diff --git a/docs/API.md b/docs/API.md
index df93bff0..e54aa054 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -881,10 +881,8 @@ The following values are currently defined for the `head` field:
  * `attachments`: an array of paths indicating media attached to this message `["/v0/file/s/sJOD_tZDPz0.jpg"]`.
  * `auto`: `true` when the message was sent automatically, i.e. by a chatbot or an auto-responder.
  * `forwarded`: an indicator that the message is a forwarded message, a unique ID of the original message, `"grp1XUtEhjv6HND:123"`.
- * `hashtags`: an array of hashtags in the message without the leading `#` symbol: `["onehash", "twohash"]`.
  * `mentions`: an array of user IDs mentioned (`@alice`) in the message: `["usr1XUtEhjv6HND", "usr2il9suCbuko"]`.
  * `mime`: MIME-type of the message content, `"text/x-drafty"`; a `null` or a missing value is interpreted as `"text/plain"`.
- * `priority`: message display priority: hint for the client that the message should be displayed more prominently for a set period of time; only `"high"` is currently defined; `{"level": "high", "expires": "2019-10-06T18:07:30.038Z"}`; `priority` can be set by the topic owner or administrator (`A` permission) only. The `"expires"` qualifier is optional.
  * `replace`: an indicator that the message is a correction/replacement for another message, a topic-unique ID of the message being updated/replaced, `":123"`
  * `reply`: an indicator that the message is a reply to another message, a unique ID of the original message, `"grp1XUtEhjv6HND:123"`.
  * `sender`: a user ID of the sender added by the server when the message is sent on behalf of another user, `"usr1XUtEhjv6HND"`.
diff --git a/server/calls.go b/server/calls.go
index 5c7010fc..63876ec6 100644
--- a/server/calls.go
+++ b/server/calls.go
@@ -171,13 +171,20 @@ func initVideoCalls(jsconfig json.RawMessage) error {
 	return nil
 }
 
-func (call *videoCall) messageHead(newState string, duration int) map[string]interface{} {
-	head := map[string]interface{}{
-		"replace": ":" + strconv.Itoa(call.seq),
-		"webrtc":  newState,
+// Add webRTC-related headers to message Head. The original Head may already contain some entries,
+// like 'sender', preserve them.
+func (call *videoCall) messageHead(head map[string]interface{}, newState string, duration int) map[string]interface{} {
+	if head == nil {
+		head = map[string]interface{}{}
 	}
+
+	head["replace"] = ":" + strconv.Itoa(call.seq)
+	head["webrtc"] = newState
+
 	if duration > 0 {
 		head["webrtc-duration"] = duration
+	} else {
+		delete(head, "webrtc-duration")
 	}
 	if call.contentMime != nil {
 		head["mime"] = call.contentMime
@@ -281,10 +288,10 @@ func (t *Topic) handleCallEvent(msg *ClientComMessage) {
 		if call.Event == constCallEventAccept {
 			// The call has been accepted.
 			// Send a replacement {data} message to the topic.
-			replaceWith := constCallMsgAccepted
-			head := t.currentCall.messageHead(replaceWith, 0)
 			msgCopy := *msg
 			msgCopy.AsUser = originatorUid.UserId()
+			replaceWith := constCallMsgAccepted
+			head := t.currentCall.messageHead(msgCopy.Pub.Head, replaceWith, 0)
 			if err := t.saveAndBroadcastMessage(&msgCopy, originatorUid, false, nil,
 				head, t.currentCall.content); err != nil {
 				return
@@ -397,9 +404,9 @@ func (t *Topic) maybeEndCallInProgress(from string, msg *ClientComMessage, callD
 	}
 
 	// Send a message indicating the call has ended.
-	head := t.currentCall.messageHead(replaceWith, int(callDuration))
 	msgCopy := *msg
 	msgCopy.AsUser = originatorUid.UserId()
+	head := t.currentCall.messageHead(msgCopy.Pub.Head, replaceWith, int(callDuration))
 	if err := t.saveAndBroadcastMessage(&msgCopy, originatorUid, false, nil, head, t.currentCall.content); err != nil {
 		logs.Err.Printf("topic[%s]: failed to write finalizing message for call seq id %d - '%s'", t.name, t.currentCall.seq, err)
 	}
diff --git a/server/topic.go b/server/topic.go
index b2741f4d..9ca33688 100644
--- a/server/topic.go
+++ b/server/topic.go
@@ -964,6 +964,17 @@ func (t *Topic) saveAndBroadcastMessage(msg *ClientComMessage, asUid types.Uid,
 		}
 	}
 
+	if msg.sess != nil && msg.sess.uid != asUid {
+		// The "sender" header contains ID of the user who sent the message on behalf of asUid.
+		if head == nil {
+			head = map[string]interface{}{}
+		}
+		head["sender"] = msg.sess.uid.UserId()
+	} else if head != nil {
+		// Make sure the received Head does not include a fake "sender" header.
+		delete(head, "sender")
+	}
+
 	markedReadBySender := false
 	if err, unreadUpdated := store.Messages.Save(
 		&types.Message{