diff --git a/requests.go b/requests.go
index cc8b7266..e0020679 100644
--- a/requests.go
+++ b/requests.go
@@ -425,6 +425,8 @@ type ReqSetReadMarkers struct {
 	Read        id.EventID `json:"m.read,omitempty"`
 	ReadPrivate id.EventID `json:"m.read.private,omitempty"`
 	FullyRead   id.EventID `json:"m.fully_read,omitempty"`
+	// Allow moving m.fully_read backwards via MSC4446.
+	AllowBackward bool `json:"com.beeper.allow_backward,omitempty"`
 
 	BeeperReadExtra        interface{} `json:"com.beeper.read.extra,omitempty"`
 	BeeperReadPrivateExtra interface{} `json:"com.beeper.read.private.extra,omitempty"`
@@ -444,6 +446,8 @@ type ReqSetBeeperInboxState struct {
 
 type ReqSendReceipt struct {
 	ThreadID string `json:"thread_id,omitempty"`
+	// Allow moving m.fully_read backwards via MSC4446.
+	AllowBackward bool `json:"com.beeper.allow_backward,omitempty"`
 }
 
 type ReqPublicRooms struct {
diff --git a/versions.go b/versions.go
index 7ecf9ca8..fdfafbcd 100644
--- a/versions.go
+++ b/versions.go
@@ -74,6 +74,7 @@ var (
 	FeatureRedactSendAsEvent         = UnstableFeature{UnstableFlag: "com.beeper.msc4169"}
 	FeatureUnstableReplaceProfile    = UnstableFeature{UnstableFlag: "com.beeper.msc4437"}
 	FeatureStableReplaceProfile      = UnstableFeature{UnstableFlag: "com.beeper.msc4437.stable"}
+	FeatureFullyReadBackward         = UnstableFeature{UnstableFlag: "com.beeper.msc4446"}
 
 	BeeperFeatureHungry                = UnstableFeature{UnstableFlag: "com.beeper.hungry"}
 	BeeperFeatureBatchSending          = UnstableFeature{UnstableFlag: "com.beeper.batch_sending"}