diff --git a/internal/app/channels/service_test.go b/internal/app/channels/service_test.go
index 5e5d4b6a..03f75789 100644
--- a/internal/app/channels/service_test.go
+++ b/internal/app/channels/service_test.go
@@ -2854,6 +2854,61 @@ func TestChannelUsernameReservedBlocksNewClaimsButKeepsExisting(t *testing.T) {
 	}
 }
 
+func TestUpdateUsernameForcesPreHistoryVisible(t *testing.T) {
+	ctx := context.Background()
+	const ownerID int64 = 1001
+	service := NewService(memory.NewChannelStore())
+	created, err := service.CreateMegagroupFromCreateChat(ctx, ownerID, domain.CreateChannelRequest{
+		Title:         "Private First",
+		MemberUserIDs: []int64{1002},
+		Date:          10,
+	})
+	if err != nil {
+		t.Fatalf("CreateMegagroupFromCreateChat: %v", err)
+	}
+
+	hidden, err := service.SetPreHistoryHidden(ctx, ownerID, created.Channel.ID, true)
+	if err != nil {
+		t.Fatalf("SetPreHistoryHidden: %v", err)
+	}
+	if !hidden.PreHistoryHidden {
+		t.Fatalf("hidden channel = %+v, want pre-history hidden", hidden)
+	}
+
+	// Assigning a public username must force pre-history back to visible.
+	public, err := service.UpdateUsername(ctx, ownerID, domain.UpdateChannelUsernameRequest{
+		ChannelID: created.Channel.ID,
+		Username:  "private_first_pub",
+	})
+	if err != nil {
+		t.Fatalf("UpdateUsername: %v", err)
+	}
+	if public.PreHistoryHidden {
+		t.Fatalf("public channel = %+v, want pre-history visible after publish", public)
+	}
+
+	// Removing the username leaves the flag alone (still visible).
+	private, err := service.UpdateUsername(ctx, ownerID, domain.UpdateChannelUsernameRequest{
+		ChannelID: created.Channel.ID,
+		Username:  "",
+	})
+	if err != nil {
+		t.Fatalf("UpdateUsername clear: %v", err)
+	}
+	if private.PreHistoryHidden {
+		t.Fatalf("re-privated channel = %+v, want pre-history still visible", private)
+	}
+
+	// ...and the creator can hide it again once private.
+	rehidden, err := service.SetPreHistoryHidden(ctx, ownerID, created.Channel.ID, true)
+	if err != nil {
+		t.Fatalf("SetPreHistoryHidden after re-privating: %v", err)
+	}
+	if !rehidden.PreHistoryHidden {
+		t.Fatalf("re-hidden channel = %+v, want pre-history hidden again", rehidden)
+	}
+}
+
 func TestListStoryPostableChannelsFiltersPostStoryRights(t *testing.T) {
 	ctx := context.Background()
 	service := NewService(memory.NewChannelStore())
diff --git a/internal/store/memory/channel_settings.go b/internal/store/memory/channel_settings.go
index 961f6bb1..15c32288 100644
--- a/internal/store/memory/channel_settings.go
+++ b/internal/store/memory/channel_settings.go
@@ -175,6 +175,13 @@ func (s *ChannelStore) UpdateUsername(ctx context.Context, req domain.UpdateChan
 	}
 	prevUsername := channel.Username
 	channel.Username = username
+	// A public group cannot keep pre-history hidden: assigning a username forces
+	// "chat history for new members" back to visible (matches the official
+	// server). Removing the username leaves the flag untouched.
+	clearedPrehistory := username != "" && channel.PreHistoryHidden
+	if clearedPrehistory {
+		channel.PreHistoryHidden = false
+	}
 	s.channels[req.ChannelID] = channel
 	s.appendChannelAdminLogLocked(domain.ChannelAdminLogEvent{
 		ChannelID:  req.ChannelID,
@@ -184,6 +191,16 @@ func (s *ChannelStore) UpdateUsername(ctx context.Context, req domain.UpdateChan
 		PrevString: prevUsername,
 		NewString:  username,
 	})
+	if clearedPrehistory {
+		s.appendChannelAdminLogLocked(domain.ChannelAdminLogEvent{
+			ChannelID: req.ChannelID,
+			UserID:    req.UserID,
+			Date:      int(time.Now().Unix()),
+			Type:      domain.ChannelAdminLogTogglePreHistoryHidden,
+			PrevBool:  true,
+			NewBool:   false,
+		})
+	}
 	return channel, nil
 }
 
diff --git a/internal/store/postgres/channel_settings.go b/internal/store/postgres/channel_settings.go
index 1cca00ac..098aecc5 100644
--- a/internal/store/postgres/channel_settings.go
+++ b/internal/store/postgres/channel_settings.go
@@ -241,12 +241,29 @@ func (s *ChannelStore) UpdateUsername(ctx context.Context, req domain.UpdateChan
 	if err := replacePeerUsernameTx(ctx, tx, peerUsernameTypeChannel, req.ChannelID, username, usernameLower); err != nil {
 		return domain.Channel{}, err
 	}
-	if _, err := tx.Exec(ctx, `UPDATE channels SET username = NULLIF($2,''), updated_at = now() WHERE id = $1`, req.ChannelID, username); err != nil {
+	// A public group cannot keep pre-history hidden: assigning a username forces
+	// "chat history for new members" back to visible (matches the official
+	// server). Removing the username leaves the flag untouched, so the creator
+	// can hide history again once the group is private.
+	if _, err := tx.Exec(ctx, `UPDATE channels SET username = NULLIF($2,''), pre_history_hidden = (pre_history_hidden AND $2 = ''), updated_at = now() WHERE id = $1`, req.ChannelID, username); err != nil {
 		return domain.Channel{}, fmt.Errorf("update channel username: %w", err)
 	}
 	if err := markUserChannelMemberIndexPublicTx(ctx, tx, req.ChannelID, username != ""); err != nil {
 		return domain.Channel{}, err
 	}
+	if username != "" && channel.PreHistoryHidden {
+		if err := s.insertChannelAdminLogTx(ctx, tx, domain.ChannelAdminLogEvent{
+			ChannelID: req.ChannelID,
+			UserID:    req.UserID,
+			Date:      nowUnix(),
+			Type:      domain.ChannelAdminLogTogglePreHistoryHidden,
+			PrevBool:  true,
+			NewBool:   false,
+		}); err != nil {
+			return domain.Channel{}, err
+		}
+		channel.PreHistoryHidden = false
+	}
 	prevUsername := channel.Username
 	channel.Username = username
 	if err := s.insertChannelAdminLogTx(ctx, tx, domain.ChannelAdminLogEvent{