diff --git a/src/core/services/ipc.test.ts b/src/core/services/ipc.test.ts
index 7196d037..8fcfb2e6 100644
--- a/src/core/services/ipc.test.ts
+++ b/src/core/services/ipc.test.ts
@@ -1037,7 +1037,13 @@ test('registerIpcHandlers accepts per-controller profile config updates', async
     },
   };
   await saveHandler({}, update);
-  assert.deepEqual(controllerSaves, [update]);
+  assert.deepEqual(controllerSaves, [
+    {
+      ...update,
+      // Validation uses a null prototype to safely store arbitrary profile IDs.
+      profiles: { __proto__: null, ...update.profiles },
+    },
+  ]);
 
   await assert.rejects(async () => {
     await saveHandler(
diff --git a/src/core/services/tokenizer/golden-corpus-harness.ts b/src/core/services/tokenizer/golden-corpus-harness.ts
index ce17fa35..4a9b7d9e 100644
--- a/src/core/services/tokenizer/golden-corpus-harness.ts
+++ b/src/core/services/tokenizer/golden-corpus-harness.ts
@@ -396,7 +396,8 @@ function createInjectedScriptVm(store: ReplayMessageStore): (script: string) =>
     Set,
     String,
   });
-  return async (script: string) => await vm.runInContext(script, context);
+  // Clone results into the host realm, matching Electron's process boundary.
+  return async (script: string) => structuredClone(await vm.runInContext(script, context));
 }
 
 export function createReplayTokenizerDeps(fixture: GoldenFixture): TokenizerServiceDeps {
diff --git a/src/core/services/tokenizer/yomitan-scan-test-harness.ts b/src/core/services/tokenizer/yomitan-scan-test-harness.ts
index 01745a96..8c9efeaa 100644
--- a/src/core/services/tokenizer/yomitan-scan-test-harness.ts
+++ b/src/core/services/tokenizer/yomitan-scan-test-harness.ts
@@ -64,7 +64,8 @@ export async function runInjectedYomitanScript(
   script: string,
   handler: (action: string, params: unknown) => unknown,
 ): Promise<unknown> {
-  return await vm.runInNewContext(script, createYomitanScriptSandbox(handler));
+  // Clone results into the host realm, matching Electron's process boundary.
+  return structuredClone(await vm.runInNewContext(script, createYomitanScriptSandbox(handler)));
 }
 
 // Persistent page context shared across executeJavaScript calls, matching the
@@ -75,7 +76,8 @@ function createPersistentYomitanScriptRunner(
   handler: (action: string, params: unknown) => unknown,
 ): (script: string) => Promise<unknown> {
   const context = vm.createContext(createYomitanScriptSandbox(handler));
-  return async (script: string) => await vm.runInContext(script, context);
+  // Clone results into the host realm, matching Electron's process boundary.
+  return async (script: string) => structuredClone(await vm.runInContext(script, context));
 }
 
 // Deps whose parser window executes every injected script (profile metadata,