diff --git a/docker-compose.shared.yml b/docker-compose.shared.yml
index 53675ba..7849fea 100644
--- a/docker-compose.shared.yml
+++ b/docker-compose.shared.yml
@@ -12,9 +12,13 @@
 #   5. mkdir -p ~/<signum>/.config/opencode
 #   6. docker compose up -d
 #
-# Named volumes are automatically isolated per user because Docker Compose
-# prefixes them with the project directory name (e.g. opencode-devbox_devbox-data).
-# Since each user runs from ~/<signum>/opencode-devbox/, volumes don't collide.
+# Volume isolation: the top-level 'name:' field uses SIGNUM to derive a
+# unique project name (devbox-<signum>), which Docker Compose uses as the
+# prefix for all named volumes. Without this, two users whose compose file
+# lives in a directory with the same basename would share volumes — the
+# Docker daemon is system-wide and doesn't scope by OS user.
+
+name: devbox-${SIGNUM:?Set SIGNUM in .env}
 
 services:
   devbox: