diff --git a/tests/AcDream.Core.Tests/Textures/SurfaceDecoderTests.cs b/tests/AcDream.Core.Tests/Textures/SurfaceDecoderTests.cs
index 8110bfc..7a440e0 100644
--- a/tests/AcDream.Core.Tests/Textures/SurfaceDecoderTests.cs
+++ b/tests/AcDream.Core.Tests/Textures/SurfaceDecoderTests.cs
@@ -56,12 +56,10 @@ public class SurfaceDecoderTests
     }
 
     [Fact]
-    public void Decode_A8_ExpandsSingleByteToRgbaWithAlphaInAllChannels()
+    public void Decode_A8_NonAdditive_ProducesWhitePlusAlpha()
     {
-        // PFID_A8 is single-byte-per-pixel alpha. AC terrain blending alpha maps
-        // are stored this way. WorldBuilder's GetExpandedAlphaTexture replicates
-        // the byte into all four RGBA channels so fragment shaders can read the
-        // blend value from any channel (convention: the alpha channel).
+        // Default (isAdditive: false) = WB FillA8 semantics: R=G=B=255, A=val.
+        // Used for non-additive entity surfaces where A8 is a pure alpha channel.
         var src = new byte[] { 0x00, 0x40, 0x80, 0xFF };  // 2x2 image
         var rs = new RenderSurface
         {
@@ -76,7 +74,34 @@ public class SurfaceDecoderTests
         Assert.Equal(2, decoded.Width);
         Assert.Equal(2, decoded.Height);
         Assert.Equal(16, decoded.Rgba8.Length);
-        // Each input byte expands to (b, b, b, b) in RGBA output
+        // Each input byte expands to (255, 255, 255, val) — white with varying alpha
+        Assert.Equal(new byte[]
+        {
+            255, 255, 255, 0x00,
+            255, 255, 255, 0x40,
+            255, 255, 255, 0x80,
+            255, 255, 255, 0xFF,
+        }, decoded.Rgba8);
+    }
+
+    [Fact]
+    public void Decode_A8_Additive_ReplicatesByteToAllChannels()
+    {
+        // isAdditive=true = WB FillA8Additive semantics: R=G=B=A=val.
+        // Used for terrain blending alpha masks (TerrainAtlas always passes isAdditive:true).
+        var src = new byte[] { 0x00, 0x40, 0x80, 0xFF };  // 2x2 image
+        var rs = new RenderSurface
+        {
+            Width = 2,
+            Height = 2,
+            Format = PixelFormat.PFID_A8,
+            SourceData = src,
+        };
+
+        var decoded = SurfaceDecoder.DecodeRenderSurface(rs, palette: null, isClipMap: false, isAdditive: true);
+
+        Assert.Equal(16, decoded.Rgba8.Length);
+        // Each input byte fans out to all four channels
         Assert.Equal(new byte[]
         {
             0x00, 0x00, 0x00, 0x00,
@@ -92,7 +117,7 @@ public class SurfaceDecoderTests
         // PFID_CUSTOM_LSCAPE_ALPHA (0xF4) is AC's custom format for terrain
         // blending alpha maps. Pixel layout is identical to PFID_A8 — one
         // byte of alpha per pixel — so the decoder routes both through the
-        // same DecodeA8 implementation.
+        // same DecodeA8 implementation. Default (isAdditive:false) → R=G=B=255, A=val.
         var src = new byte[] { 0x10, 0x20, 0x30, 0x40 };  // 2x2
         var rs = new RenderSurface
         {
@@ -107,10 +132,10 @@ public class SurfaceDecoderTests
         Assert.Equal(16, decoded.Rgba8.Length);
         Assert.Equal(new byte[]
         {
-            0x10, 0x10, 0x10, 0x10,
-            0x20, 0x20, 0x20, 0x20,
-            0x30, 0x30, 0x30, 0x30,
-            0x40, 0x40, 0x40, 0x40,
+            255, 255, 255, 0x10,
+            255, 255, 255, 0x20,
+            255, 255, 255, 0x30,
+            255, 255, 255, 0x40,
         }, decoded.Rgba8);
     }