Prevent crash when using @tailwindcss/cli using --watch on Windows

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Fixed
+
+- Ensure `@tailwindcss/cli` in `--watch` mode doesn't crash on Windows when `@source` points to a directory that doesn't exist ([#20242](https://github.com/tailwindlabs/tailwindcss/pull/20242))
 
 ## [4.3.1] - 2026-06-12
 

integrations/cli/index.test.ts

@@ -375,6 +375,35 @@ describe.each([
     },
   )
 
+  test(
+    "watch mode with unknown @source paths shouldn't crash on Windows",
+    {
+      fs: {
+        'package.json': json`
+          {
+            "dependencies": {
+              "tailwindcss": "workspace:^",
+              "@tailwindcss/cli": "workspace:^"
+            }
+          }
+        `,
+        'index.html': html`
+          <div class="underline"></div>
+        `,
+        'src/index.css': css`
+          @import 'tailwindcss';
+          @source "unknown-folder/**/*";
+        `,
+      },
+    },
+    async ({ fs, spawn }) => {
+      let process = await spawn(`${command} --input src/index.css --output dist/out.css --watch`)
+      await process.onStderr((m) => m.includes('Done in'))
+
+      await fs.expectFileToContain('dist/out.css', [candidate`underline`])
+    },
+  )
+
   test(
     'production build (stdin)',
     {

packages/@tailwindcss-cli/src/commands/build/index.ts

@@ -272,7 +272,7 @@ export async function handle(args: Result<ReturnType<typeof options>>) {
   if (args['--watch']) {
     let cleanupWatchers: (() => Promise<void>)[] = []
     cleanupWatchers.push(
-      await createWatchers(watchDirectories(scanner), async function handle(files) {
+      await createWatchers(await watchDirectories(scanner), async function handle(files) {
         try {
           // If the only change happened to the output file, then we don't want to
           // trigger a rebuild because that will result in an infinite loop.
@@ -347,7 +347,7 @@ export async function handle(args: Result<ReturnType<typeof options>>) {
 
             // Setup new watchers
             DEBUG && I.start('Setup new watchers')
-            let newCleanupFunction = await createWatchers(watchDirectories(scanner), handle)
+            let newCleanupFunction = await createWatchers(await watchDirectories(scanner), handle)
             DEBUG && I.end('Setup new watchers')
 
             // Clear old watchers
@@ -586,8 +586,22 @@ async function createWatchers(dirs: string[], cb: (files: string[]) => void) {
   }
 }
 
-function watchDirectories(scanner: Scanner) {
-  return [...new Set(scanner.normalizedSources.flatMap((globEntry) => globEntry.base))]
+async function watchDirectories(scanner: Scanner) {
+  let directories = (
+    await Promise.all(
+      scanner.normalizedSources.map(async (globEntry) => {
+        let resolvedPath = path.resolve(globEntry.base)
+        let realPath = await fs.realpath(resolvedPath).catch(() => resolvedPath)
+
+        return fs
+          .stat(realPath)
+          .then((stat) => (stat.isDirectory() ? [realPath] : []))
+          .catch(() => [])
+      }),
+    )
+  ).flat(1)
+
+  return Array.from(new Set(directories))
 }
 
 function dim(str: string) {