diff --git a/docker/android/assemble-apk.sh b/docker/android/assemble-apk.sh
index 1fc388e..8dca2ef 100755
--- a/docker/android/assemble-apk.sh
+++ b/docker/android/assemble-apk.sh
@@ -139,6 +139,25 @@ fi
     --dir "${REPO}/apps/darkroom-android/android/res" \
     -o "${OUT}/res.zip"
 
+# `android:debuggable`, when asked for, and never otherwise.
+#
+# Without it `adb shell run-as` refuses — "package not debuggable" — and the
+# app's own storage cannot be looked at from the host at all. That storage is
+# where the face shards, the thumbnail store and the catalog live, so when a
+# device disagrees with the desktop about what it has synced, there is no way
+# to find out which of them is wrong.
+#
+# Set through aapt2 rather than in `AndroidManifest.xml` deliberately: the flag
+# then exists only for the build that opted in, and a release build cannot
+# inherit it by someone forgetting to take it back out again. A debuggable APK
+# lets any process on the device read this app's private files, so it is a
+# thing to install on a test tablet and not a thing to publish.
+DEBUG_FLAG=()
+if [ -n "${DARKROOM_DEBUGGABLE:-}" ]; then
+    echo "==> debuggable build (run-as enabled; do not publish)"
+    DEBUG_FLAG=(--debug-mode)
+fi
+
 "${BT}/aapt2" link \
     -I "${ANDROID_JAR}" \
     --manifest "${REPO}/apps/darkroom-android/android/AndroidManifest.xml" \
@@ -147,6 +166,7 @@ fi
     --target-sdk-version "${TARGET_API}" \
     --version-name "${VERSION_NAME}" \
     --version-code "${VERSION_CODE}" \
+    "${DEBUG_FLAG[@]}" \
     -o "${OUT}/base.apk" \
     --auto-add-overlay
 
diff --git a/docker/android/package.sh b/docker/android/package.sh
index d5d14ed..61edcda 100755
--- a/docker/android/package.sh
+++ b/docker/android/package.sh
@@ -71,6 +71,7 @@ SO="${CACHE}/target/jniLibs/${ABI}/libdarkroom.so"
 echo "==> packaging APK"
 "${HERE}/build.sh" env \
     ABI="${ABI}" RUST_TARGET="${RUST_TARGET}" \
+    DARKROOM_DEBUGGABLE="${DARKROOM_DEBUGGABLE:-}" \
     /work/docker/android/assemble-apk.sh
 
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