diff --git a/Jint.Tests/Runtime/PropertyDescriptorTests.cs b/Jint.Tests/Runtime/PropertyDescriptorTests.cs
index 7c2c7ac23..a953138c5 100644
--- a/Jint.Tests/Runtime/PropertyDescriptorTests.cs
+++ b/Jint.Tests/Runtime/PropertyDescriptorTests.cs
@@ -98,6 +98,21 @@ public void AllForbiddenDescriptor()
         Assert.Equal(true, pd.IsDataDescriptor());
     }
 
+    [Fact]
+    public void FastSetPropertyIsVisibleOnBuiltinShapedHost()
+    {
+        // Math uses builtin-shape storage; a raw property store must deopt it to dictionary
+        // mode — without that, the write lands in a side dictionary the shape-mode read
+        // paths never consult and the property silently doesn't exist.
+        Assert.Equal(4, _engine.Evaluate("Math.floor(4.7)").AsNumber()); // initialize the shaped host
+        var math = _engine.Evaluate("Math").AsObject();
+        math.FastSetProperty("custom", new PropertyDescriptor(42, PropertyFlag.ConfigurableEnumerableWritable));
+
+        Assert.Equal(42, _engine.Evaluate("Math.custom").AsNumber());
+        Assert.True(_engine.Evaluate("Object.getOwnPropertyNames(Math).includes('custom')").AsBoolean());
+        Assert.Equal(4, _engine.Evaluate("Math.floor(4.7)").AsNumber());
+    }
+
     [Fact]
     public void LazyPropertyDescriptor()
     {
diff --git a/Jint/Native/Object/ObjectInstance.cs b/Jint/Native/Object/ObjectInstance.cs
index 68c886dc2..53940a743 100644
--- a/Jint/Native/Object/ObjectInstance.cs
+++ b/Jint/Native/Object/ObjectInstance.cs
@@ -245,10 +245,16 @@ internal void SetProperty(string property, PropertyDescriptor value)
     internal void SetProperty(Key property, PropertyDescriptor value)
     {
         // Storing a raw descriptor is a dictionary-mode operation; deopt first if needed.
+        // Without the builtin-shape deopt, the write would land in a side dictionary that the
+        // shape-mode read paths never consult — the property would silently not exist.
         if ((_type & InternalTypes.ShapeMode) != InternalTypes.Empty)
         {
             ConvertToDictionaryMode();
         }
+        else if ((_type & InternalTypes.BuiltinShapeMode) != InternalTypes.Empty)
+        {
+            DeoptBuiltinShape();
+        }
         _properties ??= new PropertyDictionary();
         _properties[property] = value;
         unchecked { _propertiesVersion++; }