diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/CompileSurfaceParityTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/CompileSurfaceParityTests.cs
index 685ff5a8..df3d872f 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/CompileSurfaceParityTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/CompileSurfaceParityTests.cs
@@ -41,6 +41,68 @@ public class CompileSurfaceParityTests
             globals: typeof(TriggerExpressionGlobals));
     }
 
+    /// <summary>
+    /// Deeper guard than the top-level name-superset check above: for each
+    /// runtime script-accessor type (<see cref="AttributeAccessor"/>,
+    /// <see cref="CompositionAccessor"/>, <see cref="ChildrenAccessor"/>) every
+    /// public instance METHOD must have a same-name / same-arity counterpart on
+    /// its compile-surface mirror. This catches the regression that motivated this
+    /// test (the <c>WaitAsync</c> overloads were present on the runtime
+    /// <see cref="AttributeAccessor"/> but missing from
+    /// <see cref="ScriptCompileSurface.CompileAttributeAccessor"/>, so scripts that
+    /// awaited them passed the design-time gate yet would have failed at the site).
+    ///
+    /// <para>
+    /// Matching is by NAME + PARAMETER COUNT only — the mirror uses
+    /// <c>object?</c>-vs-mirror-type substitutions, so exact parameter-type identity
+    /// is intentionally NOT required; the guard is about API presence, not type
+    /// equality (the design-time compile catches deeper signature drift itself).
+    /// To still catch a single dropped overload within an overload set that shares
+    /// an arity (e.g. the value-form vs predicate-form <c>WaitAsync</c>, both
+    /// 4-parameter), the assertion is by COUNT: the mirror must expose at least as
+    /// many overloads for each (name, arity) as the runtime accessor declares.
+    /// </para>
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(AccessorMirrorPairs))]
+    public void CompileAccessorMirror_Covers_RuntimeAccessor_MethodArities(
+        Type runtimeAccessor, Type compileMirror)
+    {
+        var mirrorCounts = PublicInstanceMethodArityCounts(compileMirror);
+        var runtimeCounts = PublicInstanceMethodArityCounts(runtimeAccessor);
+
+        var shortfalls = runtimeCounts
+            .Where(kvp => mirrorCounts.GetValueOrDefault(kvp.Key) < kvp.Value)
+            .OrderBy(kvp => kvp.Key.Name, StringComparer.Ordinal)
+            .ThenBy(kvp => kvp.Key.Arity)
+            .Select(kvp =>
+                $"{kvp.Key.Name}({kvp.Key.Arity} param(s)): runtime has {kvp.Value} "
+                + $"overload(s), mirror has {mirrorCounts.GetValueOrDefault(kvp.Key)}")
+            .ToList();
+
+        Assert.True(
+            shortfalls.Count == 0,
+            $"Compile surface mirror '{compileMirror.Name}' under-covers {shortfalls.Count} "
+            + $"method group(s) on runtime accessor '{runtimeAccessor.Name}': "
+            + $"{string.Join("; ", shortfalls)}. "
+            + "The compile-only mirror must expose a same-name / same-arity method for "
+            + "every public instance method a script can call on the runtime accessor — "
+            + "add the missing method(s) to the ScriptAnalysis compile surface so a "
+            + "script using them cannot pass the design-time gate then fail at the site.");
+    }
+
+    /// <summary>
+    /// Runtime script-accessor type ↔ compile-surface mirror pairs guarded by
+    /// <see cref="CompileAccessorMirror_Covers_RuntimeAccessor_MethodArities"/>.
+    /// </summary>
+    public static IEnumerable<object[]> AccessorMirrorPairs() =>
+        new[]
+        {
+            new object[] { typeof(AttributeAccessor), typeof(ScriptCompileSurface.CompileAttributeAccessor) },
+            new object[] { typeof(CompositionAccessor), typeof(ScriptCompileSurface.CompileCompositionAccessor) },
+            new object[] { typeof(ChildrenAccessor), typeof(ScriptCompileSurface.CompileChildrenAccessor) },
+        };
+
     /// <summary>
     /// Asserts that the public instance property + method member names of
     /// <paramref name="surface"/> are a superset of those of
@@ -102,4 +164,50 @@ public class CompileSurfaceParityTests
 
         return names;
     }
+
+    /// <summary>
+    /// Names of the inherited <see cref="object"/> instance methods that are not
+    /// part of any script-reachable accessor API and must be excluded from parity.
+    /// </summary>
+    private static readonly HashSet<string> ObjectMethodNames = new(StringComparer.Ordinal)
+    {
+        nameof(ToString),
+        nameof(GetHashCode),
+        nameof(Equals),
+        nameof(GetType),
+    };
+
+    /// <summary>
+    /// (Name, parameter count) of each public instance METHOD declared on or
+    /// inherited by <paramref name="type"/>, excluding inherited <see cref="object"/>
+    /// methods and compiler-generated special-name methods (property/indexer
+    /// get_/set_ accessors, operators). These are the methods a script can call
+    /// directly on the accessor, the surface this guard requires the mirror to cover.
+    /// </summary>
+    private static IEnumerable<(string Name, int Arity)> PublicInstanceMethodSignatures(Type type)
+    {
+        const BindingFlags flags = BindingFlags.Public | BindingFlags.Instance;
+
+        foreach (var method in type.GetMethods(flags))
+        {
+            if (method.IsSpecialName)
+                continue;
+            if (ObjectMethodNames.Contains(method.Name))
+                continue;
+            yield return (method.Name, method.GetParameters().Length);
+        }
+    }
+
+    /// <summary>
+    /// Number of public instance method overloads on <paramref name="type"/> per
+    /// (Name, parameter count) key — used to test method-arity parity against a
+    /// runtime accessor by count (so a single dropped overload within a shared-arity
+    /// overload set is caught, not just a wholly-missing method group). Built with
+    /// <see cref="PublicInstanceMethodSignatures"/> so the same object/special-name
+    /// exclusions apply on both sides.
+    /// </summary>
+    private static Dictionary<(string Name, int Arity), int> PublicInstanceMethodArityCounts(Type type) =>
+        PublicInstanceMethodSignatures(type)
+            .GroupBy(sig => sig)
+            .ToDictionary(g => g.Key, g => g.Count());
 }