diff --git a/docker-compose.yml b/docker-compose.yml
index a3930a9..c97fea7 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -112,7 +112,11 @@ services:
       WORKERS: ${WORKERS:-4}
       # One worker wins this lock and runs the subscription notifier / homepage
       # sweep; it lives on the appdata volume so it's shared across workers.
-      THERMOGRAPH_SINGLETON_LOCK: /app/data/notifier.lock
+      THERMOGRAPH_DATA_DIR: /state
+      # notifier.lock lives in the state volume, which is now mounted at /state
+      # (NOT /app/data) so it never shadows the Python `data/` package -- see the
+      # volumes: note below and thermograph-backend paths.py.
+      THERMOGRAPH_SINGLETON_LOCK: /state/notifier.lock
     # Prod secrets live in /etc/thermograph.env: POSTGRES_PASSWORD,
     # THERMOGRAPH_AUTH_SECRET, THERMOGRAPH_VAPID_PRIVATE_KEY/_PUBLIC_KEY,
     # THERMOGRAPH_COOKIE_SECURE=1, mail/Discord keys, ... (see
@@ -123,7 +127,11 @@ services:
         required: false
     volumes:
       # Parquet cache, notifier.lock, homepage.json, vapid.json persist here.
-      - appdata:/app/data
+      # /state, NOT /app/data: after the repo split the backend's Python package
+      # `data/` sits at /app/data, so mounting the runtime volume there erased
+      # data/*.py and broke `import data.climate` at boot. THERMOGRAPH_DATA_DIR=/state
+      # (above) points runtime state here instead, clear of the code.
+      - appdata:/state
       - applogs:/app/logs
     # No compose-level healthcheck override -- the image's own Dockerfile
     # HEALTHCHECK (port-aware via ${PORT}) already covers this, and frontend's