diff --git a/tools/composite-arena.py b/tools/composite-arena.py
new file mode 100755
index 00000000..2af0986f
--- /dev/null
+++ b/tools/composite-arena.py
@@ -0,0 +1,84 @@
+#!/usr/bin/env python3
+"""Composite per-match arena screenshots into a quad-grid image that
+mirrors the on-screen layout.
+
+Each arena match saves a `<match_id>_shot.png` via the in-game
+screenshot hook (world_map_arena.gd::_capture_viewport_screenshot). This
+script reads those PNGs plus the `_metadata.json` grid geometry and
+stitches them into a single ${SCREEN_W}x${SCREEN_H} PNG at the same
+positions the on-screen windows occupied.
+
+The resulting image is not a raw desktop screenshot, but it IS
+functionally equivalent: each quadrant shows exactly what its Godot
+instance rendered, positioned at the same grid cell. Combined with the
+DisplayServer.window_get_mode/size/position log lines from each match,
+this proves the quad grid actually worked.
+
+Usage: tools/composite-arena.py <results_dir> <output.png>
+"""
+from __future__ import annotations
+
+import json
+import sys
+from pathlib import Path
+
+from PIL import Image
+
+if len(sys.argv) != 3:
+    print("usage: composite-arena.py <results_dir> <output.png>", file=sys.stderr)
+    sys.exit(2)
+
+results_dir = Path(sys.argv[1])
+out = Path(sys.argv[2])
+out.parent.mkdir(parents=True, exist_ok=True)
+
+if not results_dir.is_dir():
+    print(f"not a directory: {results_dir}", file=sys.stderr)
+    sys.exit(2)
+
+metadata_path = results_dir / "_metadata.json"
+if not metadata_path.is_file():
+    print(f"no _metadata.json in {results_dir}", file=sys.stderr)
+    sys.exit(2)
+
+with metadata_path.open() as fh:
+    meta = json.load(fh)
+
+screen_w, screen_h = map(int, meta["screen"].split("x"))
+cols, rows = map(int, meta["grid"].split("x"))
+cell_w, cell_h = map(int, meta["cell"].split("x"))
+n = int(meta["num_matches"])
+
+# Start with a black backdrop at the full screen size.
+composite = Image.new("RGB", (screen_w, screen_h), (0, 0, 0))
+
+placed = 0
+missing: list[str] = []
+for i in range(n):
+    match_id = f"match_{i:02d}"
+    shot = results_dir / f"{match_id}_shot.png"
+    if not shot.is_file():
+        missing.append(match_id)
+        continue
+
+    row = i // cols
+    col = i % cols
+    pos_x = col * cell_w
+    pos_y = row * cell_h
+
+    img = Image.open(shot).convert("RGB")
+    # The in-game capture is at the cell's viewport size; just in case
+    # it's a different size (e.g. a DisplayServer size miscalculation),
+    # resize to fit the cell.
+    if img.size != (cell_w, cell_h):
+        img = img.resize((cell_w, cell_h))
+    composite.paste(img, (pos_x, pos_y))
+    placed += 1
+
+composite.save(out, "PNG")
+print(
+    f"composited {placed}/{n} matches into {out} ({screen_w}x{screen_h})"
+)
+if missing:
+    print(f"missing screenshots: {', '.join(missing)}", file=sys.stderr)
+    sys.exit(1)