diff --git a/src/config.ts b/src/config.ts
index 525ec39ca..8c5ecec5b 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -11,7 +11,7 @@ import { createResolver, findPath } from '@nuxt/kit'
 import { resolveModulePath } from 'exsolve'
 import { getPackageInfoSync } from 'local-pkg'
 
-import { applyEnv, loadKit } from './utils.ts'
+import { applyEnv, deepCopy, loadKit } from './utils.ts'
 import { NuxtVitestEnvironmentOptionsPlugin } from './module/plugins/options.ts'
 
 interface GetVitestConfigOptions {
@@ -192,7 +192,7 @@ export async function getVitestConfigFromNuxt(
       },
       test: {
         environmentOptions: {
-          nuxtRuntimeConfig: applyEnv(structuredClone(options.nuxt.options.runtimeConfig), {
+          nuxtRuntimeConfig: applyEnv(deepCopy(options.nuxt.options.runtimeConfig), {
             prefix: 'NUXT_',
             env: await setupDotenv(defu(loadNuxtOptions.dotenv, {
               cwd: rootDir,
@@ -426,7 +426,7 @@ async function resolveConfig<T extends ViteUserConfig & { test?: VitestConfig }
     await getVitestConfigFromNuxt(undefined, {
       dotenv: config.test?.environmentOptions?.nuxt?.dotenv,
       nitroEnvironment: config.test?.environmentOptions?.nuxt?.nitroEnvironment,
-      overrides: structuredClone(overrides),
+      overrides: deepCopy(overrides),
     }) satisfies ViteUserConfig & { test: NonNullable<T['test']> },
   ) as T & { test: NonNullable<T['test']> }
 
diff --git a/src/utils.ts b/src/utils.ts
index c184d17ab..dbd3d350e 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -55,6 +55,46 @@ export function applyEnv(obj: Record<string, any>, opts: EnvOptions, parentKey =
   return obj
 }
 
+/**
+ * Deep-copy plain objects and arrays, passing anything else through by reference.
+ *
+ * `structuredClone` throws `DataCloneError` on proxies and functions, both of which turn up in
+ * `nuxt.options` (module mutation tracking wraps options in proxies) and in user-provided config
+ * overrides. Only plain containers need copying here; the clone exists to avoid mutating the
+ * caller's objects.
+ */
+export function deepCopy<T>(input: T, seen = new WeakMap<object, any>()): T {
+  if (typeof input !== 'object' || input === null) {
+    return input
+  }
+
+  const proto = Object.getPrototypeOf(input)
+  if (proto !== Object.prototype && proto !== Array.prototype && proto !== null) {
+    return input
+  }
+
+  const existing = seen.get(input)
+  if (existing) {
+    return existing
+  }
+
+  if (Array.isArray(input)) {
+    const copy: any[] = []
+    seen.set(input, copy)
+    for (const item of input) {
+      copy.push(deepCopy(item, seen))
+    }
+    return copy as T
+  }
+
+  const copy: Record<string, any> = {}
+  seen.set(input, copy)
+  for (const key in input) {
+    copy[key] = deepCopy((input as Record<string, any>)[key], seen)
+  }
+  return copy as T
+}
+
 export async function loadKit(rootDir: string): Promise<typeof import('@nuxt/kit')> {
   try {
     const kitPath = resolveModulePath('@nuxt/kit', { from: tryResolveNuxt(rootDir) || rootDir })
diff --git a/test/unit/config.spec.ts b/test/unit/config.spec.ts
new file mode 100644
index 000000000..041e5801b
--- /dev/null
+++ b/test/unit/config.spec.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it } from 'vitest'
+import type { Nuxt, ViteConfig } from '@nuxt/schema'
+
+import { getVitestConfigFromNuxt } from '../../src/config.ts'
+import { deepCopy } from '../../src/utils.ts'
+
+describe('deepCopy', () => {
+  it('should copy through proxies', () => {
+    const source = new Proxy({ nested: new Proxy({ a: 1 }, {}) }, {})
+    const copy = deepCopy(source)
+    expect(copy).toEqual({ nested: { a: 1 } })
+    copy.nested.a = 2
+    expect(source.nested.a).toBe(1)
+  })
+
+  it('should preserve functions and class instances by reference', () => {
+    const fn = () => 'hi'
+    const date = new Date(0)
+    const copy = deepCopy({ fn, date, list: [fn] })
+    expect(copy.fn).toBe(fn)
+    expect(copy.date).toBe(date)
+    expect(copy.list[0]).toBe(fn)
+  })
+
+  it('should handle circular references', () => {
+    const source: Record<string, unknown> = { a: 1 }
+    source.self = source
+    const copy = deepCopy(source)
+    expect(copy.self).toBe(copy)
+  })
+})
+
+describe('getVitestConfigFromNuxt', () => {
+  it('should resolve a runtimeConfig containing a proxy', async () => {
+    const nuxt = {
+      options: {
+        appDir: process.cwd(),
+        modulesDir: [process.cwd()],
+        runtimeConfig: {
+          public: new Proxy({ debug: { hydration: true } }, {}),
+        },
+        routeRules: {},
+        app: {},
+        build: { transpile: [] },
+      },
+    } as unknown as Nuxt
+
+    const config = await getVitestConfigFromNuxt({
+      nuxt,
+      viteConfig: { plugins: [] } as unknown as ViteConfig,
+    })
+
+    expect(config.test.environmentOptions!.nuxtRuntimeConfig).toEqual({
+      public: { debug: { hydration: true } },
+    })
+  })
+})