diff --git a/AyCode.Core.Serializers.SourceGenerator/AcBinarySourceGenerator.cs b/AyCode.Core.Serializers.SourceGenerator/AcBinarySourceGenerator.cs
index 35a4b01..dbc9369 100644
--- a/AyCode.Core.Serializers.SourceGenerator/AcBinarySourceGenerator.cs
+++ b/AyCode.Core.Serializers.SourceGenerator/AcBinarySourceGenerator.cs
@@ -53,6 +53,33 @@ public class AcBinarySourceGenerator : IIncrementalGenerator
         defaultSeverity: DiagnosticSeverity.Warning,
         isEnabledByDefault: true);
 
+    /// <summary>
+    /// ACCORE-BIN-I-T7K3 compile-time guard: a property declared as <c>System.Object</c> requires
+    /// polymorphic-prefix emit (<c>ObjectWithTypeName</c>) so the deserializer can resolve the
+    /// concrete runtime type. When <see cref="UsePolymorphType"/> is <c>false</c>, the prefix is
+    /// suppressed and the wire silently corrupts on round-trip (FixObj slot byte against
+    /// <c>typeof(object)</c> at read-time → 0-byte object wrapper → reader position drifts →
+    /// downstream <c>DECIMAL_DRIFT</c> / <c>IndexOutOfRangeException</c>).
+    ///
+    /// Surface the misconfiguration at build time so the silent corruption is structurally
+    /// impossible. Three escape hatches for the developer:
+    ///   1. Enable the polymorphic feature (<see cref="UsePolymorphType"/> = true, or — once the
+    ///      planned <c>[AcBinarySerializable(EnablePolymorphicFeature = true)]</c> flag lands — set
+    ///      it on the type).
+    ///   2. Change the property type to a concrete type (no polymorphism needed).
+    ///   3. Mark the property with <c>[AcBinaryIgnore]</c> — ignored properties are filtered
+    ///      out at property enumeration, so this diagnostic does not fire for them.
+    /// </summary>
+    private static readonly DiagnosticDescriptor PolymorphicPropertyWithFeatureDisabledError = new(
+        id: "ACBIN002",
+        title: "Polymorphic property requires polymorphic feature enabled",
+        messageFormat: "Type '{0}' contains property '{1}' declared as System.Object, but polymorphic serialization in the source generator is disabled (UsePolymorphType=false). " +
+                       "The generated writer would silently corrupt the wire on round-trip. " +
+                       "To fix: (1) enable polymorphic serialization, (2) change '{1}' to a concrete type, or (3) exclude it with [AcBinaryIgnore].",
+        category: "AcBinarySerializer",
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
+
     public void Initialize(IncrementalGeneratorInitializationContext context)
     {
         var classDeclarations = context.SyntaxProvider
@@ -406,6 +433,7 @@ public class AcBinarySourceGenerator : IIncrementalGenerator
         if (valid.Count == 0) return;
 
         DetectAndReportCycles(valid, context);
+        DetectAndReportPolymorphicMisuse(valid, context);
 
         foreach (var ci in valid)
         {
@@ -416,6 +444,30 @@ public class AcBinarySourceGenerator : IIncrementalGenerator
         context.AddSource("AcBinaryGeneratedWriters_Init.g.cs", SourceText.From(GenInit(valid), Encoding.UTF8));
     }
 
+    /// <summary>
+    /// ACCORE-BIN-I-T7K3 guard: emits <see cref="PolymorphicPropertyWithFeatureDisabledError"/>
+    /// (ACBIN002) for every <c>System.Object</c>-declared property on any
+    /// <c>[AcBinarySerializable]</c> type while <see cref="UsePolymorphType"/> is <c>false</c>.
+    /// Short-circuits when the feature is enabled — no per-property work needed.
+    /// </summary>
+    private static void DetectAndReportPolymorphicMisuse(List<SerializableClassInfo> classes, SourceProductionContext spc)
+    {
+        if (UsePolymorphType) return; // Feature enabled → polymorphic prefix is emitted, no misuse possible.
+
+        foreach (var ci in classes)
+        {
+            foreach (var p in ci.Properties)
+            {
+                if (p.IsObjectDeclaredType)
+                {
+                    spc.ReportDiagnostic(Diagnostic.Create(
+                        PolymorphicPropertyWithFeatureDisabledError, Location.None,
+                        ci.ClassName, p.Name));
+                }
+            }
+        }
+    }
+
     /// <summary>
     /// Detects circular reference chains among [AcBinarySerializable] types at compile time
     /// and reports ACBIN001 warnings. Uses DFS with 3-color marking to find back-edges.