diff --git a/tools/grab-screen.py b/tools/grab-screen.py
new file mode 100755
index 00000000..5d15b1c8
--- /dev/null
+++ b/tools/grab-screen.py
@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+"""Grab the full screen to a PNG. Used by the arena verification flow to
+prove the quad grid actually tiles on screen — without this we only have
+DisplayServer's inside-Godot view of its own window state, which is the
+kind of circumstantial evidence that burned us earlier.
+
+Uses ffmpeg's x11grab (via XWayland on Wayland sessions). `mss` fails on
+pure Wayland because its Linux backend is Xlib-only.
+
+Usage: tools/grab-screen.py <output.png> [WIDTH] [HEIGHT]
+"""
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+if len(sys.argv) < 2 or len(sys.argv) > 4:
+    print("usage: grab-screen.py <output.png> [WIDTH] [HEIGHT]", file=sys.stderr)
+    sys.exit(2)
+
+out = Path(sys.argv[1])
+width = int(sys.argv[2]) if len(sys.argv) >= 3 else 3440
+height = int(sys.argv[3]) if len(sys.argv) >= 4 else 1440
+display = os.environ.get("DISPLAY", ":0")
+
+out.parent.mkdir(parents=True, exist_ok=True)
+
+cmd = [
+    "ffmpeg",
+    "-hide_banner",
+    "-loglevel", "error",
+    "-f", "x11grab",
+    "-video_size", f"{width}x{height}",
+    "-i", display,
+    "-frames:v", "1",
+    "-update", "1",
+    "-y",
+    str(out),
+]
+result = subprocess.run(cmd, check=False, capture_output=True, text=True)
+if result.returncode != 0:
+    print(f"ffmpeg failed (exit {result.returncode}):", file=sys.stderr)
+    print(result.stderr, file=sys.stderr)
+    sys.exit(result.returncode)
+
+size = out.stat().st_size
+print(f"saved {out} ({width}x{height}, {size} bytes)")