diff --git a/plugins/libs/plugins-runtime/src/lib/load-plugin.spec.ts b/plugins/libs/plugins-runtime/src/lib/load-plugin.spec.ts
index a46bbc8573..c8d31131a0 100644
--- a/plugins/libs/plugins-runtime/src/lib/load-plugin.spec.ts
+++ b/plugins/libs/plugins-runtime/src/lib/load-plugin.spec.ts
@@ -22,6 +22,7 @@ vi.mock('./create-plugin', () => ({
 vi.mock('./ses.js', () => ({
   ses: {
     harden: vi.fn().mockImplementation((obj) => obj),
+    hardenIntrinsics: vi.fn(),
   },
 }));
 
diff --git a/plugins/libs/plugins-runtime/src/lib/load-plugin.ts b/plugins/libs/plugins-runtime/src/lib/load-plugin.ts
index 990a9148a1..df62cd4fca 100644
--- a/plugins/libs/plugins-runtime/src/lib/load-plugin.ts
+++ b/plugins/libs/plugins-runtime/src/lib/load-plugin.ts
@@ -52,6 +52,16 @@ export const loadPlugin = async function (
 
     closeAllPlugins();
 
+    // hardenIntrinsics must be called BEFORE harden() to ensure that
+    // override taming (enablePropertyOverrides) converts prototype
+    // properties like Function.prototype.toString into accessor pairs.
+    // Without this, harden() would freeze Function.prototype with plain
+    // data properties, making them non-configurable, which causes
+    // enablePropertyOverrides to silently skip them when hardenIntrinsics
+    // runs later — resulting in "Cannot assign to read only property
+    // 'toString'" errors.
+    ses.hardenIntrinsics();
+
     const plugin = await createPlugin(
       ses.harden(context) as Context,
       manifest,