diff --git a/packages/plugin-rsc/e2e/basic.test.ts b/packages/plugin-rsc/e2e/basic.test.ts
index 30ffc231d..6e6aef111 100644
--- a/packages/plugin-rsc/e2e/basic.test.ts
+++ b/packages/plugin-rsc/e2e/basic.test.ts
@@ -46,6 +46,28 @@ test.describe('build-default', () => {
   defineTest(f)
 })
 
+test.describe('dev-non-optimized-cjs', () => {
+  test.beforeAll(async () => {
+    // remove explicitly added optimizeDeps.include
+    const editor = f.createEditor('vite.config.ts')
+    editor.edit((s) =>
+      s.replace(
+        `'@vitejs/test-dep-transitive-cjs > use-sync-external-store/shim/index.js',`,
+        ``,
+      ),
+    )
+  })
+
+  const f = useFixture({ root: 'examples/basic', mode: 'dev' })
+
+  test('show warning', async ({ page }) => {
+    await page.goto(f.url())
+    expect(f.proc().stderr()).toContain(
+      `[vite-rsc] found non-optimized CJS dependency in 'ssr' environment.`,
+    )
+  })
+})
+
 function defineTest(f: Fixture) {
   test('basic', async ({ page }) => {
     using _ = expectNoPageError(page)
diff --git a/packages/plugin-rsc/e2e/fixture.ts b/packages/plugin-rsc/e2e/fixture.ts
index b21ac65ca..aa61f332a 100644
--- a/packages/plugin-rsc/e2e/fixture.ts
+++ b/packages/plugin-rsc/e2e/fixture.ts
@@ -10,12 +10,16 @@ function runCli(options: { command: string; label?: string } & SpawnOptions) {
   const [name, ...args] = options.command.split(' ')
   const child = x(name!, args, { nodeOptions: options }).process!
   const label = `[${options.label ?? 'cli'}]`
+  let stdout = ''
+  let stderr = ''
   child.stdout!.on('data', (data) => {
+    stdout += stripVTControlCharacters(String(data))
     if (process.env.TEST_DEBUG) {
       console.log(styleText('cyan', label), data.toString())
     }
   })
   child.stderr!.on('data', (data) => {
+    stderr += stripVTControlCharacters(String(data))
     console.log(styleText('magenta', label), data.toString())
   })
   const done = new Promise<void>((resolve) => {
@@ -48,7 +52,14 @@ function runCli(options: { command: string; label?: string } & SpawnOptions) {
     }
   }
 
-  return { proc: child, done, findPort, kill }
+  return {
+    proc: child,
+    done,
+    findPort,
+    kill,
+    stdout: () => stdout,
+    stderr: () => stderr,
+  }
 }
 
 export type Fixture = ReturnType<typeof useFixture>
@@ -64,12 +75,13 @@ export function useFixture(options: {
   let baseURL!: string
 
   const cwd = path.resolve(options.root)
+  let proc!: ReturnType<typeof runCli>
 
   // TODO: `beforeAll` is called again on any test failure.
   // https://playwright.dev/docs/test-retries
   test.beforeAll(async () => {
     if (options.mode === 'dev') {
-      const proc = runCli({
+      proc = runCli({
         command: options.command ?? `pnpm dev`,
         label: `${options.root}:dev`,
         cwd,
@@ -144,6 +156,7 @@ export function useFixture(options: {
     root: cwd,
     url: (url: string = './') => new URL(url, baseURL).href,
     createEditor,
+    proc: () => proc,
   }
 }
 
diff --git a/packages/plugin-rsc/examples/basic/vite.config.ts b/packages/plugin-rsc/examples/basic/vite.config.ts
index b0eec5511..d4ee08d3b 100644
--- a/packages/plugin-rsc/examples/basic/vite.config.ts
+++ b/packages/plugin-rsc/examples/basic/vite.config.ts
@@ -151,8 +151,6 @@ export default { fetch: handler };
     },
     ssr: {
       optimizeDeps: {
-        // TODO: this should be somehow auto inferred or at least show a warning
-        // to guide users to `optimizeDeps.include`
         include: [
           '@vitejs/test-dep-transitive-cjs > use-sync-external-store/shim/index.js',
         ],
diff --git a/packages/plugin-rsc/src/plugin.ts b/packages/plugin-rsc/src/plugin.ts
index cc3861b52..c89213f94 100644
--- a/packages/plugin-rsc/src/plugin.ts
+++ b/packages/plugin-rsc/src/plugin.ts
@@ -20,7 +20,7 @@ import {
   normalizePath,
   parseAstAsync,
 } from 'vite'
-import { crawlFrameworkPkgs } from 'vitefu'
+import { crawlFrameworkPkgs, findClosestPkgJsonPath } from 'vitefu'
 import vitePluginRscCore from './core/plugin'
 import {
   type TransformWrapExportFilter,
@@ -838,9 +838,42 @@ globalThis.AsyncLocalStorage = __viteRscAyncHooks.AsyncLocalStorage;
       ? [validateImportPlugin()]
       : []),
     scanBuildStripPlugin(),
+    detectNonOptimizedCjsPlugin(),
   ]
 }
 
+function detectNonOptimizedCjsPlugin(): Plugin {
+  return {
+    name: 'rsc:detect-non-optimized-cjs',
+    apply: 'serve',
+    async transform(code, id) {
+      if (
+        id.includes('/node_modules/') &&
+        !id.startsWith(this.environment.config.cacheDir) &&
+        /\b(require|exports)\b/.test(code)
+      ) {
+        id = parseIdQuery(id).filename
+        let isEsm = id.endsWith('.mjs')
+        if (id.endsWith('.js')) {
+          const pkgJsonPath = await findClosestPkgJsonPath(path.dirname(id))
+          if (pkgJsonPath) {
+            const pkgJson = JSON.parse(
+              fs.readFileSync(pkgJsonPath, 'utf-8'),
+            ) as { type?: string }
+            isEsm = pkgJson.type === 'module'
+          }
+        }
+        if (!isEsm) {
+          this.warn(
+            `[vite-rsc] found non-optimized CJS dependency in '${this.environment.name}' environment. ` +
+              `It is recommended to manually add the dependency to 'environments.${this.environment.name}.optimizeDeps.include'.`,
+          )
+        }
+      }
+    },
+  }
+}
+
 function scanBuildStripPlugin(): Plugin {
   return {
     name: 'rsc:scan-strip',
@@ -1900,7 +1933,7 @@ function evalValue<T = any>(rawValue: string): T {
 // https://github.com/vitejs/vite-plugin-vue/blob/06931b1ea2b9299267374cb8eb4db27c0626774a/packages/plugin-vue/src/utils/query.ts#L13
 function parseIdQuery(id: string) {
   if (!id.includes('?')) return { filename: id, query: {} }
-  const [filename, rawQuery] = id.split(`?`, 2)
+  const [filename, rawQuery] = id.split(`?`, 2) as [string, string]
   const query = Object.fromEntries(new URLSearchParams(rawQuery))
   return { filename, query }
 }