diff --git a/flake.nix b/flake.nix
index 009faa7..66a9bef 100644
--- a/flake.nix
+++ b/flake.nix
@@ -147,6 +147,29 @@
 						};
 					};
 
+
+					# Create and loop-mount an ext4 image on the 9p data volume
+					# for the overlay upper/work (9p can't serve as overlayfs upper)
+					systemd.services.claude-overlay-img = {
+						description = "Create ext4 image for Claude overlay";
+						wantedBy = [ "var-lib-claude\\x2drw.mount" ];
+						before = [ "var-lib-claude\\x2drw.mount" ];
+						after = [ "var-lib-cc\\x2dsandbox.mount" ];
+						requires = [ "var-lib-cc\\x2dsandbox.mount" ];
+						unitConfig.DefaultDependencies = false;
+						serviceConfig = {
+							Type = "oneshot";
+							RemainAfterExit = true;
+							ExecStart = pkgs.writeShellScript "claude-overlay-img" ''
+								img=/var/lib/cc-sandbox/claude-overlay.img
+								if [ ! -f "$img" ]; then
+									${pkgs.coreutils}/bin/truncate -s 128M "$img"
+									${pkgs.e2fsprogs}/bin/mkfs.ext4 -q "$img"
+								fi
+							'';
+						};
+					};
+
 					fileSystems = {
 						# tmpfs backing for the writable nix store overlay
 						"/nix/.rw-store" = {
@@ -155,11 +178,14 @@
 							neededForBoot = true;
 						};
 
-						# Claude config: ephemeral overlay on top of host's ~/.claude
+						# Loop-mounted ext4 image for overlay upper/work
 						"/var/lib/claude-rw" = {
-							fsType = "tmpfs";
-							options = [ "size=128M" "mode=0700" ];
+							device = "/var/lib/cc-sandbox/claude-overlay.img";
+							fsType = "ext4";
+							options = [ "loop" ];
 						};
+
+						# Claude config: persistent overlay on top of host's ~/.claude
 						"/root/.claude".overlay = {
 							lowerdir = [ "/var/lib/claude-lower" ];
 							upperdir = "/var/lib/claude-rw/upper";