diff --git a/sure/templates/_helpers.tpl b/sure/templates/_helpers.tpl
index 86387fb..a86ea0a 100644
--- a/sure/templates/_helpers.tpl
+++ b/sure/templates/_helpers.tpl
@@ -336,6 +336,13 @@ probe on "/" reach the database at all (see values.yaml).
   value: "true"
 - name: RAILS_ENV
   value: "production"
+# Not cosmetic and not optional: /tmp is an emptyDir, which Kubernetes creates
+# mode 0777 without a sticky bit, and Ruby skips exactly that shape when it
+# looks for a temporary directory. Without this the app aborts during boot with
+# "could not find a temporary directory". The directory is created by the
+# init container in the StatefulSet, which explains the whole case.
+- name: TMPDIR
+  value: "/tmp/ruby"
 - name: PORT
   value: "3000"
 # Sure's production.rb defaults BOTH of these to true. The chart renders them
diff --git a/sure/templates/sure.sfs.yaml b/sure/templates/sure.sfs.yaml
index 8d668a3..27ba61f 100644
--- a/sure/templates/sure.sfs.yaml
+++ b/sure/templates/sure.sfs.yaml
@@ -34,6 +34,71 @@ spec:
       automountServiceAccountToken: false
       securityContext:
         {{- include "sure.podSecurityContext" .Values.sure.securityContext | nindent 8 }}
+      initContainers:
+        # Ruby refuses a world-writable directory that has no sticky bit as its
+        # temporary directory (ruby 3.4, tmpdir.rb: a candidate is skipped when
+        # "world_writable? && !sticky?"), and a Kubernetes emptyDir is created
+        # mode 0777 WITHOUT the sticky bit. With /tmp mounted as an emptyDir and
+        # the image's root filesystem read-only, Ruby then finds no candidate at
+        # all and both containers die during boot - measured in this chart's
+        # first install-test, which logged
+        #
+        #   system temporary path is world-writable: /tmp
+        #   /tmp is world-writable: /tmp
+        #   . is not writable: /rails
+        #   ArgumentError: could not find a temporary directory
+        #
+        # raised from MiniMagick, which Active Storage loads while Rails boots.
+        # Note where it does NOT come from: nothing in the app asked for a temp
+        # file yet.
+        #
+        # The fix is a directory that is writable without being world-writable.
+        # This init container creates one per app container inside the same
+        # emptyDirs (mode 0700, owned by the pod's uid) and both containers get
+        # TMPDIR pointed at it. It runs as the same non-root uid as the app and
+        # needs no capability at all: the emptyDir root is 0777, so creating an
+        # entry in it is allowed, and chmod on a directory one has just created
+        # needs no privilege. That is deliberately weaker than the chown-as-root
+        # init container paperless-ngx needs.
+        #
+        # The image is the app's own, so there is no second image to pin, no
+        # Renovate block to maintain for it and nothing extra to pull.
+        - name: {{ .Release.Name }}-{{ .Chart.Name }}-init-tmp
+          image: "ghcr.io/we-promise/sure:{{ .Chart.AppVersion }}"
+          imagePullPolicy: IfNotPresent
+          # command, not args: the image entrypoint would start the application.
+          command:
+            - /bin/bash
+            - -ec
+            - |
+              for dir in /web-tmp/ruby /worker-tmp/ruby; do
+                mkdir -p "$dir"
+                chmod 700 "$dir"
+              done
+          securityContext:
+            allowPrivilegeEscalation: false
+            readOnlyRootFilesystem: true
+            runAsNonRoot: true
+            capabilities:
+              drop:
+                - ALL
+          volumeMounts:
+            - name: web-tmp
+              mountPath: /web-tmp
+            - name: worker-tmp
+              mountPath: /worker-tmp
+          # Fixed, deliberately small: this container runs two mkdirs and two
+          # chmods. Not values-configurable, for the same reason as the init
+          # container in paperless-ngx - there is nothing here to tune.
+          resources:
+            requests:
+              cpu: 10m
+              memory: 32Mi
+              ephemeral-storage: 1Mi
+            limits:
+              cpu: 100m
+              memory: 64Mi
+              ephemeral-storage: 10Mi
       containers:
         # The web process. Its command is the image's default
         # (CMD ["./bin/rails", "server"]), which is exactly what makes the
@@ -74,8 +139,11 @@ spec:
             # between the two processes, and a shared scratch directory would
             # only create coupling nobody asked for.
             #
-            # /tmp: Rack writes multipart uploads (CSV imports, attachments) to
-            # Dir.tmpdir before Active Storage ever sees them.
+            # /tmp: Rack writes multipart uploads (CSV imports, attachments)
+            # here before Active Storage ever sees them. The app does not use
+            # this path directly, though - TMPDIR points at the private
+            # subdirectory the init container creates inside it, because Ruby
+            # rejects a world-writable emptyDir as a temporary directory.
             - name: web-tmp
               mountPath: /tmp
             # /rails/tmp: Rails' own scratch directory - the Bootsnap cache,
diff --git a/sure/values.yaml b/sure/values.yaml
index b9d7271..b8eae6b 100644
--- a/sure/values.yaml
+++ b/sure/values.yaml
@@ -36,10 +36,18 @@ sure:
   # need a ReadWriteMany volume. Above 1 the extra pods normally stay Pending on
   # "Multi-Attach error for volume" - see NOTES.txt.
   replicas: 1
-  # Pod-level hardening, shared by both containers. The upstream image declares
-  # USER 1000:1000 (group and user "rails", created in the Dockerfile), so the
-  # chart needs no deviation from the repo hardening standard; fsGroup 1000
-  # makes the storage PVC writable for that uid.
+  # Pod-level hardening, shared by both containers and by the init container.
+  # The upstream image declares USER 1000:1000 (group and user "rails", created
+  # in the Dockerfile), so the chart needs no deviation from the repo hardening
+  # standard; fsGroup 1000 makes the storage PVC writable for that uid.
+  #
+  # readOnlyRootFilesystem: true holds, and the writable paths are emptyDirs:
+  # /rails/tmp, /rails/log and /tmp, one set per container. /tmp needs one extra
+  # step that is easy to mistake for decoration - a tiny init container creates
+  # a private subdirectory in it and the containers run with TMPDIR pointing
+  # there, because Ruby refuses a world-writable directory without a sticky bit
+  # as its temporary directory and an emptyDir is exactly that. Without it the
+  # app aborts during boot; the StatefulSet carries the measured evidence.
   securityContext:
     pod:
       runAsNonRoot: true