diff --git a/README.md b/README.md
index d041512..c2f1356 100644
--- a/README.md
+++ b/README.md
@@ -20,11 +20,11 @@ This runs the host-side unit tests (`test/`, `native` environment), then builds
 
 ## Flash
 
-1. Connect the Cardputer by USB-C. It shows up as `/dev/ttyACM0`.
+1. Connect the Cardputer by USB-C.
 2. Run:
 
    ```sh
-   scripts/flash.sh            # or: scripts/flash.sh /dev/ttyACM1
+   scripts/flash.sh            # auto-detects the port; or: scripts/flash.sh /dev/ttyACM1
    ```
 
    This uploads the firmware, then opens the serial monitor. Quit the monitor with `Ctrl+C`.
diff --git a/scripts/flash.sh b/scripts/flash.sh
index ca689a2..7a962ef 100755
--- a/scripts/flash.sh
+++ b/scripts/flash.sh
@@ -1,8 +1,9 @@
 #!/usr/bin/env bash
 # Flash the firmware over USB, then open the serial monitor.
-# Usage: scripts/flash.sh [/dev/ttyACM0]
+# Usage: scripts/flash.sh [port]   (default: the first Espressif device found)
 set -euo pipefail
-PORT="${1:-/dev/ttyACM0}"
+PORT="${1:-$(readlink -f /dev/serial/by-id/*Espressif* 2>/dev/null | head -1)}"
+[ -n "$PORT" ] || { echo "No Cardputer found on USB (is it plugged in / attached to the VM?)" >&2; exit 1; }
 source "$(dirname "$0")/_docker.sh"
 DOCKER_EXTRA=(--device "$PORT" --group-add "$(stat -c %g "$PORT")" -it)