diff --git a/.size-limit.json b/.size-limit.json
index 9491dca7cd..8f6b1a4be7 100644
--- a/.size-limit.json
+++ b/.size-limit.json
@@ -30,7 +30,7 @@
   {
     "name": "all",
     "path": "build/*",
-    "limit": "833.1 kB",
+    "limit": "833.6 kB",
     "brotli": false,
     "gzip": false
   }
diff --git a/src/cli.ts b/src/cli.ts
index b3edaaddbe..be53189a90 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -86,6 +86,7 @@ export const argv = minimist(process.argv.slice(2), {
 
 export async function main() {
   await import('./globals.js')
+  argv.ext = normalizeExt(argv.ext)
   if (argv.cwd) $.cwd = argv.cwd
   if (argv.verbose) $.verbose = true
   if (argv.quiet) $.quiet = true
@@ -302,3 +303,9 @@ export function isMain(
 
   return false
 }
+
+export function normalizeExt(ext?: string) {
+  if (!ext) return
+  if (!/^\.?\w+(\.\w+)*$/.test(ext)) throw new Error(`Invalid extension ${ext}`)
+  return ext[0] === '.' ? ext : `.${ext}`
+}
diff --git a/test/cli.test.js b/test/cli.test.js
index e9a9cce303..27201dbf67 100644
--- a/test/cli.test.js
+++ b/test/cli.test.js
@@ -16,7 +16,7 @@ import assert from 'node:assert'
 import { test, describe, before, after } from 'node:test'
 import { fileURLToPath } from 'node:url'
 import '../build/globals.js'
-import { isMain } from '../build/cli.js'
+import { isMain, normalizeExt } from '../build/cli.js'
 
 const __filename = fileURLToPath(import.meta.url)
 const spawn = $.spawn
@@ -267,4 +267,11 @@ describe('cli', () => {
       }
     })
   })
+
+  test('normalizeExt()', () => {
+    assert.equal(normalizeExt('.ts'), '.ts')
+    assert.equal(normalizeExt('ts'), '.ts')
+    assert.equal(normalizeExt(), undefined)
+    assert.throws(() => normalizeExt('.'))
+  })
 })