diff --git a/client.c b/client.c
index e2ffa546..48109613 100644
--- a/client.c
+++ b/client.c
@@ -222,7 +222,7 @@ client_main(int argc, char **argv, int flags)
 		cmdflags = CMD_STARTSERVER;
 	} else if (argc == 0) {
 		msg = MSG_COMMAND;
-		cmdflags = CMD_STARTSERVER|CMD_CANTNEST;
+		cmdflags = CMD_STARTSERVER;
 	} else {
 		msg = MSG_COMMAND;
 
@@ -240,24 +240,10 @@ client_main(int argc, char **argv, int flags)
 		TAILQ_FOREACH(cmd, &cmdlist->list, qentry) {
 			if (cmd->entry->flags & CMD_STARTSERVER)
 				cmdflags |= CMD_STARTSERVER;
-			if (cmd->entry->flags & CMD_CANTNEST)
-				cmdflags |= CMD_CANTNEST;
 		}
 		cmd_list_free(cmdlist);
 	}
 
-	/*
-	 * Check if this could be a nested session, if the command can't nest:
-	 * if the socket path matches $TMUX, this is probably the same server.
-	 */
-	if (shell_cmd == NULL && environ_path != NULL &&
-	    (cmdflags & CMD_CANTNEST) &&
-	    strcmp(socket_path, environ_path) == 0) {
-		fprintf(stderr, "sessions should be nested with care, "
-		    "unset $TMUX to force\n");
-		return (1);
-	}
-
 	/* Set process title, log and signals now this is the client. */
 	setproctitle("client (%s)", socket_path);
 	logfile("client");
diff --git a/cmd-attach-session.c b/cmd-attach-session.c
index 46d568b4..f889019c 100644
--- a/cmd-attach-session.c
+++ b/cmd-attach-session.c
@@ -36,7 +36,7 @@ const struct cmd_entry cmd_attach_session_entry = {
 	"attach-session", "attach",
 	"c:drt:", 0, 0,
 	"[-dr] [-c working-directory] " CMD_TARGET_SESSION_USAGE,
-	CMD_CANTNEST|CMD_STARTSERVER,
+	CMD_STARTSERVER,
 	cmd_attach_session_exec
 };
 
@@ -81,6 +81,11 @@ cmd_attach_session(struct cmd_q *cmdq, const char *tflag, int dflag, int rflag,
 
 	if (cmdq->client == NULL)
 		return (CMD_RETURN_NORMAL);
+	if (server_client_check_nested(cmdq->client)) {
+		cmdq_error(cmdq, "sessions should be nested with care, "
+		    "unset $TMUX to force");
+		return (CMD_RETURN_ERROR);
+	}
 
 	if (wl != NULL) {
 		if (wp != NULL)
diff --git a/cmd-new-session.c b/cmd-new-session.c
index 9c767489..0fb849e2 100644
--- a/cmd-new-session.c
+++ b/cmd-new-session.c
@@ -41,7 +41,7 @@ const struct cmd_entry cmd_new_session_entry = {
 	"[-AdDP] [-c start-directory] [-F format] [-n window-name] "
 	"[-s session-name] " CMD_TARGET_SESSION_USAGE " [-x width] "
 	"[-y height] [command]",
-	CMD_STARTSERVER|CMD_CANTNEST,
+	CMD_STARTSERVER,
 	cmd_new_session_exec
 };
 
@@ -145,15 +145,20 @@ cmd_new_session_exec(struct cmd *self, struct cmd_q *cmdq)
 	}
 
 	/*
-	 * Save the termios settings, part of which is used for new windows in
-	 * this session.
+	 * If this is a new client, check for nesting and save the termios
+	 * settings (part of which is used for new windows in this session).
 	 *
-	 * This is read again with tcgetattr() rather than using tty.tio as if
-	 * detached, tty_open won't be called. Because of this, it must be done
-	 * before opening the terminal as that calls tcsetattr() to prepare for
-	 * tmux taking over.
+	 * tcgetattr() is used rather than using tty.tio since if the client is
+	 * detached, tty_open won't be called. It must be done before opening
+	 * the terminal as that calls tcsetattr() to prepare for tmux taking
+	 * over.
 	 */
 	if (!detached && !already_attached && c->tty.fd != -1) {
+		if (server_client_check_nested(cmdq->client)) {
+			cmdq_error(cmdq, "sessions should be nested with care, "
+			    "unset $TMUX to force");
+			return (CMD_RETURN_ERROR);
+		}
 		if (tcgetattr(c->tty.fd, &tio) != 0)
 			fatal("tcgetattr failed");
 		tiop = &tio;
diff --git a/server-client.c b/server-client.c
index 36408e91..01c8b313 100644
--- a/server-client.c
+++ b/server-client.c
@@ -46,6 +46,27 @@ void	server_client_msg_command(struct client *, struct imsg *);
 void	server_client_msg_identify(struct client *, struct imsg *);
 void	server_client_msg_shell(struct client *);
 
+/* Check if this client is inside this server. */
+int
+server_client_check_nested(struct client *c)
+{
+	struct environ_entry	*envent;
+	struct window_pane	*wp;
+
+	if (c->tty.path == NULL)
+		return (0);
+
+	envent = environ_find(&c->environ, "TMUX");
+	if (envent == NULL || *envent->value == '\0')
+		return (0);
+
+	RB_FOREACH(wp, window_pane_tree, &all_window_panes) {
+		if (strcmp(wp->tty, c->tty.path) == 0)
+			return (1);
+	}
+	return (0);
+}
+
 /* Set client key table. */
 void
 server_client_key_table(struct client *c, const char *name)
diff --git a/tmux.h b/tmux.h
index ae1682af..8406955b 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1374,8 +1374,7 @@ struct cmd_entry {
 	const char	*usage;
 
 #define CMD_STARTSERVER 0x1
-#define CMD_CANTNEST 0x2
-#define CMD_READONLY 0x4
+#define CMD_READONLY 0x2
 	int		 flags;
 
 	enum cmd_retval	 (*exec)(struct cmd *, struct cmd_q *);
@@ -1868,6 +1867,7 @@ void	 server_update_socket(void);
 void	 server_add_accept(int);
 
 /* server-client.c */
+int	 server_client_check_nested(struct client *);
 void	 server_client_handle_key(struct client *, int);
 void	 server_client_create(int);
 int	 server_client_open(struct client *, char **);