diff --git a/examples/folio/justfile b/examples/folio/justfile
index fcfc615..82d0502 100644
--- a/examples/folio/justfile
+++ b/examples/folio/justfile
@@ -79,6 +79,51 @@ _ensure-device:
     echo "emulator did not finish booting in time (see /tmp/folio-emulator.log)" >&2
     exit 1
 
+# Print the serial every device-affecting recipe must act on, or refuse. A run
+# installs the app, clears its state and fuzzes it, so the target is never
+# inferred from "whatever adb resolved to": the one case it picks on its own is
+# a single emulator on the local adb server, which is cheap to rebuild.
+_require-device:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    adb="$(just _android-home)/platform-tools/adb"
+    listing="$("$adb" devices)"
+    online="$(echo "$listing" | awk 'NR>1 && $2=="device"{print $1}')"
+    count="$(printf '%s' "$online" | grep -c . || true)"
+
+    refuse() {
+      cat >&2 <<EOF
+    folio: $1
+
+    $listing
+
+    Set ANDROID_DEVICE to the serial to use, and ADB_SERVER_SOCKET when that adb
+    server runs on another host. Either can live in examples/folio/.env:
+
+      ANDROID_DEVICE=emulator-5554 just test
+      ADB_SERVER_SOCKET=tcp:10.0.0.5:5037 ANDROID_DEVICE=emulator-5554 just test
+    EOF
+      exit 1
+    }
+
+    if [[ -n "{{android_device}}" ]]; then
+      if printf '%s\n' "$online" | grep -qxF "{{android_device}}"; then
+        echo "{{android_device}}"
+        exit 0
+      fi
+      refuse "ANDROID_DEVICE={{android_device}} is not an online device."
+    fi
+    if [[ "$count" -eq 0 ]]; then
+      refuse "no device is online."
+    fi
+    if [[ -z "${ADB_SERVER_SOCKET:-}" && "$count" -eq 1 && "$online" =~ ^emulator-[0-9]+$ ]]; then
+      echo "$online"
+      exit 0
+    fi
+    refuse "refusing to install on and fuzz a device nobody named. A run installs
+    the app, clears its state and drives it, so the only target it picks on its
+    own is a single emulator on the local adb server."
+
 # Run folio's own unit tests. Named test-unit because `test` is the fuzz run.
 test-unit:
     #!/usr/bin/env bash
@@ -101,9 +146,9 @@ install: _ensure-device
     #!/usr/bin/env bash
     set -euo pipefail
     export ANDROID_HOME="$(just _android-home)"
+    serial="$(just _require-device)"
+    export ANDROID_SERIAL="$serial"
     ./gradlew :app:androidApp:assembleDebug
-    # ANDROID_SERIAL routes the install to the chosen device when several attach.
-    [[ -n "{{android_device}}" ]] && export ANDROID_SERIAL="{{android_device}}"
     "$ANDROID_HOME/platform-tools/adb" install -r "{{apk}}"
 
 # Remove the folio APK from the connected device.
@@ -111,7 +156,8 @@ uninstall:
     #!/usr/bin/env bash
     set -euo pipefail
     export ANDROID_HOME="$(just _android-home)"
-    [[ -n "{{android_device}}" ]] && export ANDROID_SERIAL="{{android_device}}"
+    serial="$(just _require-device)"
+    export ANDROID_SERIAL="$serial"
     "$ANDROID_HOME/platform-tools/adb" uninstall app.folio
 
 # Remove gradle + iOS build directories.
@@ -176,17 +222,15 @@ ios-device:
 test: install
     #!/usr/bin/env bash
     set -euo pipefail
-    avd_flag=()
+    serial="$(just _require-device)"
+    device_flag=(--device "$serial")
     if [[ -n "{{avd}}" ]]; then
-      avd_flag=(--avd "{{avd}}")
-    fi
-    if [[ -n "{{android_device}}" ]]; then
-      avd_flag+=(--device "{{android_device}}")
+      device_flag+=(--avd "{{avd}}")
     fi
     "{{sanderling}}" test \
       --spec "{{justfile_directory()}}/sanderling/spec.ts" \
       --bundle-id app.folio \
-      "${avd_flag[@]}" \
+      "${device_flag[@]}" \
       --android-app-path "{{apk}}" \
       --duration "{{duration}}" \
       --seed "{{seed}}" \
@@ -198,18 +242,16 @@ test: install
 test-llm: install
     #!/usr/bin/env bash
     set -euo pipefail
-    avd_flag=()
+    serial="$(just _require-device)"
+    device_flag=(--device "$serial")
     if [[ -n "{{avd}}" ]]; then
-      avd_flag=(--avd "{{avd}}")
-    fi
-    if [[ -n "{{android_device}}" ]]; then
-      avd_flag+=(--device "{{android_device}}")
+      device_flag+=(--avd "{{avd}}")
     fi
     "{{sanderling}}" test \
       --spec "{{justfile_directory()}}/sanderling/spec.ts" \
       --bundle-id app.folio \
       --generator llm \
-      "${avd_flag[@]}" \
+      "${device_flag[@]}" \
       --android-app-path "{{apk}}" \
       --duration "{{duration}}" \
       --seed "{{seed}}" \