diff --git a/src/vulca/providers/gemini.py b/src/vulca/providers/gemini.py
index 4fb16a2f..e9206854 100644
--- a/src/vulca/providers/gemini.py
+++ b/src/vulca/providers/gemini.py
@@ -6,6 +6,7 @@
 
 import asyncio
 import base64
+import io
 import math
 import os
 
@@ -66,6 +67,20 @@ def _detect_mime_type(data: bytes) -> str:
     return "image/png"
 
 
+def _build_visible_mask_reference(mask_bytes: bytes) -> bytes:
+    """Render an alpha edit mask as visible black/white guidance for Gemini."""
+    from PIL import Image
+
+    with Image.open(io.BytesIO(mask_bytes)) as mask_img:
+        alpha = mask_img.convert("RGBA").split()[-1]
+        visible = alpha.point(lambda value: 255 if value < 128 else 0)
+    rgb = Image.new("RGB", visible.size, (0, 0, 0))
+    rgb.paste((255, 255, 255), mask=visible)
+    buf = io.BytesIO()
+    rgb.save(buf, format="PNG")
+    return buf.getvalue()
+
+
 class GeminiImageProvider:
     """Image generation via Google Gemini API.
 
@@ -279,6 +294,7 @@ async def inpaint_with_mask(
             image_bytes = image_fh.read()
         with open(mask_path, "rb") as mask_fh:
             mask_bytes = mask_fh.read()
+        visible_mask_bytes = _build_visible_mask_reference(mask_bytes)
 
         if size and "x" in size:
             try:
@@ -295,11 +311,15 @@ async def inpaint_with_mask(
         aspect_ratio = _dims_to_aspect_ratio(width, height)
         full_prompt = (
             f"{prompt}\n\n"
-            "Use the first image as the source crop. Use the second image as an "
-            "RGBA edit mask: transparent mask pixels mark the edit region, and "
-            "opaque mask pixels mark source context that should stay visually "
-            "preserved. Repaint only the transparent mask pixels. Do not create "
-            "a new scene outside the masked replacement area."
+            "Use the first image as the source crop. Use the second image as a "
+            "visible binary edit mask rendered from the original RGBA mask: "
+            "white mask pixels (original transparent mask pixels) mark the edit "
+            "region, and black mask pixels (original opaque mask pixels) mark "
+            "source context that should stay visually preserved. Repaint only "
+            "the white edit region. Do not create a new scene outside the masked "
+            "replacement area. The mask image is not part of the output: do not "
+            "draw the mask, do not copy its white or black shapes, and do not "
+            "leave mask-colored background behind."
         )
         if tradition and tradition != "default":
             full_prompt += (
@@ -314,8 +334,8 @@ async def inpaint_with_mask(
                 mime_type=_detect_mime_type(image_bytes),
             ),
             types.Part.from_bytes(
-                data=mask_bytes,
-                mime_type=_detect_mime_type(mask_bytes),
+                data=visible_mask_bytes,
+                mime_type="image/png",
             ),
             full_prompt,
         ]
diff --git a/tests/test_gemini_image_size.py b/tests/test_gemini_image_size.py
index 23ffc98a..ec187fa4 100644
--- a/tests/test_gemini_image_size.py
+++ b/tests/test_gemini_image_size.py
@@ -202,8 +202,17 @@ def __init__(self, api_key):
         assert recorded["model"] == "gemini-3.1-flash-image-preview"
         assert recorded["contents"][0].mime_type == "image/png"
         assert recorded["contents"][1].mime_type == "image/png"
+        visible_mask = Image.open(io.BytesIO(recorded["contents"][1].data)).convert(
+            "RGB"
+        )
+        assert visible_mask.getpixel((8, 6)) == (255, 255, 255)
+        assert visible_mask.getpixel((0, 0)) == (0, 0, 0)
         assert "transparent mask pixels" in recorded["contents"][2]
         assert "opaque mask pixels" in recorded["contents"][2]
+        assert "white mask pixels" in recorded["contents"][2]
+        assert "black mask pixels" in recorded["contents"][2]
+        assert "do not draw the mask" in recorded["contents"][2].lower()
+        assert "not part of the output" in recorded["contents"][2].lower()
         assert "paint one compact yellow flower head" in recorded["contents"][2]
         assert result.mime == "image/png"
         assert base64.b64decode(result.image_b64).startswith(b"\x89PNG")