diff --git a/packages/compiler/src/backend/llvm/emitter.ts b/packages/compiler/src/backend/llvm/emitter.ts
index 52ee5cf8..e1abc132 100644
--- a/packages/compiler/src/backend/llvm/emitter.ts
+++ b/packages/compiler/src/backend/llvm/emitter.ts
@@ -4679,14 +4679,12 @@ export class LlEmitter {
       throw new InternalCompilerError(`llvm emitter bug: live dyn ref of ${key}`);
     }
     const prefix = `sc_ldr_${this.liveDynRefAdapters.size}`;
-    const adapter = streamTypedRefMaterializeAdapter(
+    return streamTypedRefMaterializeAdapter(
       this,
       t,
-      { prefix, adapters: new Map() },
+      { prefix, adapters: this.liveDynRefAdapters },
       `${prefix}_materialize`,
     );
-    this.liveDynRefAdapters.set(key, adapter);
-    return adapter;
   }
 
   liveDynUnionRefAdapter(
diff --git a/packages/compiler/src/backend/llvm/expr-dynamic.ts b/packages/compiler/src/backend/llvm/expr-dynamic.ts
index 24ac5683..a32deaa5 100644
--- a/packages/compiler/src/backend/llvm/expr-dynamic.ts
+++ b/packages/compiler/src/backend/llvm/expr-dynamic.ts
@@ -48,16 +48,7 @@ export function emitDynamicExpr(host: LlvmEmitterContext, e: ExprOf<"dynFrom" |
             throw new InternalCompilerError(`llvm emitter bug: live dyn ref of ${typeKey(v.type)}`);
           }
           const key = typeKey(v.type);
-          let adapter = host.liveDynRefAdapters.get(key);
-          if (!adapter) {
-            const prefix = `sc_ldr_${host.liveDynRefAdapters.size}`;
-            adapter = host.streamTypedRefMaterializeAdapter(
-              v.type,
-              { prefix, adapters: new Map() },
-              `${prefix}_materialize`,
-            );
-            host.liveDynRefAdapters.set(key, adapter);
-          }
+          const adapter = host.liveDynRefAdapter(v.type);
           const rc = vAdapters(host.shapeHost, v.type);
 
           const boxed = B.tmp();
diff --git a/packages/compiler/src/backend/llvm/expr-stream-bridges.ts b/packages/compiler/src/backend/llvm/expr-stream-bridges.ts
index 6baa05f9..5ffd0d4a 100644
--- a/packages/compiler/src/backend/llvm/expr-stream-bridges.ts
+++ b/packages/compiler/src/backend/llvm/expr-stream-bridges.ts
@@ -559,6 +559,9 @@ export function streamTypedRefMaterializeAdapter(host: LlvmEmitterContext,
         return adapter;
       }
     }
+    // Publish both symbols before their bodies: class-array and callback
+    // converters can request this same adapter while emitting the commit.
+    adapter.commit = `@${snapshot}_commit`;
     adapter.commit = host.streamTypedRefCommitAdapter(t, snapshot);
     const B = new BlockBuilder();
 
diff --git a/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts b/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts
index 223844e6..0f8bdec1 100644
--- a/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts
+++ b/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts
@@ -16,6 +16,12 @@ interface Dispatch {
   classes: Set<string>;
 }
 
+interface PropertyReceiver {
+  info: ClassInfo;
+  capsule: ClassInfo;
+  key: string;
+}
+
 interface PropertyDispatch {
   name: string;
   write: boolean;
@@ -316,16 +322,18 @@ export class ClassDynamicDispatch {
       }
     }
     for (const dispatch of this.properties.values()) {
-      for (const info of this.propertyCandidates(lowerer, dispatch.name)) {
-        if (dispatch.classes.has(info.def.name)) continue;
-        dispatch.classes.add(info.def.name);
+      for (const plan of this.propertyReceivers(lowerer)) {
+        const { info } = plan;
+        if (!isClassOwnEnumerableFieldName(dispatch.name) ||
+            !info.fields.has(dispatch.name) && !findMethodOn(lowerer, info, `get:${dispatch.name}`) && !findMethodOn(lowerer, info, `set:${dispatch.name}`)) continue;
+        if (dispatch.classes.has(plan.key)) continue;
+        dispatch.classes.add(plan.key);
         const loc = dispatch.fn.loc;
-        const type: IrType = { kind: "object", className: info.def.name };
-        const receiver = varRef("p.0", DYN, loc);
+        const { receiver, condition } = this.propertyReceiver(plan, loc);
         const before = lowerer.diags.length;
         let branch: IrStmt[];
         try {
-          branch = this.propertyBody(lowerer, dispatch, info, { kind: "dynCheck", value: receiver, type, loc });
+          branch = this.propertyBody(lowerer, dispatch, info, receiver);
         } catch (error) {
           if (!(error instanceof PoisonError) || !info.decl) throw error;
           const fence = lowerer.deferToRuntimeFence(before, info.decl, { kind: "statement" });
@@ -333,23 +341,18 @@ export class ClassDynamicDispatch {
           branch = [fence];
         }
         dispatch.fn.body.unshift({
-          kind: "if", cond: {
-            kind: "libCall", fn: "dyn.typedRefIs", args: [receiver, { kind: "strLit", value: typeKey(type), type: STRING, loc }], type: BOOL, loc,
-          }, then: branch, else_: null, loc,
+          kind: "if", cond: condition, then: branch, else_: null, loc,
         });
         changed = true;
       }
     }
     for (const dispatch of this.computed.values()) {
-      for (const className of this.boxed) {
-        if (dispatch.classes.has(className)) continue;
-        const info = lowerer.classes.get(className);
-        if (!info || info.builtinEmitter || info.builtinStream || info.builtinError) continue;
-        dispatch.classes.add(className);
+      for (const plan of this.propertyReceivers(lowerer)) {
+        if (dispatch.classes.has(plan.key)) continue;
+        const { info } = plan;
+        dispatch.classes.add(plan.key);
         const loc = dispatch.fn.loc;
-        const type: IrType = { kind: "object", className };
-        const value = varRef("p.0", DYN, loc);
-        const receiver: IrExpr = { kind: "dynCheck", value, type, loc };
+        const { receiver, condition } = this.propertyReceiver(plan, loc);
         const names = new Set([...info.fields.keys()].filter(isClassOwnEnumerableFieldName));
         for (let owner: ClassInfo | null = info; owner; owner = owner.base) {
           for (const method of owner.methods.keys()) {
@@ -372,9 +375,7 @@ export class ClassDynamicDispatch {
             kind: "strEq", left: varRef(dispatch.keyLocal, STRING, loc), right: { kind: "strLit", value: name, type: STRING, loc }, negated: false, type: BOOL, loc,
           }, then: body, else_: null, loc });
         }
-        dispatch.fn.body.splice(dispatch.branchIndex, 0, { kind: "if", cond: {
-          kind: "libCall", fn: "dyn.typedRefIs", args: [value, { kind: "strLit", value: typeKey(type), type: STRING, loc }], type: BOOL, loc,
-        }, then: branch, else_: null, loc });
+        dispatch.fn.body.splice(dispatch.branchIndex, 0, { kind: "if", cond: condition, then: branch, else_: null, loc });
         changed = true;
       }
     }
@@ -400,13 +401,34 @@ export class ClassDynamicDispatch {
     add(root);
   }
 
-  private propertyCandidates(lowerer: Lowerer, name: string): ClassInfo[] {
-    if (!isClassOwnEnumerableFieldName(name)) return [];
-    return [...this.boxed].flatMap((className) => {
-      const info = lowerer.classes.get(className);
-      if (!info || info.builtinEmitter || info.builtinStream || info.builtinError) return [];
-      return info.fields.has(name) || findMethodOn(lowerer, info, `get:${name}`) || findMethodOn(lowerer, info, `set:${name}`) ? [info] : [];
-    });
+  /** A capsule retains its static type, while its object may be a subclass.
+   * Preorder insertion lets later branches test the most derived layout first. */
+  private propertyReceivers(lowerer: Lowerer): PropertyReceiver[] {
+    const plans: PropertyReceiver[] = [];
+    for (const name of this.boxed) {
+      const capsule = lowerer.classes.get(name);
+      if (!capsule || capsule.builtinEmitter || capsule.builtinStream || capsule.builtinError) continue;
+      const visit = (info: ClassInfo): void => {
+        plans.push({ info, capsule, key: JSON.stringify([name, info.def.name]) });
+        for (const child of info.subclasses) visit(child);
+      };
+      visit(capsule);
+    }
+    return plans;
+  }
+
+  private propertyReceiver(plan: PropertyReceiver, loc: SrcLoc): { receiver: IrExpr; condition: IrExpr } {
+    const value = varRef("p.0", DYN, loc);
+    const type: IrType = { kind: "object", className: plan.capsule.def.name };
+    const checked: IrExpr = { kind: "dynCheck", value, type, loc };
+    const matches: IrExpr = { kind: "libCall", fn: "dyn.typedRefIs", args: [value,
+      { kind: "strLit", value: typeKey(type), type: STRING, loc }], type: BOOL, loc };
+    if (plan.info === plan.capsule) return { receiver: checked, condition: matches };
+    return {
+      receiver: { kind: "downcast", value: checked, type: { kind: "object", className: plan.info.def.name }, loc },
+      condition: { kind: "ternary", cond: matches, then: { kind: "instanceOf", value: checked,
+        className: plan.info.def.name, type: BOOL, loc }, else_: { kind: "boolLit", value: false, type: BOOL, loc }, type: BOOL, loc },
+    };
   }
 
   private propertyBody(lowerer: Lowerer, dispatch: PropertyDispatch, info: ClassInfo, receiver: IrExpr): IrStmt[] {
diff --git a/packages/compiler/src/frontend/lowering/expressions/object-literals.ts b/packages/compiler/src/frontend/lowering/expressions/object-literals.ts
index 134d0b1f..ef3e4e27 100644
--- a/packages/compiler/src/frontend/lowering/expressions/object-literals.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/object-literals.ts
@@ -325,6 +325,11 @@ export function lowerDynObjectLiteral(
     let v = boxValue
       ? boxValue(valueExpr as ts.Expression, raw)
       : lowerer.coerceToExpected(raw, DYN);
+    // A JS object's array-valued properties retain the same source array.
+    // Repeated references must compare equal and mutations remain shared.
+    if (!boxValue && isJsSourceFile(prop.getSourceFile()) && v.kind === "dynFrom" && v.value.type.kind === "array") {
+      v = { ...v, liveRef: true };
+    }
     if (v.type.kind !== "dyn") {
       const convDiagsBefore = lowerer.diags.length;
       try {
diff --git a/packages/compiler/src/frontend/lowering/lower-calls.ts b/packages/compiler/src/frontend/lowering/lower-calls.ts
index 1fd9c756..e942e981 100644
--- a/packages/compiler/src/frontend/lowering/lower-calls.ts
+++ b/packages/compiler/src/frontend/lowering/lower-calls.ts
@@ -26,7 +26,7 @@ import { lowerAbsenceProbe, lowerPromiseAllTupleCall, lowerPromiseRejectCall, st
 import { isSafeToDiscard } from "./expressions/evaluation-safety.js";
 import { tryLowerExpression } from "./expressions/try-lower-expression.js";
 import { httpClientFnBindingOf, isStreamUndefCallExpr, lowerCompatReqStreamOptionalCall, lowerHttpClientFnCall } from "./lower-server.js";
-import { EMITTER_API_MEMBERS, exactInstanceClassOf, findGenericMethodOn, lowerClassGenericMethodCall, lowerStaticMethodCall, type ClassInfo } from "./lower-classes.js";
+import { EMITTER_API_MEMBERS, exactInstanceClassOf, findGenericMethodOn, lowerClassGenericMethodCall, lowerStaticMethodCall, returnsOnlyThis, type ClassInfo } from "./lower-classes.js";
 import { classCallbackCall, isClassCallback } from "./class-callbacks.js";
 import { emitterRooted, lowerEmitterMethodCall } from "./lower-event-emitter.js";
 import { lowerConsoleInspectArg, lowerFormatCall } from "./lower-inspect.js";
@@ -6768,6 +6768,12 @@ const inliningPredicates = new Set<ts.Symbol>();
   function reconcileOverloadReturn(lowerer: Lowerer, expr: ts.CallExpression | ts.TaggedTemplateExpression, call: IrExpr): IrExpr {
     const rsig = lowerer.checker.getResolvedSignature(expr);
     const rdecl = rsig ? lowerer.checker.signatureDeclaration(rsig) : undefined;
+    // Fluent JS overrides share the base's return ABI, but their result
+    // still has the receiver's subclass layout at the call site.
+    if (call.type.kind === "object" && rdecl && ts.isMethodDeclaration(rdecl) &&
+        isJsSourceFile(rdecl.getSourceFile()) && returnsOnlyThis(rdecl)) {
+      call = lowerer.maybeNarrow(call, expr);
+    }
     const resolvedOverload =
       rsig && rdecl &&
       (ts.isFunctionDeclaration(rdecl) || ts.isMethodDeclaration(rdecl)) &&
diff --git a/packages/compiler/src/frontend/lowering/lower-classes.ts b/packages/compiler/src/frontend/lowering/lower-classes.ts
index 68e99fb7..bf5f666e 100644
--- a/packages/compiler/src/frontend/lowering/lower-classes.ts
+++ b/packages/compiler/src/frontend/lowering/lower-classes.ts
@@ -30,6 +30,24 @@ import { isNativeProxyInitializer, lowerNativeProxy } from "./expressions/native
 import { classStaticDataFor } from "./class-static-data.js";
 import { lowerInstanceConstructorNew } from "./class-instance-constructor.js";
 
+export function returnsOnlyThis(member: ts.MethodDeclaration): boolean {
+  const body = member.body;
+  const last = body?.statements.at(-1);
+  if (!body || !last || !ts.isReturnStatement(last)) return false;
+  let valid = true;
+  const visit = (node: ts.Node): void => {
+    if (ts.isFunctionLike(node)) return;
+    if (ts.isReturnStatement(node)) {
+      let value = node.expression;
+      while (value && ts.isParenthesizedExpression(value)) value = value.expression;
+      if (!value || value.kind !== ts.SyntaxKind.ThisKeyword) valid = false;
+    }
+    node.forEachChild(visit);
+  };
+  body.forEachChild(visit);
+  return valid;
+}
+
 function functionLocalClass(decl: ts.ClassLikeDeclaration): boolean {
   if (!ts.isClassExpression(decl)) return false;
   for (let parent: ts.Node | undefined = decl.parent; parent && !ts.isSourceFile(parent); parent = parent.parent) {
@@ -545,13 +563,16 @@ export interface GenericClassInfo {
 /** The undefined-armed union of a JS class property's inferred type — the
    * honest slot for a field first assigned outside the constructor's top
    * level (undefined until the write runs, Node-exact). Null when the
-   * inference is unmappable, checked-dynamic (dyn stays out of class
-   * fields — KEEP NARROW), or an arm-less kind that cannot join a union
+   * inference is unmappable or an arm-less kind that cannot join a union
    * (genResultRecord's list, including scalar-backed Date values). */
   function undefArmedFieldType(lowerer: Lowerer, p: ts.Symbol): IrType | null {
     const t = lowerer.checker.getTypeOfSymbol(p);
-    const mapped = lowerer.mapTypeOf(t);
-    if (!mapped || mapped.kind === "void" || mapped.kind === "dyn") return null;
+    const site = lowerer.checker.declarationsOf(p)[0];
+    let mapped = lowerer.mapTypeOf(t) ?? (site ? dynFallbackType(lowerer, site, t) : null);
+    if (mapped && site) mapped = jsOpenObjectType(site, mapped, lowerer.shapes, lowerer.unions);
+    if (!mapped || mapped.kind === "void") return null;
+    // An implicit JS value already carries undefined in its native box.
+    if (mapped.kind === "dyn") return mapped;
     const byKey = new Map<string, IrType>();
     const arms = mapped.kind === "union" ? (lowerer.unions.get(mapped.unionId)?.arms ?? []) : [mapped];
     for (const a of arms) {
@@ -1911,6 +1932,11 @@ export function collectClassShapeInner(lowerer: Lowerer, decl: ts.ClassLikeDecla
             );
           }
           if (overridden && isJsSourceFile(member.getSourceFile())) {
+            // Fluent overrides return the same native receiver. Keep the
+            // inherited return ABI and upcast `this` when lowering returns.
+            if (overridden.sig.ret.kind === "object" && base &&
+                (base.def.name === overridden.sig.ret.className || lowerer.isSubclassOf(base.def.name, overridden.sig.ret.className)) &&
+                returnsOnlyThis(member)) ft.ret = overridden.sig.ret;
             // An unannotated JS parameter can use the base's typed ABI.
             // Defaults stay local to each declaration, so only inherited
             // required/optional slots participate in this refinement.
@@ -2386,6 +2412,10 @@ export function collectClassShapeInner(lowerer: Lowerer, decl: ts.ClassLikeDecla
           if (isJsSourceFile(decl.getSourceFile())) {
             const armed = undefArmedFieldType(lowerer, p);
             if (armed !== null) {
+              // Unknown JS properties use the instance bag: they do not
+              // exist until assigned, and a subclass may declare the same
+              // name as an ordinary layout field.
+              if (armed.kind === "dyn") continue;
               fields.set(p.name, armed);
               fieldOrder.push({ name: p.name, type: armed, initializer: undefined });
               continue;
diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index 112b11a6..250995cd 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -2163,8 +2163,12 @@ function lowerExprInner(lowerer: Lowerer, expr: ts.Expression): IrExpr {
           `reading the abstract property '${expr.name.text}' through a '${lowerer.checker.typeToString(lowerer.typeOf(expr.expression))}'-typed receiver (abstract property declarations are erased at runtime, so no shared slot exists — type the receiver as the concrete class, or declare an abstract getter instead)`,
         );
       }
-      if (isDynTypedRefType(recvLowered.type)) {
-        const info = lowerer.classes.get(recvLowered.type.className);
+      // Array reads may add an undefined arm even when the checker still
+      // reports a class. Its property bag must remain reachable, while
+      // an absent element takes the normal checked-value TypeError.
+      const nativeReceiver = lowerer.stripUndefinedArm(recvLowered.type);
+      if (isDynTypedRefType(nativeReceiver)) {
+        const info = lowerer.classes.get(nativeReceiver.className);
         if (info && !info.def.runtime && !info.builtinError && !info.builtinEmitter && !info.builtinStream) {
           return lowerer.maybeNarrow({
             kind: "dynKeyGet", value: lowerer.coerceToExpected(recvLowered, DYN),
@@ -4679,7 +4683,7 @@ export function lowerOptionalNumber(
     // (`pkg.workspaces.packages["0"]` — the lowering world types the
     // unknown-rooted chain `any`); a non-dyn lowering falls through to
     // the fences below (re-lowering is pure IR construction).
-    if (receiverIr?.kind === "dyn" || receiverIr === null) {
+    if (receiverIr?.kind === "dyn" || receiverIr?.kind === "object" || receiverIr?.kind === "union" || receiverIr === null) {
       const obj = lowerer.lowerExpr(expr.expression);
       // A generic mapped type can stay unresolved at this body use even
       // though its instantiated parameter has a concrete record ABI.
@@ -4690,7 +4694,7 @@ export function lowerOptionalNumber(
           return lowerer.lowerRecordKeyRead(expr, obj.type.shapeId, shape);
         }
       }
-      if (obj.type.kind === "dyn") {
+      if (obj.type.kind === "dyn" || isDynTypedRefType(obj.type)) {
         const rawKey = lowerer.lowerExpr(expr.argumentExpression);
         // Number, bool, and DYN keys stringify (ToPropertyKey) — the
         // dyn-keyed read `catchWarning[warning.name]` where the property
@@ -4699,7 +4703,7 @@ export function lowerOptionalNumber(
         if (key.type.kind === "string") {
           const opt = hasOptionalChainGuard(expr.expression);
           return lowerer.maybeNarrow(
-            { kind: "dynKeyGet", key, ...(opt ? { optional: true as const } : {}), value: obj, type: DYN, loc: locOf(expr) },
+            { kind: "dynKeyGet", key, ...(opt ? { optional: true as const } : {}), value: lowerer.coerceToExpected(obj, DYN), type: DYN, loc: locOf(expr) },
             expr,
           );
         }
@@ -9913,6 +9917,9 @@ function representedClassFieldTarget(
     if (receiverIr?.kind === "object") {
       return classFieldTarget(lowerer, access.expression, receiverIr, access.name.text);
     }
+    if (receiverIr?.kind === "union" && hasClassPayload(lowerer, receiverIr)) {
+      return representedClassFieldTarget(lowerer, access.expression, access.name.text, lowerer.lowerExpr(access.expression));
+    }
     if (receiverIr?.kind === "record") {
       const shape = lowerer.shapes.get(receiverIr.shapeId);
       const fieldType = shape?.fields.find((f) => f.name === access.name.text)?.type;
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index 580aad8b..225ba666 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -39,7 +39,7 @@ import { isNativeProxyInitializer } from "./expressions/native-proxy.js";
 import { tryLowerExpression } from "./expressions/try-lower-expression.js";
 import { lowerUnionFieldWrite } from "./expressions/union-field-write.js";
 import { UNSUPPORTED, checkerPanicDiag, isCheckerPanic, requiresDynamicDiag } from "../../diagnostics/diagnostic.js";
-import { isParseArgsDynTypeName, isUnitOnlyTsType, unitOnlyUnion } from "../type-mapper.js";
+import { isParseArgsDynTypeName, isUnitOnlyTsType, jsOpenObjectType, unitOnlyUnion } from "../type-mapper.js";
 import { canonicalBuiltinModule } from "../builtin-modules.js";
 import { isRelativeSpecifier } from "../workspace-registry.js";
 import { probeNodeRequireRefusal } from "../npm.js";
@@ -3830,7 +3830,8 @@ export function lowerVarDecl(lowerer: Lowerer, decl: ts.VariableDeclaration, isL
     }
     const preservesObjectIdentity = init.type.kind === "dyn" &&
       (inferredObjectType?.kind === "record" ||
-        (isJsSourceFile(decl.getSourceFile()) && inferredObjectType?.kind === "func") ||
+        (isJsSourceFile(decl.getSourceFile()) && inferredObjectType !== null &&
+          (inferredObjectType.kind === "func" || jsOpenObjectType(decl, inferredObjectType, lowerer.shapes, lowerer.unions).kind === "dyn")) ||
         (isJsSourceFile(decl.getSourceFile()) && inferredObjectType !== null &&
           (isUnitType(inferredObjectType) || inferredObjectType.kind === "union" && lowerer.unions.get(inferredObjectType.unionId)?.arms.every(isUnitType)))) &&
       !decl.type && !hasJsTypeAnnotation(decl);
diff --git a/packages/compiler/src/ir/ir.ts b/packages/compiler/src/ir/ir.ts
index 3b4320ec..6ae11b61 100644
--- a/packages/compiler/src/ir/ir.ts
+++ b/packages/compiler/src/ir/ir.ts
@@ -6333,6 +6333,8 @@ function canBoxDynComposite(
     case "regex":
     case "url":
       return true;
+    case "object":
+      return isDynTypedRefType(t);
     case "func":
       return canBoxFuncIntoDyn(t, getRecord, getUnion);
     case "set":
@@ -6386,6 +6388,10 @@ export function canDynCheckTo(
   if (t.kind === "bytes") return true;
   if (t.kind === "classval") return true;
   if (t.kind === "object" && t.className === "%Error") return true;
+  // Native class arrays validate each branded reference, never fabricate
+  // class instances from plain object data.
+  if (isDynTypedRefType(t)) return true;
+  if (t.kind === "array" && isDynTypedRefType(t.elem)) return true;
   if (t.kind === "func") return canAdaptDynFuncTo(t, getRecord, getUnion);
   if (DYN_HANDLE_KINDS.has(t.kind)) return true;
   if (t.kind === "union") {
diff --git a/packages/compiler/test/native-class-array-boundaries.test.ts b/packages/compiler/test/native-class-array-boundaries.test.ts
new file mode 100644
index 00000000..30e3e52c
--- /dev/null
+++ b/packages/compiler/test/native-class-array-boundaries.test.ts
@@ -0,0 +1,42 @@
+import { spawnSync } from "node:child_process";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { expect, test } from "vitest";
+import { compile } from "../src/index.js";
+
+test("checked class arrays reject unbranded and differently branded elements", async () => {
+  const dir = mkdtempSync("/tmp/scriptc-class-arrays-");
+  try {
+    const entry = join(dir, "main.ts");
+    writeFileSync(entry, `
+class Item { value = 1; }
+class Other { value = 1; }
+function check(input: unknown): void {
+  try { const items = input as Item[]; console.log('accepted', items.length); }
+  catch (error) { console.log('rejected', error instanceof TypeError); }
+}
+check([new Item()]);
+check([new Other()]);
+check([{value: 1}]);
+const mixed: unknown[] = [];
+mixed.push(new Item()); mixed.push({value: 1});
+check(mixed);
+check(null);
+check([1]);
+check([]);
+`);
+    const result = await compile(entry, {
+      dynamic: false, outDir: dir, outPath: join(dir, "program"),
+      sanitize: process.env["SCRIPTC_SAN"] === "1",
+    });
+    if (!result.ok) throw new Error(JSON.stringify(result.diagnostics));
+    const child = spawnSync(result.binaryPath, [], {encoding: "utf8", timeout: 30_000});
+    expect(child.error).toBeUndefined();
+    expect({status: child.status, signal: child.signal, stdout: child.stdout, stderr: child.stderr}).toEqual({
+      status: 0, signal: null, stderr: "",
+      stdout: "accepted 1\nrejected true\nrejected true\nrejected true\nrejected true\nrejected true\naccepted 0\n",
+    });
+  } finally {
+    rmSync(dir, {recursive: true, force: true});
+  }
+});
diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index 7ca1cd6e..c366ad0e 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -11823,6 +11823,42 @@
     "<repo>/tests/corpus/js-nested-native-descriptors.js"
    ],
    "diags": []
+  },
+  "<repo>/tests/corpus/js-class-fluent-overrides.js": {
+   "order": [
+    "<repo>/tests/corpus/js-class-fluent-overrides.js"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/js-class-subclass-properties.js": {
+   "order": [
+    "<repo>/tests/corpus/js-class-subclass-properties.js"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/js-object-shared-arrays.js": {
+   "order": [
+    "<repo>/tests/corpus/js-object-shared-arrays.js"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/js-optional-class-fields.js": {
+   "order": [
+    "<repo>/tests/corpus/js-optional-class-fields.js"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/native-class-array-boundaries.ts": {
+   "order": [
+    "<repo>/tests/corpus/native-class-array-boundaries.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/native-class-type-predicates.ts": {
+   "order": [
+    "<repo>/tests/corpus/native-class-type-predicates.ts"
+   ],
+   "diags": []
   }
  }
 }
diff --git a/packages/runtime/src/scr_json.c b/packages/runtime/src/scr_json.c
index 98a1922a..273a879e 100644
--- a/packages/runtime/src/scr_json.c
+++ b/packages/runtime/src/scr_json.c
@@ -1951,6 +1951,9 @@ const ScrDynJsvalOps *scr_dyn_jsval_ops(void) {
 
 bool scr_dyn_isl_typeof_is(const ScrDyn *d, const char *name) {
   if (d->kind == SCR_DYN_TYPED_REF) {
+    if (d->v.typed_ref.type_key_len >= 7 &&
+        memcmp(d->v.typed_ref.type_key, "object:", 7) == 0)
+      return strcmp(name, "object") == 0;
     ScrDyn *materialized = scr_dyn_typed_ref_materialize(d);
     bool out = scr_dyn_isl_typeof_is(materialized, name);
     if (materialized->kind != SCR_DYN_JSVAL) {
@@ -1978,6 +1981,8 @@ bool scr_dyn_isl_typeof_is(const ScrDyn *d, const char *name) {
 
 bool scr_dyn_isl_is_array(const ScrDyn *d) {
   if (d->kind == SCR_DYN_TYPED_REF) {
+    if (d->v.typed_ref.type_key_len >= 7 &&
+        memcmp(d->v.typed_ref.type_key, "object:", 7) == 0) return false;
     ScrDyn *materialized = scr_dyn_typed_ref_materialize(d);
     bool out = materialized->kind == SCR_DYN_ARR ||
                scr_dyn_isl_is_array(materialized);
@@ -2425,6 +2430,9 @@ bool scr_dyn_truthy(const ScrDyn *d) {
 ScrStr *scr_dyn_typeof(const ScrDyn *d) {
   const char *s;
   if (d->kind == SCR_DYN_TYPED_REF) {
+    if (d->v.typed_ref.type_key_len >= 7 &&
+        memcmp(d->v.typed_ref.type_key, "object:", 7) == 0)
+      return scr_str_new("object", 6);
     ScrDyn *materialized = scr_dyn_typed_ref_materialize(d);
     ScrStr *out = scr_dyn_typeof(materialized);
     scr_dyn_release(materialized);
@@ -5963,11 +5971,18 @@ static bool scr_ffi_signature_equal(const ScrDyn *entry, const ScrDyn *definitio
   if (!definition || definition->kind != SCR_DYN_OBJ) return false;
   ScrDyn *expected = scr_dyn_obj_get(entry, "arguments", 9);
   ScrDyn *actual = scr_dyn_obj_get(definition, "arguments", 9);
-  if (!actual || actual->kind != SCR_DYN_ARR || actual->v.arr.len != expected->v.arr.len ||
-      !scr_ffi_abi_equal(scr_dyn_obj_get(entry, "return", 6), scr_dyn_obj_get(definition, "return", 6))) return false;
-  for (size_t i = 0; i < actual->v.arr.len; i++)
-    if (!scr_ffi_abi_equal(actual->v.arr.items[i], expected->v.arr.items[i])) return false;
-  return true;
+  if (!actual) return false;
+  // JS object properties can retain a native array by reference. Read its
+  // current contents when checking the requested ABI, just like a dyn array.
+  ScrDyn *view = actual->kind == SCR_DYN_TYPED_REF
+    ? scr_dyn_typed_ref_materialize(actual) : scr_dyn_retain(actual);
+  bool matches = view && view->kind == SCR_DYN_ARR && view->v.arr.len == expected->v.arr.len &&
+    scr_ffi_abi_equal(scr_dyn_obj_get(entry, "return", 6), scr_dyn_obj_get(definition, "return", 6));
+  if (matches) for (size_t i = 0; i < view->v.arr.len; i++) {
+    if (!scr_ffi_abi_equal(view->v.arr.items[i], expected->v.arr.items[i])) { matches = false; break; }
+  }
+  scr_dyn_release(view);
+  return matches;
 }
 
 static void scr_ffi_callback_remove(ScrDyn *entry, ScrDyn *catalog) {
diff --git a/tests/corpus/js-class-fluent-overrides.js b/tests/corpus/js-class-fluent-overrides.js
new file mode 100644
index 00000000..c3ffbf4e
--- /dev/null
+++ b/tests/corpus/js-class-fluent-overrides.js
@@ -0,0 +1,28 @@
+class Base {
+  constructor() { this.value = 1; }
+  copy(source) { this.value = source.value; return this; }
+  dispatch(source) { return this.copy(source); }
+}
+class Child extends Base {
+  constructor() { super(); this.extra = 2; }
+  copy(source) { super.copy(source); this.extra = source.extra; return this; }
+  childOnly() { return this.extra; }
+}
+class Leaf extends Child {
+  constructor() { super(); this.leaf = 3; }
+  copy(source) {
+    super.copy(source);
+    if (source.leaf === 0) return (this);
+    this.leaf = source.leaf;
+    return this;
+  }
+}
+const source = new Leaf();
+source.value = 4; source.extra = 5; source.leaf = 6;
+const target = new Leaf();
+console.log('direct', target.copy(source) === target, target.value, target.extra, target.leaf);
+console.log('chain', target.copy(source).childOnly(), target.copy(source).leaf);
+source.extra = 8;
+console.log('virtual', target.dispatch(source) === target, target.extra);
+source.leaf = 0;
+console.log('early', target.copy(source) === target, target.leaf);
diff --git a/tests/corpus/js-class-subclass-properties.js b/tests/corpus/js-class-subclass-properties.js
new file mode 100644
index 00000000..19e76030
--- /dev/null
+++ b/tests/corpus/js-class-subclass-properties.js
@@ -0,0 +1,43 @@
+class Base {
+  constructor() { this.base = 1; }
+  read(key) { return this[key]; }
+  write(key, value) { this[key] = value; }
+  named() { return this.extra; }
+  attach(value) { this.extra = value; }
+  remember(key, value) {
+    if (this.cache === undefined) this.cache = {};
+    this.cache[key] = value;
+  }
+  recall(key) { return this.cache[key]; }
+  saved() {
+    const cache = this.cache;
+    if (cache === undefined) return 'empty';
+    return Object.keys(cache).join(',');
+  }
+}
+class Child extends Base {
+  constructor() { super(); this.extra = 2; this.storage = 3; }
+  get doubled() { return this.storage * 2; }
+  set doubled(value) { this.storage = value / 2; }
+}
+class Leaf extends Child {
+  constructor() { super(); this.leaf = 4; }
+  get doubled() { return this.storage * 3; }
+  set doubled(value) { this.storage = value / 3; }
+}
+const base = new Base(), child = new Child(), leaf = new Leaf();
+console.log('absent', base.read('extra'), Object.keys(base).join(','));
+base.attach({answer: 42});
+console.log('added', base.read('extra').answer, Object.keys(base).join(','));
+console.log('inherited', child.read('extra'), child.named(), leaf.read('leaf'));
+child.write('extra', 9); leaf.write('leaf', 10);
+child.write('doubled', 12); leaf.write('doubled', 12);
+console.log('writes', child.extra, leaf.leaf, child.storage, leaf.storage);
+console.log('accessors', child.read('doubled'), leaf.read('doubled'));
+let order = '';
+function receiver() { order += 'r'; return leaf; }
+function key() { order += 'k'; return 'leaf'; }
+console.log('read', receiver()[key()], order);
+base.remember('first', 1); base.remember('second', {value: 2});
+console.log('cache', base.recall('first'), base.recall('second').value, Object.keys(base.cache).join(','));
+console.log('saved', base.saved(), child.saved());
diff --git a/tests/corpus/js-object-shared-arrays.js b/tests/corpus/js-object-shared-arrays.js
new file mode 100644
index 00000000..1fb79aa3
--- /dev/null
+++ b/tests/corpus/js-object-shared-arrays.js
@@ -0,0 +1,11 @@
+const shared = [1, 2, 3];
+const a = {values: shared}, b = {nested: {values: shared}};
+console.log('identity', a.values === b.nested.values, a.values === shared);
+a.values[1] = 8;
+console.log('write', shared[1], b.nested.values[1]);
+shared.push(4);
+console.log('push', a.values.length, b.nested.values.join(','));
+b.nested.values.pop();
+console.log('pop', shared.length, a.values.join(','));
+const separate = {values: [1, 8, 3]};
+console.log('distinct', separate.values === a.values);
diff --git a/tests/corpus/js-optional-class-fields.js b/tests/corpus/js-optional-class-fields.js
new file mode 100644
index 00000000..f51890b4
--- /dev/null
+++ b/tests/corpus/js-optional-class-fields.js
@@ -0,0 +1,26 @@
+class Vector {
+  constructor() { this.x = 1; this.y = 2; }
+  get doubled() { return this.x * 2; }
+  set doubled(value) { this.x = value / 2; }
+}
+class Holder {
+  initialize() { this.vector = new Vector(); }
+}
+const holder = new Holder();
+holder.initialize();
+let calls = 0;
+function receiver() { calls++; return holder; }
+receiver().vector.x = 7;
+holder.vector.y += 3;
+holder.vector.doubled = 18;
+console.log('nested', holder.vector.x, holder.vector.y, calls);
+
+class Positioned {
+  constructor() { Object.defineProperty(this, 'position', {value: new Vector(), enumerable: true}); }
+}
+const objects = [new Positioned()];
+console.log('array', objects[0].position.x);
+objects[0].position.x = 4;
+console.log('write', objects[0].position.x);
+try { console.log(objects[1].position.x); }
+catch (error) { console.log('missing', error instanceof TypeError, error.message); }
diff --git a/tests/corpus/native-class-array-boundaries.ts b/tests/corpus/native-class-array-boundaries.ts
new file mode 100644
index 00000000..452528a0
--- /dev/null
+++ b/tests/corpus/native-class-array-boundaries.ts
@@ -0,0 +1,29 @@
+class Plane {
+  constant: number;
+  constructor(value: number) { this.constant = value; }
+}
+function roundtrip(value: unknown): Plane[] { return value as Plane[]; }
+function optional(value: unknown): Plane[] | null { return value as Plane[] | null; }
+const plane = new Plane(2);
+const planes = [plane];
+const copy = roundtrip(planes);
+console.log('array', copy.length, copy[0] === plane, copy[0].constant);
+copy[0].constant = 4;
+console.log('mutation', plane.constant, optional(planes)![0] === plane, optional(null) === null);
+function boxedRecord(value: unknown) { return value; }
+const boxed = boxedRecord({plane, planes});
+const view = boxed as {plane: unknown; planes: unknown[]};
+console.log('record', view.plane === plane, view.planes[0] === plane);
+const callback: unknown = (value: Plane): number => value.constant;
+console.log('callback', (callback as (value: Plane) => number)(plane));
+
+class Tree {
+  children: Tree[] = [];
+  select: (node: Tree) => Tree[] = (node) => node.children;
+}
+const root = new Tree(), child = new Tree();
+root.children.push(child);
+const tree: unknown = root;
+const restored = tree as Tree;
+console.log('recursive', restored === root, restored.select(root)[0] === child);
+console.log('view', Object.keys(tree as object).sort().join(','));
diff --git a/tests/corpus/native-class-type-predicates.ts b/tests/corpus/native-class-type-predicates.ts
new file mode 100644
index 00000000..80c08c7b
--- /dev/null
+++ b/tests/corpus/native-class-type-predicates.ts
@@ -0,0 +1,15 @@
+class Opaque {
+  entries: Map<string, number> = new Map();
+  self: Opaque | null = null;
+}
+function inspect(value: unknown): void {
+  console.log(typeof value, typeof value === 'object', typeof value === 'function', Array.isArray(value));
+}
+const value = new Opaque();
+value.self = value;
+inspect(value);
+inspect([value]);
+inspect(null);
+inspect(1);
+inspect('text');
+value.self = null;
diff --git a/tests/ffi/catalog.mjs b/tests/ffi/catalog.mjs
index 521b03fa..7c0e16ae 100644
--- a/tests/ffi/catalog.mjs
+++ b/tests/ffi/catalog.mjs
@@ -2,8 +2,9 @@ import { createRequire } from "node:module";
 const requireModule = createRequire(import.meta.url);
 const ffi = requireModule("node:ffi");
 console.log(ffi === requireModule("node:ffi"));
+const u64Arguments = ["u64"];
 const definitions = {
-  nativeU64: { arguments: ["u64"], return: "u64" },
+  nativeU64: { arguments: u64Arguments, return: "u64" },
   nativePointer: { arguments: ["pointer"], return: "pointer" },
   nativeI64: { arguments: ["int64"], return: "int64" },
   nativeByte: { arguments: ["bool"], return: "uint8" },
@@ -44,4 +45,9 @@ second.lib.close();
 if (process.env.FFI_STATIC === "1") {
   try { ffi.dlopen(process.env.FFI_LIBRARY, { nativeU64: { arguments: ["i64"], return: "u64" } }); }
   catch { console.log("signature mismatch"); }
+  u64Arguments[0] = "i64";
+  try { ffi.dlopen(process.env.FFI_LIBRARY, definitions); }
+  catch { console.log("mutated signature"); }
+  try { ffi.dlopen(process.env.FFI_LIBRARY, { nativeU64: { return: "u64" } }); }
+  catch (error) { console.log("missing arguments", error.name, error.code); }
 }
diff --git a/tests/fixtures/three/materials.mjs b/tests/fixtures/three/materials.mjs
new file mode 100644
index 00000000..3bbe66d9
--- /dev/null
+++ b/tests/fixtures/three/materials.mjs
@@ -0,0 +1,45 @@
+import { MeshNormalMaterial } from 'three/src/materials/MeshNormalMaterial.js';
+import { MeshBasicMaterial } from 'three/src/materials/MeshBasicMaterial.js';
+import { MeshStandardMaterial } from 'three/src/materials/MeshStandardMaterial.js';
+import { Plane } from 'three/src/math/Plane.js';
+import { Vector3 } from 'three/src/math/Vector3.js';
+
+const normal = new MeshNormalMaterial({wireframe: true, opacity: 0.5, transparent: true});
+console.log('normal', normal.type, normal.wireframe, normal.opacity, normal.transparent);
+normal.setValues({wireframe: false, opacity: 0.8});
+normal.needsUpdate = true;
+console.log('updated', normal.wireframe, normal.opacity, normal.version);
+const copy = normal.clone();
+console.log('clone', copy !== normal, copy.type, copy.opacity, copy.normalScale.x);
+copy.normalScale.x = 2;
+console.log('independent', normal.normalScale.x, copy.normalScale.x);
+
+const plane = new Plane(new Vector3(1, 0, 0), 2);
+normal.setValues({clippingPlanes: [plane], clipIntersection: true});
+if (normal.clippingPlanes !== null) {
+  console.log('clip', normal.clippingPlanes.length, normal.clippingPlanes[0] === plane, normal.clipIntersection);
+  normal.clippingPlanes[0].constant = 3;
+}
+console.log('plane', plane.constant);
+normal.setValues({clippingPlanes: null});
+console.log('unclipped', normal.clippingPlanes === null);
+
+const basic = new MeshBasicMaterial({color: 0xff0000});
+console.log('red', basic.color.r, basic.color.g, basic.color.b);
+basic.setValues({color: 0x0000ff});
+console.log('blue', basic.color.r, basic.color.g, basic.color.b);
+console.log('basic-copy', basic.clone().color.b);
+const standard = new MeshStandardMaterial({roughness: 0.25, metalness: 0.8, color: 0x00ff00});
+console.log('standard', standard.type, standard.roughness, standard.metalness, standard.color.g);
+console.log('standard-copy', standard.clone().roughness, standard.clone().metalness);
+
+let count = 0;
+function disposed(event) {
+  count++;
+  console.log('event', event.type, event.target === normal, this === normal);
+}
+normal.addEventListener('dispose', disposed);
+normal.dispose();
+normal.removeEventListener('dispose', disposed);
+normal.dispose();
+console.log('events', count);
diff --git a/tests/fixtures/three/mesh.mjs b/tests/fixtures/three/mesh.mjs
new file mode 100644
index 00000000..ccfc6f79
--- /dev/null
+++ b/tests/fixtures/three/mesh.mjs
@@ -0,0 +1,33 @@
+import { Mesh } from 'three/src/objects/Mesh.js';
+import { BoxGeometry } from 'three/src/geometries/BoxGeometry.js';
+import { MeshNormalMaterial } from 'three/src/materials/MeshNormalMaterial.js';
+import { Scene } from 'three/src/scenes/Scene.js';
+import { Vector3 } from 'three/src/math/Vector3.js';
+
+const geometry = new BoxGeometry(2, 4, 6);
+const material = new MeshNormalMaterial();
+const mesh = new Mesh(geometry, material);
+console.log('mesh', mesh.type, mesh.isMesh, mesh.geometry === geometry, mesh.material === material);
+const scene = new Scene();
+scene.add(mesh);
+mesh.position.set(1, 2, 3);
+scene.updateMatrixWorld();
+const point = mesh.getVertexPosition(0, new Vector3());
+console.log('vertex', point.x, point.y, point.z);
+console.log('world', mesh.matrixWorld.elements[12], mesh.matrixWorld.elements[13], mesh.matrixWorld.elements[14]);
+const copy = mesh.clone();
+console.log('copy', copy !== mesh, copy.geometry === geometry, copy.material === material, copy.position.x);
+copy.position.x = 7;
+console.log('independent', mesh.position.x, copy.position.x);
+const other = new Mesh(geometry, material);
+console.log('fluent', other.copy(mesh) === other, other.copy(mesh).geometry === geometry);
+const child = new Mesh(geometry, material);
+child.position.y = 3;
+mesh.add(child);
+const recursive = mesh.clone();
+console.log('children', recursive.children.length, recursive.children[0] !== child, recursive.children[0].position.y);
+console.log('shallow', mesh.clone(false).children.length);
+const materials = [material, new MeshNormalMaterial({wireframe: true})];
+const multi = new Mesh(geometry, materials);
+const multiCopy = multi.clone();
+console.log('materials', multiCopy.material !== multi.material, multiCopy.material.length, multiCopy.material[0] === material, multiCopy.material[1].wireframe);
diff --git a/tests/harness/ffi-catalog.test.ts b/tests/harness/ffi-catalog.test.ts
index ccbdf484..8a88c886 100644
--- a/tests/harness/ffi-catalog.test.ts
+++ b/tests/harness/ffi-catalog.test.ts
@@ -31,7 +31,7 @@ describe.each(["llvm"] as const)("static node:ffi catalog, %s", backend => {
     expect(native.error).toBeUndefined();
     expect(native.stderr).toBe("");
     expect(native.status).toBe(0);
-    expect(native.stdout).toBe("true\n9007199254740993n\ntrue\n0n 0n\n65 255\n-9223372036854775808n 18446744073709551615n\n" + "TypeError ERR_INVALID_ARG_VALUE\n".repeat(12) + "5\narity\narity\nsignature\nclosed\n42n\nsignature mismatch\n");
+    expect(native.stdout).toBe("true\n9007199254740993n\ntrue\n0n 0n\n65 255\n-9223372036854775808n 18446744073709551615n\n" + "TypeError ERR_INVALID_ARG_VALUE\n".repeat(12) + "5\narity\narity\nsignature\nclosed\n42n\nsignature mismatch\nmutated signature\nmissing arguments Error SC2020\n");
     // A separately linked Node library is only the oracle. scriptc links
     // the object file and must work before this dynamic library exists.
     if (oracle) {
@@ -42,7 +42,7 @@ describe.each(["llvm"] as const)("static node:ffi catalog, %s", backend => {
       expect(node.error).toBeUndefined();
       expect(node.stderr).toBe(native.stderr);
       expect(node.status).toBe(native.status);
-      expect(node.stdout + "signature mismatch\n").toBe(native.stdout);
+      expect(node.stdout + "signature mismatch\nmutated signature\nmissing arguments Error SC2020\n").toBe(native.stdout);
     }
   });
 });
diff --git a/tests/harness/three.test.ts b/tests/harness/three.test.ts
index 2ee73914..617bb8ac 100644
--- a/tests/harness/three.test.ts
+++ b/tests/harness/three.test.ts
@@ -21,7 +21,7 @@ function run(command: string, args: string[]) {
   return { stdout: result.stdout, stderr: result.stderr, status: result.status, signal: result.signal };
 }
 
-const cases = ["math", "spatial", "attributes", "geometry", "scene"].flatMap((fixture) =>
+const cases = ["math", "spatial", "attributes", "geometry", "scene", "materials", "mesh"].flatMap((fixture) =>
   ["native", "wasm32-wasi"].map((target) => ({ fixture, target })));
 test.for(cases)("published three.js $fixture runs statically through LLVM on $target", async ({ fixture, target }, context) => {
   const entry = join(import.meta.dirname, `../fixtures/three/${fixture}.mjs`);