diff --git a/helpers/render.py b/helpers/render.py
index 0d02cff..430a597 100644
--- a/helpers/render.py
+++ b/helpers/render.py
@@ -515,7 +515,14 @@ def build_final_composite(
     inputs: list[str] = ["-i", str(base_path)]
     for ov in overlays:
         ov_path = resolve_path(ov["file"], edit_dir)
-        inputs += ["-i", str(ov_path)]
+        # VP9/WebM overlays carry alpha as a side-data plane. ffmpeg's native vp9
+        # decoder drops it (decodes to yuv420p → opaque), which turns a full-frame
+        # transparent overlay into an opaque black layer. Force the alpha-aware
+        # libvpx-vp9 decoder so overlay alpha is honored.
+        if ov_path.suffix.lower() == ".webm":
+            inputs += ["-c:v", "libvpx-vp9", "-i", str(ov_path)]
+        else:
+            inputs += ["-i", str(ov_path)]
 
     filter_parts: list[str] = []
     # PTS-shift every overlay so its frame 0 lands at start_in_output