MSC3575 does not include support for receipts. This extension will add support for both public
+and private room receipts.
+
+The proposal is to introduce a new extension called `receipts`. 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 `receipts` extension response:
+```json5
+{
+    "rooms": {
+        "!foo:bar": {
+            // m.receipt EDU
+        },
+        "!foo2:bar": {
+            // m.receipt 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.receipt` event in this response is the same event that would appear in the array
+`rooms.join["!foo:bar"].ephemeral.events` under `/sync`. This includes the `m.read.private` key in the
+receipt EDU for private read receipts.
+
+Receipts MUST only be sent for rooms returned in the sliding sync response. On initial sync, receipts MUST only be
+returned for events sent in the `timeline` section for each room of the sliding sync response. In addition,
+public and private receipts sent by the client MUST be returned, even if the events in question are not in the `timeline`.
+Delta tokens MUST be ignored when calculating the events in the `timeline`, or else additional read receipts
+for the same event would never be returned to the client. The receipts themeselves MAY be subject to
+delta tokens such that only read receipt deltas are returned to the client.
+
+Overall, these steps prevent the sliding sync response from scaling with the amount of rooms on the client.
+This contrasts with the existing `/sync` mechanism which sends all receipts for all users in all rooms.
+
+
+## Potential issues
+
+By not sending receipts for events not in the returned `timeline`, the client will not have receipt
+information for old events (e.g retrieved via `/messages`). An additional MSC would be required to add
+receipt information to events returned from `/messages`. It is unacceptable to return receipts not in
+the returned `timeline` because this can be extremely large e.g for Matrix HQ this is 50,000+ read receipts
+just for that room, for events that the client will potentially never view!
+
+## 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 read receipts, 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": {
+        "receipts": {
+            "enabled": true
+        }
+    }
+}
+```
+
+## Dependencies
+
+This MSC builds on MSC3575, which at the time of writing has not yet been accepted into the spec.