diff --git a/src/__tests__/utils.js b/src/__tests__/utils.js
index 68be33c..f637d6f 100644
--- a/src/__tests__/utils.js
+++ b/src/__tests__/utils.js
@@ -4,6 +4,8 @@ export const IS_JSDOM = window.navigator.userAgent.includes('jsdom')
 
 export const IS_HAPPYDOM = !IS_JSDOM // right now it's happy or js
 
+export const IS_JEST = Boolean(process.env.JEST_WORKER_ID)
+
 export const IS_SVELTE_5 = SVELTE_VERSION >= '5'
 
 export const MODE_LEGACY = 'legacy'
diff --git a/src/__tests__/vite-plugin.test.js b/src/__tests__/vite-plugin.test.js
new file mode 100644
index 0000000..f87f713
--- /dev/null
+++ b/src/__tests__/vite-plugin.test.js
@@ -0,0 +1,186 @@
+import { beforeEach, describe, expect, test, vi } from 'vitest'
+
+import { svelteTesting } from '../vite.js'
+import { IS_JEST } from './utils.js'
+
+describe.skipIf(IS_JEST)('vite plugin', () => {
+  beforeEach(() => {
+    vi.stubEnv('VITEST', '1')
+  })
+
+  test('does not modify config if disabled', () => {
+    const subject = svelteTesting({
+      resolveBrowser: false,
+      autoCleanup: false,
+      noExternal: false,
+    })
+
+    const config = {}
+    subject.config(config)
+
+    expect(config).toEqual({})
+  })
+
+  test('does not modify config if not Vitest', () => {
+    vi.stubEnv('VITEST', '')
+
+    const subject = svelteTesting()
+    const config = {}
+
+    subject.config(config)
+
+    expect(config).toEqual({})
+  })
+
+  test.each([
+    {
+      config: { resolve: { conditions: ['node'] } },
+      expectedConditions: ['browser', 'node'],
+    },
+    {
+      config: { resolve: { conditions: ['svelte', 'node'] } },
+      expectedConditions: ['svelte', 'browser', 'node'],
+    },
+  ])(
+    'adds browser condition if necessary',
+    ({ config, expectedConditions }) => {
+      const subject = svelteTesting()
+      const viteConfig = structuredClone(config)
+
+      subject.config(viteConfig)
+
+      expect(viteConfig).toMatchObject({
+        resolve: {
+          conditions: expectedConditions,
+        },
+      })
+    }
+  )
+
+  test.each([
+    {
+      config: {},
+      expectedConditions: [],
+    },
+    {
+      config: { resolve: { conditions: [] } },
+      expectedConditions: [],
+    },
+    {
+      config: { resolve: { conditions: ['svelte'] } },
+      expectedConditions: ['svelte'],
+    },
+  ])(
+    'skips browser condition if possible',
+    ({ config, expectedConditions }) => {
+      const subject = svelteTesting()
+      const viteConfig = structuredClone(config)
+
+      subject.config(viteConfig)
+
+      expect(viteConfig).toMatchObject({
+        resolve: {
+          conditions: expectedConditions,
+        },
+      })
+    }
+  )
+
+  test.each([
+    {
+      config: {},
+      expectedSetupFiles: [expect.stringMatching(/src\/vitest.js$/u)],
+    },
+    {
+      config: { test: { setupFiles: [] } },
+      expectedSetupFiles: [expect.stringMatching(/src\/vitest.js$/u)],
+    },
+    {
+      config: { test: { setupFiles: 'other-file.js' } },
+      expectedSetupFiles: [
+        'other-file.js',
+        expect.stringMatching(/src\/vitest.js$/u),
+      ],
+    },
+  ])('adds cleanup', ({ config, expectedSetupFiles }) => {
+    const subject = svelteTesting()
+    const viteConfig = structuredClone(config)
+
+    subject.config(viteConfig)
+
+    expect(viteConfig).toMatchObject({
+      test: {
+        setupFiles: expectedSetupFiles,
+      },
+    })
+  })
+
+  test.each([
+    {
+      config: { ssr: { noExternal: [] } },
+      expectedNoExternal: ['@testing-library/svelte'],
+    },
+    {
+      config: {},
+      expectedNoExternal: ['@testing-library/svelte'],
+    },
+    {
+      config: { ssr: { noExternal: 'other-file.js' } },
+      expectedNoExternal: ['other-file.js', '@testing-library/svelte'],
+    },
+    {
+      config: { ssr: { noExternal: /other/u } },
+      expectedNoExternal: [/other/u, '@testing-library/svelte'],
+    },
+  ])('adds noExternal rule', ({ config, expectedNoExternal }) => {
+    const subject = svelteTesting()
+    const viteConfig = structuredClone(config)
+
+    subject.config(viteConfig)
+
+    expect(viteConfig).toMatchObject({
+      ssr: {
+        noExternal: expectedNoExternal,
+      },
+    })
+  })
+
+  test.each([
+    {
+      config: { ssr: { noExternal: true } },
+      expectedNoExternal: true,
+    },
+    {
+      config: { ssr: { noExternal: '@testing-library/svelte' } },
+      expectedNoExternal: '@testing-library/svelte',
+    },
+    {
+      config: { ssr: { noExternal: /svelte/u } },
+      expectedNoExternal: /svelte/u,
+    },
+  ])('skips noExternal if able', ({ config, expectedNoExternal }) => {
+    const subject = svelteTesting()
+    const viteConfig = structuredClone(config)
+
+    subject.config(viteConfig)
+
+    expect(viteConfig).toMatchObject({
+      ssr: {
+        noExternal: expectedNoExternal,
+      },
+    })
+  })
+
+  test('bails on noExternal if input is unexpected', () => {
+    const subject = svelteTesting()
+    const viteConfig = structuredClone({ ssr: { noExternal: false } })
+
+    subject.config(viteConfig)
+
+    expect(viteConfig).toMatchObject({
+      ssr: {
+        noExternal: false,
+      },
+    })
+  })
+})
diff --git a/src/vite.js b/src/vite.js
index 0062b89..d8b8e20 100644
--- a/src/vite.js
+++ b/src/vite.js
@@ -7,12 +7,13 @@ import { fileURLToPath } from 'node:url'
  * Ensures Svelte is imported correctly in tests
  * and that the DOM is cleaned up after each test.
  *
- * @param {{resolveBrowser?: boolean, autoCleanup?: boolean}} options
+ * @param {{resolveBrowser?: boolean, autoCleanup?: boolean, noExternal?: boolean}} options
  * @returns {import('vite').Plugin}
  */
 export const svelteTesting = ({
   resolveBrowser = true,
   autoCleanup = true,
+  noExternal = true,
 } = {}) => ({
   name: 'vite-plugin-svelte-testing-library',
   config: (config) => {
@@ -27,6 +28,10 @@ export const svelteTesting = ({
     if (autoCleanup) {
       addAutoCleanup(config)
     }
+
+    if (noExternal) {
+      addNoExternal(config)
+    }
   },
 })
 
@@ -73,3 +78,43 @@ const addAutoCleanup = (config) => {
   test.setupFiles = setupFiles
   config.test = test
 }
+
+/**
+ * Add `@testing-library/svelte` to Vite's noExternal rules, if not present.
+ *
+ * This ensures `@testing-library/svelte` is processed by `@sveltejs/vite-plugin-svelte`
+ * in certain monorepo setups.
+ */
+const addNoExternal = (config) => {
+  const ssr = config.ssr ?? {}
+  let noExternal = ssr.noExternal ?? []
+
+  if (noExternal === true) {
+    return
+  }
+
+  if (typeof noExternal === 'string' || noExternal instanceof RegExp) {
+    noExternal = [noExternal]
+  }
+
+  if (!Array.isArray(noExternal)) {
+    return
+  }
+
+  for (const rule of noExternal) {
+    if (typeof rule === 'string' && rule === '@testing-library/svelte') {
+      return
+    }
+
+    if (
+      noExternal instanceof RegExp &&
+      noExternal.test('@testing-library/svelte')
+    ) {
+      return
+    }
+  }
+
+  noExternal.push('@testing-library/svelte')
+  ssr.noExternal = noExternal
+  config.ssr = ssr
+}
diff --git a/vite.config.js b/vite.config.js
index 76baf61..1ddeea5 100644
--- a/vite.config.js
+++ b/vite.config.js
@@ -11,6 +11,7 @@ export default defineConfig({
     setupFiles: ['./src/__tests__/_vitest-setup.js'],
     mockReset: true,
     unstubGlobals: true,
+    unstubEnvs: true,
     coverage: {
       provider: 'v8',
       include: ['src/**/*'],