From d7d73e1f33b5e22cd8cb21c3a8e40dac8f8f2f43 Mon Sep 17 00:00:00 2001
From: Marko Lahma <marko.lahma@gmail.com>
Date: Mon, 29 Jun 2026 21:33:41 +0300
Subject: [PATCH] Built-in shapes default-on via base class (opt-out); shape
 Temporal.Now
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Make built-in shape storage the default for any [JsObject] host that derives from
BuiltinShapeObject — deriving from the base is now the opt-in, so a built-in adopts
shapes with a one-line base change and no attribute flag. UseShape = false is the
explicit opt-out; hosts that don't derive from BuiltinShapeObject keep the dictionary
path regardless.

- Generator auto-detects the base (walks the base-type chain) and uses the shape path
  by default for derived hosts. The UseShape attribute property now defaults to true and
  only means "opt this derived host back out" when set false.
- New diagnostic JINT022: a BuiltinShapeObject-derived host that declares an unsupported
  member (accessor / intrinsic reference / thrower / instance-or-mutable property) fails
  the build instead of silently dropping it.
- Math, JSON, Reflect, Atomics drop the now-redundant UseShape = true (auto-detected).
- Temporal.Now (9 functions + Symbol.toStringTag, no properties) adopts shapes by simply
  deriving from BuiltinShapeObject — demonstrating the one-line adoption and adding a win.

Benchmarks (BuiltinShapeBenchmark, default job, vs main; EngineOnly control identical at
13.69 KB):

  EngineInitMath / EngineInitBuiltins   15.51 / 20.20 KB  ==  15.51 / 20.20 KB  (byte-identical;
                                                                the default change is neutral)
  EngineInitTemporalNow                 20.43 → 19.45 KB   (−4.8%; time −5.6%)

Conformance: Math 662/662, JSON 502/502, Reflect 306/306, Atomics 779/779, Temporal.Now
138/138; full Test262 0 new failures (the annexB RegExp-*-escape-BMP and Atomics.waitAsync
tests are pre-existing timing flakes under concurrent load — they flake identically on
main and pass in isolation). Jint.Tests 3138/3076; Jint.Tests.PublicInterface 79/79;
Jint.Tests.SourceGenerators 30/30 (snapshots updated for the attribute default).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RDS23QeSSNRkaUB2KL74U9
---
 Jint.Benchmark/BuiltinShapeBenchmark.cs       | 11 +++++
 Jint.SourceGenerators/Attributes.cs           | 14 +++---
 Jint.SourceGenerators/Diagnostics.cs          |  8 ++++
 Jint.SourceGenerators/Models.cs               | 44 +++++++++++++++++--
 ...tterAndSetter#JintAttributes.g.verified.cs | 14 +++---
 ...sorGetterOnly#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...ercedRestSpan#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...nstanceMethod#JintAttributes.g.verified.cs | 14 +++---
 ...erConversions#JintAttributes.g.verified.cs | 14 +++---
 ...nsicReference#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ....MinimalClass#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 ...oughArguments#JintAttributes.g.verified.cs | 14 +++---
 ...ertyConstants#JintAttributes.g.verified.cs | 14 +++---
 ...RestParameter#JintAttributes.g.verified.cs | 14 +++---
 ...chConversions#JintAttributes.g.verified.cs | 14 +++---
 ...sorGetterOnly#JintAttributes.g.verified.cs | 14 +++---
 ...unctionMethod#JintAttributes.g.verified.cs | 14 +++---
 ....SymbolMember#JintAttributes.g.verified.cs | 14 +++---
 ...astToCallable#JintAttributes.g.verified.cs | 14 +++---
 ...bjectInstance#JintAttributes.g.verified.cs | 14 +++---
 ...rowerAccessor#JintAttributes.g.verified.cs | 14 +++---
 ...berConversion#JintAttributes.g.verified.cs | 14 +++---
 ...cesDiagnostic#JintAttributes.g.verified.cs | 14 +++---
 Jint/Native/Atomics/AtomicsInstance.cs        |  2 +-
 Jint/Native/Json/JsonInstance.cs              |  2 +-
 Jint/Native/Math/MathInstance.cs              |  4 +-
 Jint/Native/Reflect/ReflectInstance.cs        |  2 +-
 Jint/Native/Temporal/Now/TemporalNow.cs       |  2 +-
 39 files changed, 314 insertions(+), 195 deletions(-)

diff --git a/Jint.Benchmark/BuiltinShapeBenchmark.cs b/Jint.Benchmark/BuiltinShapeBenchmark.cs
index 4835020ac..f522ec0f2 100644
--- a/Jint.Benchmark/BuiltinShapeBenchmark.cs
+++ b/Jint.Benchmark/BuiltinShapeBenchmark.cs
@@ -58,4 +58,15 @@ public Engine EngineInitBuiltins()
         engine.Evaluate(_initBuiltins);
         return engine;
     }
+
+    // Forces Temporal.Now (9 functions) to initialize — newly shape-backed here, dictionary on main.
+    private static readonly Prepared<Script> _initTemporalNow = Engine.PrepareScript("Temporal.Now.instant();");
+
+    [Benchmark]
+    public Engine EngineInitTemporalNow()
+    {
+        var engine = new Engine();
+        engine.Evaluate(_initTemporalNow);
+        return engine;
+    }
 }
diff --git a/Jint.SourceGenerators/Attributes.cs b/Jint.SourceGenerators/Attributes.cs
index b0c11b4bd..57e346f27 100644
--- a/Jint.SourceGenerators/Attributes.cs
+++ b/Jint.SourceGenerators/Attributes.cs
@@ -22,13 +22,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
             public int ExtraCapacity { get; set; }
 
             /// <summary>
-            /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-            /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-            /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-            /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-            /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+            /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+            /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+            /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+            /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+            /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+            /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+            /// BuiltinShapeObject.
             /// </summary>
-            public bool UseShape { get; set; }
+            public bool UseShape { get; set; } = true;
         }
 
         [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.SourceGenerators/Diagnostics.cs b/Jint.SourceGenerators/Diagnostics.cs
index 061d3248d..937fffc65 100644
--- a/Jint.SourceGenerators/Diagnostics.cs
+++ b/Jint.SourceGenerators/Diagnostics.cs
@@ -107,4 +107,12 @@ internal static class DiagnosticDescriptors
         category: "Jint.SourceGenerators",
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true);
+
+    public static readonly DiagnosticDescriptor UnsupportedShapeMember = new(
+        id: "JINT022",
+        title: "Built-in shape host has an unsupported member kind",
+        messageFormat: "Type '{0}' uses built-in shape storage (derives from BuiltinShapeObject) but declares an accessor / intrinsic reference / thrower / non-static or mutable property, which the shape path does not support; add [JsObject(UseShape = false)] or remove the member",
+        category: "Jint.SourceGenerators",
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
 }
diff --git a/Jint.SourceGenerators/Models.cs b/Jint.SourceGenerators/Models.cs
index c0b8acca6..d6e8c3119 100644
--- a/Jint.SourceGenerators/Models.cs
+++ b/Jint.SourceGenerators/Models.cs
@@ -98,19 +98,32 @@ public IEnumerable<Diagnostic> Diagnostics
             ? string.Empty
             : typeSymbol.ContainingNamespace.ToDisplayString();
 
-        // Read [JsObject(ExtraCapacity = N, UseShape = true)].
+        // Read [JsObject(ExtraCapacity = N, UseShape = ...)]. Shape storage is the default for any host
+        // that derives from BuiltinShapeObject (deriving from it is the opt-in); UseShape = false is the
+        // explicit opt-out. Hosts that don't derive from it always use the dictionary path regardless.
         var extraCapacity = 0;
-        var useShape = false;
+        var useShapeRequested = true;
         foreach (var attr in typeSymbol.GetAttributes())
         {
             if (attr.AttributeClass?.Name != "JsObjectAttribute") continue;
             foreach (var named in attr.NamedArguments)
             {
                 if (named.Key == "ExtraCapacity" && named.Value.Value is int v) extraCapacity = v;
-                else if (named.Key == "UseShape" && named.Value.Value is bool s) useShape = s;
+                else if (named.Key == "UseShape" && named.Value.Value is bool s) useShapeRequested = s;
             }
         }
 
+        var derivesFromBuiltinShapeObject = false;
+        for (var baseType = typeSymbol.BaseType; baseType is not null; baseType = baseType.BaseType)
+        {
+            if (baseType.Name == "BuiltinShapeObject")
+            {
+                derivesFromBuiltinShapeObject = true;
+                break;
+            }
+        }
+        var useShape = useShapeRequested && derivesFromBuiltinShapeObject;
+
         var functions = new List<FunctionDefinition>();
         var properties = new List<PropertyDefinition>();
         var symbols = new List<SymbolDefinition>();
@@ -329,6 +342,31 @@ public IEnumerable<Diagnostic> Diagnostics
                 typeSymbol.Name));
         }
 
+        // The shape path supports [JsFunction]s, static-immutable [JsProperty] constants, and symbols
+        // (incl. symbol-keyed functions). Accessors, intrinsic references, throwers and instance/mutable
+        // properties aren't representable — fail loudly rather than silently dropping them.
+        if (useShape)
+        {
+            var hasUnsupported = intrinsicReferences.Count > 0 || throwerAccessors.Count > 0;
+            foreach (var fn in functions)
+            {
+                if (fn.Registration is RegistrationKind.AccessorGet or RegistrationKind.AccessorSet
+                    or RegistrationKind.SymbolAccessorGet or RegistrationKind.SymbolAccessorSet)
+                {
+                    hasUnsupported = true;
+                    break;
+                }
+            }
+            foreach (var p in properties)
+            {
+                if (!p.IsStatic || !p.IsImmutable) { hasUnsupported = true; break; }
+            }
+            if (hasUnsupported)
+            {
+                diagnostics.Add(new DiagnosticInfo(DiagnosticDescriptors.UnsupportedShapeMember, location, typeSymbol.Name));
+            }
+        }
+
         functions.Sort(static (a, b) => string.CompareOrdinal(a.JsName, b.JsName));
         properties.Sort(static (a, b) => string.CompareOrdinal(a.JsName, b.JsName));
         symbols.Sort(static (a, b) => string.CompareOrdinal(a.SymbolName, b.SymbolName));
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterAndSetter#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterAndSetter#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterAndSetter#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterAndSetter#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterOnly#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterOnly#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterOnly#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorGetterOnly#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorWrongArity_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorWrongArity_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorWrongArity_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.AccessorWrongArity_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpan#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpan#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpan#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpan#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpanTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpanTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpanTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.CoercedRestSpanTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingAccessorFlags_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingAccessorFlags_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingAccessorFlags_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingAccessorFlags_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingConversionAttrs_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingConversionAttrs_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingConversionAttrs_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConflictingConversionAttrs_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConversionTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConversionTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConversionTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ConversionTypeMismatch_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateIntrinsicReference_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateIntrinsicReference_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateIntrinsicReference_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateIntrinsicReference_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateThrower_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateThrower_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateThrower_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.DuplicateThrower_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.InstanceMethod#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.InstanceMethod#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.InstanceMethod#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.InstanceMethod#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntegerConversions#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntegerConversions#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntegerConversions#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntegerConversions#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReference#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReference#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReference#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReference#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReferenceWithoutRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReferenceWithoutRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReferenceWithoutRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.IntrinsicReferenceWithoutRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MinimalClass#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MinimalClass#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MinimalClass#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MinimalClass#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MissingRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MissingRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MissingRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.MissingRealmField_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.NotPartial_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.NotPartial_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.NotPartial_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.NotPartial_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.OverloadCollision_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.OverloadCollision_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.OverloadCollision_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.OverloadCollision_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PassthroughArguments#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PassthroughArguments#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PassthroughArguments#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PassthroughArguments#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PropertyConstants#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PropertyConstants#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PropertyConstants#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.PropertyConstants#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RestParameter#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RestParameter#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RestParameter#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RestParameter#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RichConversions#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RichConversions#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RichConversions#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.RichConversions#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolAccessorGetterOnly#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolAccessorGetterOnly#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolAccessorGetterOnly#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolAccessorGetterOnly#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolFunctionMethod#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolFunctionMethod#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolFunctionMethod#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolFunctionMethod#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolMember#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolMember#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolMember#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.SymbolMember#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToCallable#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToCallable#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToCallable#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToCallable#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToObjectInstance#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToObjectInstance#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToObjectInstance#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThisObjectCastToObjectInstance#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThrowerAccessor#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThrowerAccessor#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThrowerAccessor#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ThrowerAccessor#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ToNumberConversion#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ToNumberConversion#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ToNumberConversion#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.ToNumberConversion#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.UnsupportedReturnType_ProducesDiagnostic#JintAttributes.g.verified.cs b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.UnsupportedReturnType_ProducesDiagnostic#JintAttributes.g.verified.cs
index a751297e7..97076b945 100644
--- a/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.UnsupportedReturnType_ProducesDiagnostic#JintAttributes.g.verified.cs
+++ b/Jint.Tests.SourceGenerators/Snapshots/ObjectGeneratorTests.UnsupportedReturnType_ProducesDiagnostic#JintAttributes.g.verified.cs
@@ -18,13 +18,15 @@ internal sealed class JsObjectAttribute : global::System.Attribute
     public int ExtraCapacity { get; set; }
 
     /// <summary>
-    /// Store this built-in's string-keyed own properties as a shared immutable BuiltinShape plus a
-    /// per-realm lazily-filled descriptor array instead of a per-realm property dictionary. The host
-    /// must derive from <c>BuiltinShapeObject</c>. Supported for hosts whose members are
-    /// [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s (namespace built-ins
-    /// such as Math/JSON/Reflect/Atomics). See BuiltinShapeObject.
+    /// Built-in shape storage (a shared immutable BuiltinShape plus a per-realm lazily-filled
+    /// descriptor array instead of a per-realm property dictionary) is used <b>by default</b> for any
+    /// host that derives from <c>BuiltinShapeObject</c> — deriving from it is the opt-in. Set this to
+    /// <c>false</c> to opt a derived host back out to the dictionary path. Hosts that do not derive
+    /// from <c>BuiltinShapeObject</c> always use the dictionary path regardless of this flag.
+    /// Supported members: [JsFunction]s, static-immutable [JsProperty] constants, and symbols. See
+    /// BuiltinShapeObject.
     /// </summary>
-    public bool UseShape { get; set; }
+    public bool UseShape { get; set; } = true;
 }
 
 [global::System.AttributeUsage(global::System.AttributeTargets.Method)]
diff --git a/Jint/Native/Atomics/AtomicsInstance.cs b/Jint/Native/Atomics/AtomicsInstance.cs
index 338c83d71..cf4660a0a 100644
--- a/Jint/Native/Atomics/AtomicsInstance.cs
+++ b/Jint/Native/Atomics/AtomicsInstance.cs
@@ -16,7 +16,7 @@ namespace Jint.Native.Atomics;
 /// <summary>
 /// https://tc39.es/ecma262/#sec-atomics-object
 /// </summary>
-[JsObject(UseShape = true)]
+[JsObject]
 internal sealed partial class AtomicsInstance : BuiltinShapeObject
 {
     private readonly Realm _realm;
diff --git a/Jint/Native/Json/JsonInstance.cs b/Jint/Native/Json/JsonInstance.cs
index dfea24196..20122d116 100644
--- a/Jint/Native/Json/JsonInstance.cs
+++ b/Jint/Native/Json/JsonInstance.cs
@@ -4,7 +4,7 @@
 
 namespace Jint.Native.Json;
 
-[JsObject(UseShape = true)]
+[JsObject]
 internal sealed partial class JsonInstance : BuiltinShapeObject
 {
     private readonly Realm _realm;
diff --git a/Jint/Native/Math/MathInstance.cs b/Jint/Native/Math/MathInstance.cs
index c27c33fa3..c7e90b502 100644
--- a/Jint/Native/Math/MathInstance.cs
+++ b/Jint/Native/Math/MathInstance.cs
@@ -6,7 +6,7 @@
 
 namespace Jint.Native.Math;
 
-[JsObject(UseShape = true)]
+[JsObject]
 internal sealed partial class MathInstance : BuiltinShapeObject
 {
     private readonly Realm _realm;
@@ -31,7 +31,7 @@ internal MathInstance(Engine engine, Realm realm, ObjectPrototype objectPrototyp
 
     protected override void Initialize()
     {
-        // CreateProperties_Generated installs the built-in shape (UseShape = true). See BuiltinShapeObject.
+        // CreateProperties_Generated installs the built-in shape (this derives from BuiltinShapeObject).
         CreateProperties_Generated();
         CreateSymbols_Generated();
     }
diff --git a/Jint/Native/Reflect/ReflectInstance.cs b/Jint/Native/Reflect/ReflectInstance.cs
index 95f1b263e..33de975a4 100644
--- a/Jint/Native/Reflect/ReflectInstance.cs
+++ b/Jint/Native/Reflect/ReflectInstance.cs
@@ -10,7 +10,7 @@ namespace Jint.Native.Reflect;
 /// <summary>
 /// https://www.ecma-international.org/ecma-262/6.0/index.html#sec-reflect-object
 /// </summary>
-[JsObject(UseShape = true)]
+[JsObject]
 internal sealed partial class ReflectInstance : BuiltinShapeObject
 {
     private readonly Realm _realm;
diff --git a/Jint/Native/Temporal/Now/TemporalNow.cs b/Jint/Native/Temporal/Now/TemporalNow.cs
index 9639aadb9..c2541ee94 100644
--- a/Jint/Native/Temporal/Now/TemporalNow.cs
+++ b/Jint/Native/Temporal/Now/TemporalNow.cs
@@ -10,7 +10,7 @@ namespace Jint.Native.Temporal;
 /// https://tc39.es/proposal-temporal/#sec-temporal-now-object
 /// </summary>
 [JsObject]
-internal sealed partial class TemporalNow : ObjectInstance
+internal sealed partial class TemporalNow : BuiltinShapeObject
 {
     private readonly Realm _realm;