diff --git a/cmd-list-panes.c b/cmd-list-panes.c
index 54c8db7d3..5bc996c2f 100644
--- a/cmd-list-panes.c
+++ b/cmd-list-panes.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: cmd-list-panes.c,v 1.40 2026/06/01 14:01:09 nicm Exp $ */
+/* $OpenBSD: cmd-list-panes.c,v 1.41 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -97,6 +97,7 @@ cmd_list_panes_window(struct cmd *self, struct session *s, struct winlink *wl,
     struct cmdq_item *item, int type)
 {
 	struct args		*args = cmd_get_args(self);
+	struct client		*c = cmdq_get_client(item);
 	struct window_pane	*wp, **l;
 	u_int			 i, n;
 	struct format_tree	*ft;
@@ -147,7 +148,7 @@ cmd_list_panes_window(struct cmd *self, struct session *s, struct winlink *wl,
 		wp = l[i];
 		ft = format_create(cmdq_get_client(item), item, FORMAT_NONE, 0);
 		format_add(ft, "line", "%u", n);
-		format_defaults(ft, NULL, s, wl, wp);
+		format_defaults(ft, c, s, wl, wp);
 
 		if (filter != NULL) {
 			expanded = format_expand(ft, filter);
diff --git a/cmd-list-sessions.c b/cmd-list-sessions.c
index 07a622979..30f2178ad 100644
--- a/cmd-list-sessions.c
+++ b/cmd-list-sessions.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: cmd-list-sessions.c,v 1.36 2026/02/27 08:25:12 nicm Exp $ */
+/* $OpenBSD: cmd-list-sessions.c,v 1.37 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -53,6 +53,7 @@ static enum cmd_retval
 cmd_list_sessions_exec(struct cmd *self, struct cmdq_item *item)
 {
 	struct args		 *args = cmd_get_args(self);
+	struct client		 *c = cmdq_get_client(item);
 	struct session		**l;
 	u_int		 	  n, i;
 	struct format_tree	 *ft;
@@ -76,7 +77,7 @@ cmd_list_sessions_exec(struct cmd *self, struct cmdq_item *item)
 	for (i = 0; i < n; i++) {
 		ft = format_create(cmdq_get_client(item), item, FORMAT_NONE, 0);
 		format_add(ft, "line", "%u", i);
-		format_defaults(ft, NULL, l[i], NULL, NULL);
+		format_defaults(ft, c, l[i], NULL, NULL);
 
 		if (filter != NULL) {
 			expanded = format_expand(ft, filter);
diff --git a/cmd-list-windows.c b/cmd-list-windows.c
index 40f8c2db5..2cbccc1b8 100644
--- a/cmd-list-windows.c
+++ b/cmd-list-windows.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: cmd-list-windows.c,v 1.50 2026/02/27 08:25:12 nicm Exp $ */
+/* $OpenBSD: cmd-list-windows.c,v 1.51 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -60,6 +60,7 @@ cmd_list_windows_exec(struct cmd *self, struct cmdq_item *item)
 {
 	struct args		*args = cmd_get_args(self);
 	struct cmd_find_state	*target = cmdq_get_target(item);
+	struct client		*c = cmdq_get_client(item);
 	struct winlink		*wl, **l;
 	struct session		*s;
 	u_int			 i, n;
@@ -94,7 +95,7 @@ cmd_list_windows_exec(struct cmd *self, struct cmdq_item *item)
 		s = wl->session;
 		ft = format_create(cmdq_get_client(item), item, FORMAT_NONE, 0);
 		format_add(ft, "line", "%u", n);
-		format_defaults(ft, NULL, s, wl, NULL);
+		format_defaults(ft, c, s, wl, NULL);
 
 		if (filter != NULL) {
 			expanded = format_expand(ft, filter);
diff --git a/cmd-parse.y b/cmd-parse.y
index 4a6ec66b1..a6541c930 100644
--- a/cmd-parse.y
+++ b/cmd-parse.y
@@ -1,4 +1,4 @@
-/* $OpenBSD: cmd-parse.y,v 1.59 2026/08/31 07:51:56 nicm Exp $ */
+/* $OpenBSD: cmd-parse.y,v 1.60 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2019 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -210,7 +210,7 @@ expanded	: format
 				cmd_find_from_client(&fs, c, 0);
 				fsp = &fs;
 			}
-			ft = format_create(NULL, pi->item, FORMAT_NONE, flags);
+			ft = format_create(c, pi->item, FORMAT_NONE, flags);
 			format_defaults(ft, c, fsp->s, fsp->wl, fsp->wp);
 
 			$$ = format_expand(ft, $1);
diff --git a/cmd-select-layout.c b/cmd-select-layout.c
index 741dc63f4..86effb03c 100644
--- a/cmd-select-layout.c
+++ b/cmd-select-layout.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: cmd-select-layout.c,v 1.43 2026/07/10 13:38:45 nicm Exp $ */
+/* $OpenBSD: cmd-select-layout.c,v 1.44 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -90,7 +90,7 @@ cmd_select_layout_exec(struct cmd *self, struct cmdq_item *item)
 		previous = 1;
 
 	oldlayout = w->old_layout;
-	w->old_layout = layout_dump(w, w->layout_root);
+	w->old_layout = layout_dump(w, w->layout_root, 0);
 
 	if (next || previous) {
 		if (next)
diff --git a/control-notify.c b/control-notify.c
index adfc9dcb0..9aa90662b 100644
--- a/control-notify.c
+++ b/control-notify.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: control-notify.c,v 1.38 2026/08/03 13:38:42 nicm Exp $ */
+/* $OpenBSD: control-notify.c,v 1.39 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2012 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -70,6 +70,7 @@ control_window_layout_changed_cb(__unused const char *name,
 	struct session		*s;
 	struct winlink		*wl;
 	struct window		*w = event_payload_get_window(ep, "window");
+	struct format_tree	*ft;
 	const char		*template;
 	char			*cp;
 
@@ -84,19 +85,25 @@ control_window_layout_changed_cb(__unused const char *name,
 	 * and we don't need to inform the client about the layout change
 	 * because the whole window will go away soon.
 	 */
-	wl = TAILQ_FIRST(&w->winlinks);
-	if (wl == NULL || w->layout_root == NULL)
+	if (TAILQ_FIRST(&w->winlinks) == NULL || w->layout_root == NULL)
 		return;
-	cp = format_single(NULL, template, NULL, NULL, wl, NULL);
 
 	TAILQ_FOREACH(c, &clients, entry) {
 		if (!CONTROL_SHOULD_NOTIFY_CLIENT(c) || c->session == NULL)
 			continue;
 		s = c->session;
-		if (winlink_find_by_window_id(&s->windows, w->id) != NULL)
-			control_notify_write(c, "%s", cp);
+		wl = winlink_find_by_window_id(&s->windows, w->id);
+		if (wl == NULL)
+			continue;
+
+		ft = format_create(c, NULL, FORMAT_NONE, 0);
+		format_defaults(ft, c, s, wl, NULL);
+		cp = format_expand(ft, template);
+		format_free(ft);
+
+		control_notify_write(c, "%s", cp);
+		free(cp);
 	}
-	free(cp);
 }
 
 /* Notify control clients that window pane changed. */
diff --git a/format.c b/format.c
index 78e3f0fb5..f8239fd02 100644
--- a/format.c
+++ b/format.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: format.c,v 1.415 2026/08/31 19:34:09 nicm Exp $ */
+/* $OpenBSD: format.c,v 1.416 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2011 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -855,26 +855,42 @@ format_cb_window_active_clients_list(struct format_tree *ft)
 static void *
 format_cb_window_layout(struct format_tree *ft)
 {
-	struct window	*w = ft->w;
+	struct client		*c = ft->c;
+	struct window		*w = ft->w;
+	struct layout_cell	*lcroot;
+	int			 flags = 0;
 
 	if (w == NULL)
 		return (NULL);
 
 	if (w->saved_layout_root != NULL)
-		return (layout_dump(w, w->saved_layout_root));
-	return (layout_dump(w, w->layout_root));
+		lcroot = w->saved_layout_root;
+	else
+		lcroot = w->layout_root;
+
+	if (c != NULL &&
+	    (c->flags & CLIENT_CONTROL) &&
+	    (~c->flags & CLIENT_CONTROL_NEWLAYOUTS))
+		flags |= LAYOUT_CUSTOM_OLD_FORMAT;
+	return (layout_dump(w, lcroot, flags));
 }
 
 /* Callback for window_visible_layout. */
 static void *
 format_cb_window_visible_layout(struct format_tree *ft)
 {
+	struct client	*c = ft->c;
 	struct window	*w = ft->w;
+	int		 flags = 0;
 
 	if (w == NULL)
 		return (NULL);
 
-	return (layout_dump(w, w->layout_root));
+	if (c != NULL &&
+	    (c->flags & CLIENT_CONTROL) &&
+	    (~c->flags & CLIENT_CONTROL_NEWLAYOUTS))
+		flags |= LAYOUT_CUSTOM_OLD_FORMAT;
+	return (layout_dump(w, w->layout_root, flags));
 }
 
 /* Callback for pane_start_command. */
@@ -5234,6 +5250,7 @@ format_loop_sessions(struct format_expand_state *es, const char *fmt)
 		format_log(es, "session loop: $%u", s->id);
 		if (active != NULL &&
 		    ft->c != NULL &&
+		    ft->c->session != NULL &&
 		    s->id == ft->c->session->id)
 			use = active;
 		else
diff --git a/layout-custom.c b/layout-custom.c
index c58e92028..3fa30347e 100644
--- a/layout-custom.c
+++ b/layout-custom.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: layout-custom.c,v 1.38 2026/07/16 12:36:58 nicm Exp $ */
+/* $OpenBSD: layout-custom.c,v 1.39 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2010 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -58,7 +58,7 @@ layout_checksum(const char *layout)
 
 /* Dump layout as a string. */
 char *
-layout_dump(struct window *w, struct layout_cell *root)
+layout_dump(struct window *w, struct layout_cell *root, __unused int flags)
 {
 	char			 layout[8192], *out;
 	int			 bracket = 0;
diff --git a/server-client.c b/server-client.c
index 27ae9307b..d4bf3cc07 100644
--- a/server-client.c
+++ b/server-client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: server-client.c,v 1.510 2026/09/01 19:50:58 nicm Exp $ */
+/* $OpenBSD: server-client.c,v 1.511 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -3084,6 +3084,8 @@ server_client_control_flags(struct client *c, const char *next)
 		return (CLIENT_CONTROL_NOOUTPUT);
 	if (strcmp(next, "wait-exit") == 0)
 		return (CLIENT_CONTROL_WAITEXIT);
+	if (strcmp(next, "new-layouts") == 0)
+		return (CLIENT_CONTROL_NEWLAYOUTS);
 	return (0);
 }
 
@@ -3150,6 +3152,8 @@ server_client_get_flags(struct client *c)
 		strlcat(s, "no-output,", sizeof s);
 	if (c->flags & CLIENT_CONTROL_WAITEXIT)
 		strlcat(s, "wait-exit,", sizeof s);
+	if (c->flags & CLIENT_CONTROL_NEWLAYOUTS)
+		strlcat(s, "new-layouts,", sizeof s);
 	if (c->flags & CLIENT_CONTROL_PAUSEAFTER) {
 		xsnprintf(tmp, sizeof tmp, "pause-after=%u,",
 		    c->pause_age / 1000);
diff --git a/tmux.1 b/tmux.1
index 71f1afc78..00b93866f 100644
--- a/tmux.1
+++ b/tmux.1
@@ -1,4 +1,4 @@
-.\" $OpenBSD: tmux.1,v 1.1166 2026/09/08 08:33:10 nicm Exp $
+.\" $OpenBSD: tmux.1,v 1.1167 2026/09/08 10:20:08 nicm Exp $
 .\"
 .\" Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
 .\"
@@ -1116,6 +1116,8 @@ The flags are:
 .Bl -tag -width Ds
 .It ignore\-size
 the client does not affect the size of other clients
+.It new\-layouts
+use the new layout string format
 .It no\-detach\-on\-destroy
 do not detach the client when the session it is attached to is destroyed if
 there are any other sessions
diff --git a/tmux.h b/tmux.h
index 3a37fb59f..4ceaceb57 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: tmux.h,v 1.1435 2026/09/08 08:37:56 nicm Exp $ */
+/* $OpenBSD: tmux.h,v 1.1436 2026/09/08 10:20:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -2297,7 +2297,7 @@ struct client {
 #define CLIENT_CONTROL_PAUSEAFTER 0x100000000ULL
 #define CLIENT_CONTROL_WAITEXIT 0x200000000ULL
 #define CLIENT_WINDOWSIZECHANGED 0x400000000ULL
-/* 0x800000000ULL unused */
+#define CLIENT_CONTROL_NEWLAYOUTS 0x800000000ULL
 #define CLIENT_BRACKETPASTING 0x1000000000ULL
 #define CLIENT_ASSUMEPASTING 0x2000000000ULL
 #define CLIENT_WRITE_ACK 0x4000000000ULL
@@ -3914,7 +3914,8 @@ int		 layout_remove_tile(struct window *, struct layout_cell *);
 int		 layout_insert_tile(struct window *, struct layout_cell *);
 
 /* layout-custom.c */
-char		*layout_dump(struct window *, struct layout_cell *);
+#define LAYOUT_CUSTOM_OLD_FORMAT 0x1
+char		*layout_dump(struct window *, struct layout_cell *, int);
 int		 layout_parse(struct window *, const char *, char **);
 
 /* layout-set.c */