diff --git a/frontend/package-lock.json b/frontend/package-lock.json
index aade7bd2..adbfcc9e 100644
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
@@ -23,6 +23,7 @@
         "@types/jsdom": "^21.1.0",
         "@typescript-eslint/eslint-plugin": "^8.0.0",
         "@typescript-eslint/parser": "^8.0.0",
+        "@ungap/structured-clone": "^1.3.1",
         "babel-loader": "^9.1.0",
         "copy-webpack-plugin": "^14.0.0",
         "css-loader": "^6.8.0",
@@ -3140,9 +3141,9 @@
       }
     },
     "node_modules/@ungap/structured-clone": {
-      "version": "1.3.0",
-      "resolved": "https://registry.npmjs.org/@ungap/structured-clone/-/structured-clone-1.3.0.tgz",
-      "integrity": "sha512-WmoN8qaIAo7WTYWbAZuG8PYEhn5fkz7dZrqTBZ7dtt//lL2Gwms1IcnQ5yHqjDfX8Ft5j4YzDM23f87zBfDe9g==",
+      "version": "1.3.1",
+      "resolved": "https://registry.npmjs.org/@ungap/structured-clone/-/structured-clone-1.3.1.tgz",
+      "integrity": "sha512-mUFwbeTqrVgDQxFveS+df2yfap6iuP20NAKAsBt5jDEoOTDew+zwLAOilHCeQJOVSvmgCX4ogqIrA0mnyr08yQ==",
       "dev": true,
       "license": "ISC"
     },
diff --git a/frontend/package.json b/frontend/package.json
index fc809f76..f57d3ceb 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -24,6 +24,7 @@
     "@types/jsdom": "^21.1.0",
     "@typescript-eslint/eslint-plugin": "^8.0.0",
     "@typescript-eslint/parser": "^8.0.0",
+    "@ungap/structured-clone": "^1.3.1",
     "babel-loader": "^9.1.0",
     "copy-webpack-plugin": "^14.0.0",
     "css-loader": "^6.8.0",
diff --git a/frontend/src/__tests__/setup.ts b/frontend/src/__tests__/setup.ts
index c5286012..329de724 100644
--- a/frontend/src/__tests__/setup.ts
+++ b/frontend/src/__tests__/setup.ts
@@ -4,6 +4,7 @@
 import '@testing-library/jest-dom';
 import { TextEncoder, TextDecoder } from 'util';
 import { webcrypto } from 'crypto';
+import structuredClonePolyfill from '@ungap/structured-clone';
 
 // Polyfill TextEncoder/TextDecoder for Node.js environment
 global.TextEncoder = TextEncoder;
@@ -61,8 +62,24 @@ Object.defineProperty(global, 'Chart', {
 // the Node.js global structuredClone to the window scope). The utils.ts
 // deepClone function delegates to structuredClone; without this the suite
 // throws "ReferenceError: structuredClone is not defined" (finding 11-N1).
+//
+// Prefer the environment-provided native implementation when present; only
+// polyfill when absent. The polyfill is @ungap/structured-clone, a faithful
+// implementation of the HTML structured clone algorithm (preserves
+// undefined-valued properties, Date, Map, Set, RegExp, cycles; throws
+// TypeError on functions), unlike the previous JSON round-trip which
+// validated weaker semantics than production browsers (TEST-07). When the
+// host Node has a native structuredClone, @ungap delegates to it; otherwise
+// it uses its own serialize/deserialize. In jsdom-on-jest, Date/Map/Set
+// primordials are shared with Node so `instanceof` keeps working on cloned
+// values regardless of which path runs.
 if (typeof globalThis.structuredClone === 'undefined') {
-  globalThis.structuredClone = <T>(val: T): T => JSON.parse(JSON.stringify(val)) as T;
+  globalThis.structuredClone = (<T>(value: T, options?: StructuredSerializeOptions): T => {
+    if (options?.transfer?.length) {
+      throw new Error('structuredClone polyfill does not support the transfer option');
+    }
+    return structuredClonePolyfill(value);
+  }) as typeof structuredClone;
 }
 
 // Mock alert and confirm
diff --git a/frontend/src/__tests__/ungap-structured-clone.d.ts b/frontend/src/__tests__/ungap-structured-clone.d.ts
new file mode 100644
index 00000000..68b01294
--- /dev/null
+++ b/frontend/src/__tests__/ungap-structured-clone.d.ts
@@ -0,0 +1,12 @@
+// Type declarations for @ungap/structured-clone, which ships no .d.ts files.
+// Used only by the Jest setup (setup.ts) to polyfill structuredClone with
+// faithful HTML structured clone semantics under jest-environment-jsdom.
+declare module '@ungap/structured-clone' {
+  interface StructuredCloneOptions {
+    transfer?: Transferable[];
+    json?: boolean;
+    lossy?: boolean;
+  }
+  const structuredClonePolyfill: <T>(value: T, options?: StructuredCloneOptions) => T;
+  export default structuredClonePolyfill;
+}
diff --git a/frontend/src/__tests__/utils.test.ts b/frontend/src/__tests__/utils.test.ts
index 14d025d6..3c5f4418 100644
--- a/frontend/src/__tests__/utils.test.ts
+++ b/frontend/src/__tests__/utils.test.ts
@@ -501,10 +501,9 @@ describe('providerBadgeHtml', () => {
 
 // ---------------------------------------------------------------------------
 // Regression tests for finding 11-N1: deepClone defaults to structuredClone.
-// Note: the jsdom test environment polyfills structuredClone with a JSON
-// round-trip (see setup.ts) because jsdom does not expose Node's built-in
-// structuredClone. As a result, undefined preservation can only be asserted
-// when the native structuredClone is available (not in jsdom).
+// The jsdom test environment polyfills structuredClone via
+// `@ungap/structured-clone` (see setup.ts), which implements the real HTML
+// structured clone algorithm, so full browser semantics are asserted here.
 // ---------------------------------------------------------------------------
 describe('deepClone (11-N1: structuredClone default)', () => {
   test('deep copy: mutation of clone does not affect source', () => {
@@ -528,6 +527,57 @@ describe('deepClone (11-N1: structuredClone default)', () => {
   });
 });
 
+// ---------------------------------------------------------------------------
+// Regression tests for TEST-07: the test-environment structuredClone must
+// implement real structured clone semantics, not a JSON round-trip. Each test
+// below fails against the previous JSON.parse(JSON.stringify(...)) polyfill
+// and passes against the browser/Node algorithm.
+// ---------------------------------------------------------------------------
+describe('deepClone (TEST-07: real structured clone semantics)', () => {
+  test('preserves undefined-valued properties (JSON round-trip drops them)', () => {
+    const obj: { a: number; b: number | undefined } = { a: 1, b: undefined };
+    const clone = deepClone(obj);
+    expect('b' in clone).toBe(true);
+    expect(clone.b).toBeUndefined();
+  });
+
+  test('preserves Date instances (JSON round-trip stringifies them)', () => {
+    const obj = { ts: new Date('2026-01-02T03:04:05.678Z') };
+    const clone = deepClone(obj);
+    expect(clone.ts).toBeInstanceOf(Date);
+    expect(clone.ts.getTime()).toBe(obj.ts.getTime());
+    expect(clone.ts).not.toBe(obj.ts);
+  });
+
+  test('preserves Map and Set (JSON round-trip degrades them to {})', () => {
+    const obj = { m: new Map([['k', 1]]), s: new Set([1, 2]) };
+    const clone = deepClone(obj);
+    expect(clone.m).toBeInstanceOf(Map);
+    expect(clone.m.get('k')).toBe(1);
+    expect(clone.m).not.toBe(obj.m);
+    expect(clone.s).toBeInstanceOf(Set);
+    expect(clone.s.has(2)).toBe(true);
+    expect(clone.s).not.toBe(obj.s);
+  });
+
+  test('handles cyclic references (JSON round-trip throws)', () => {
+    interface Cyclic {
+      name: string;
+      self?: Cyclic;
+    }
+    const obj: Cyclic = { name: 'loop' };
+    obj.self = obj;
+    const clone = deepClone(obj);
+    expect(clone).not.toBe(obj);
+    expect(clone.self).toBe(clone);
+  });
+
+  test('throws on functions like the browser implementation', () => {
+    // JSON round-trip silently drops functions; structured clone must throw.
+    expect(() => deepClone({ fn: (): number => 1 })).toThrow();
+  });
+});
+
 describe('jsonClone (explicit JSON-serialisation variant)', () => {
   test('drops undefined values (expected JSON behaviour)', () => {
     const obj = { a: 1, b: undefined };