diff --git a/helm/README.md b/helm/README.md
index 155a289..3b58d7a 100644
--- a/helm/README.md
+++ b/helm/README.md
@@ -11,11 +11,15 @@ alpine + the freshly built static binary.
 
 | Chart | When to use |
 |---|---|
-| **`zddc-server-prod/`** | Production. Pin `zddc.gitRef` to a stable tag (`zddc-server-vX.Y.Z`). Slower probe cadence; image-pull policy `IfNotPresent`. |
-| **`zddc-server-dev/`** | Development / soak. Tracks `main` by default; `helm upgrade` triggers a pod recreate so each rollout pulls the latest commit. Faster probes; debug-level logging (request headers logged — sensitive). |
+| **`zddc-server-prod/`** | Production. Pin `zddc.gitRef` to a stable tag (`zddc-server-vX.Y.Z`). Slower probe cadence; image-pull policy `IfNotPresent`. Mounts the data PVC directly RW at `ZDDC_ROOT`. |
+| **`zddc-server-dev/`** | Development / soak. Tracks `main` by default; `helm upgrade` triggers a pod recreate so each rollout pulls the latest commit. Faster probes; debug-level logging (request headers logged — sensitive). Wraps the data PVC in **OverlayFS** (lower = PVC mounted RO, upper = ephemeral `emptyDir`) so dev-side writes never mutate the underlying store. Use this shape when the dev replica points at the same data as prod. |
 
 The chart values are nearly identical between the two; the differences
-are encoded as defaults in each chart's `values.yaml.example`.
+are encoded as defaults in each chart's `values.yaml.example`. The
+dev chart's overlay-isolation layer is a structural difference, not a
+values-level toggle — see `zddc-server-dev/templates/deployment.yaml`
+for the privileged init container and the `data-readonly` /
+`overlay-scratch` / `data` volume sandwich.
 
 ## Quick start
 
@@ -55,7 +59,9 @@ helm upgrade zddc-server-dev helm/zddc-server-dev/ -f my-dev-values.yaml
   and starts the main container against that binary.
 - Wires the `ZDDC_*` environment-variable contract (root path, addr,
   email header, CORS allowlist, log level, index path).
-- Mounts a caller-supplied PersistentVolumeClaim at `ZDDC_ROOT`.
+- Mounts a caller-supplied PersistentVolumeClaim at `ZDDC_ROOT` (prod
+  chart) or as the OverlayFS lowerdir behind a merged `ZDDC_ROOT`
+  (dev chart).
 - Optionally creates an Ingress (`ingress.enabled: true`).
 
 **Does not:**
diff --git a/helm/zddc-server-dev/Chart.yaml b/helm/zddc-server-dev/Chart.yaml
index c758455..5c0b07d 100644
--- a/helm/zddc-server-dev/Chart.yaml
+++ b/helm/zddc-server-dev/Chart.yaml
@@ -1,11 +1,14 @@
 apiVersion: v2
 name: zddc-server-dev
 description: |
-  Development deployment of zddc-server. Same shape as zddc-server-prod
-  but tracks main HEAD (rebuilt on every pod restart), runs at debug
-  log level, has faster probe cadence for quick feedback, and uses
-  smaller resource limits. Intended for ephemeral / soak / preview
-  environments — not for production traffic.
+  Development deployment of zddc-server. Tracks main HEAD (rebuilt on
+  every pod restart), runs at debug log level, has faster probe
+  cadence for quick feedback, and uses smaller resource limits.
+  Wraps the data PVC in OverlayFS — lower (PVC) mounted RO, upper an
+  ephemeral emptyDir — so dev-side writes never mutate the underlying
+  store, making it safe to point at the same data PVC as prod.
+  Intended for ephemeral / soak / preview environments — not for
+  production traffic.
 type: application
 version: 0.1.0
 appVersion: "main"