diff --git a/tests/tools/test_green_screen_chromakey.py b/tests/tools/test_green_screen_chromakey.py
index 1a0cb05e..ab4bcf3a 100644
--- a/tests/tools/test_green_screen_chromakey.py
+++ b/tests/tools/test_green_screen_chromakey.py
@@ -50,16 +50,20 @@ def test_chromakey_filter_scales_background_to_frame():
             fd.mkdir()
             pd.mkdir()
             (fd / "frame_0000.png").write_bytes(b"stub")
-            tool._process_chromakey(fd, pd, "#0E172A", 1)
+            tool._process_chromakey(fd, pd, "#0E172A", 1, 320, 240)
     finally:
         GreenScreenProcessor.run_command = orig
 
     ffmpeg_cmds = [c for c in captured if c and c[0] == "ffmpeg"]
     assert ffmpeg_cmds, "no ffmpeg command built"
-    fc_idx = ffmpeg_cmds[0].index("-filter_complex")
-    fc = ffmpeg_cmds[0][fc_idx + 1]
-    assert "scale2ref" in fc, f"background not scaled to frame: {fc}"
+    cmd = ffmpeg_cmds[0]
+    fc = cmd[cmd.index("-filter_complex") + 1]
+    # Background must be sized to the frame, not the old 1x1 no-op.
+    assert "size=320x240" in " ".join(cmd), "background not sized to frame"
+    assert "size=1x1" not in " ".join(cmd), "still using the 1x1 background"
     assert "[0:v]scale=iw:ih[bg]" not in fc, "still using the 1x1 no-op scale"
+    # Alpha must be forced so keyed transparency survives on every FFmpeg build.
+    assert "format=yuva420p" in fc, f"keyed alpha not forced: {fc}"
 
 
 @pytest.mark.skipif(shutil.which("ffmpeg") is None or shutil.which("ffprobe") is None,
@@ -79,7 +83,9 @@ def test_chromakey_preserves_frame_size_and_keys(tmp_path):
         capture_output=True, check=True, timeout=60,
     )
 
-    ok = GreenScreenProcessor()._process_chromakey(frames_dir, processed_dir, "#0E172A", 1)
+    ok = GreenScreenProcessor()._process_chromakey(
+        frames_dir, processed_dir, "#0E172A", 1, 320, 240
+    )
     assert ok
 
     out = processed_dir / "frame_0000.png"
diff --git a/tools/video/green_screen_processor.py b/tools/video/green_screen_processor.py
index ab5e3ce6..fdc9b7bf 100644
--- a/tools/video/green_screen_processor.py
+++ b/tools/video/green_screen_processor.py
@@ -161,7 +161,7 @@ class GreenScreenProcessor(BaseTool):
 
             if method == "chromakey":
                 ok = self._process_chromakey(
-                    frames_dir, processed_dir, bg_color, frame_count
+                    frames_dir, processed_dir, bg_color, frame_count, width, height
                 )
             else:
                 ok = self._process_rembg(
@@ -445,6 +445,8 @@ class GreenScreenProcessor(BaseTool):
         processed_dir: Path,
         bg_color: str,
         frame_count: int,
+        width: int,
+        height: int,
     ) -> bool:
         """Process frames using FFmpeg chromakey filter.
 
@@ -461,20 +463,23 @@ class GreenScreenProcessor(BaseTool):
             out_path = processed_dir / frame.name
             cmd = [
                 "ffmpeg", "-y",
-                "-f", "lavfi", "-i", f"color=c={ffmpeg_bg}:size=1x1",
+                # Background sized to the frame up front. The old code used a 1x1
+                # color source and `[0:v]scale=iw:ih` — a no-op (iw/ih were the
+                # 1x1 source's own size) — and overlay takes the size of its
+                # FIRST input, so every frame was clipped to a single pixel and
+                # the whole video came out a solid color with the subject gone.
+                "-f", "lavfi", "-i", f"color=c={ffmpeg_bg}:size={width}x{height}",
                 "-i", str(frame),
                 "-filter_complex",
                 (
-                    # The background is a 1x1 color source; it MUST be scaled up
-                    # to the frame's dimensions. `scale=iw:ih` on [0:v] alone is a
-                    # no-op (iw/ih are the 1x1 source's own size), and overlay
-                    # takes the size of its FIRST input — so that leaves a 1x1
-                    # canvas and the keyed frame gets clipped to a single pixel,
-                    # producing a solid-color video with the subject gone.
-                    # scale2ref resizes [0:v] to match the frame [1:v] first.
-                    f"[1:v]chromakey=color=0x00FF00:similarity=0.3:blend=0.08[fg];"
-                    f"[0:v][fg]scale2ref[bg][fg2];"
-                    f"[bg][fg2]overlay=0:0"
+                    # Force an explicit alpha format after chromakey so the keyed
+                    # transparency survives filter negotiation on every FFmpeg
+                    # build — without it, some Linux builds carry the keyed frame
+                    # forward without an alpha plane and overlay draws opaque
+                    # green over the background instead of compositing.
+                    f"[1:v]chromakey=color=0x00FF00:similarity=0.3:blend=0.08,"
+                    f"format=yuva420p[fg];"
+                    f"[0:v][fg]overlay=0:0:format=auto,format=yuv420p"
                 ),
                 "-frames:v", "1",
                 str(out_path),