[BUG] Windows: ERR_UNSUPPORTED_ESM_URL_SCHEME when previewing due to raw Windows paths in import()

[cording12]

Describe the bug

Bug Description

Running opennextjs-cloudflare preview on Windows can fail with the following error:

Error [ERR_UNSUPPORTED_ESM_URL_SCHEME]: Only URLs with a scheme in: file, data, and node are supported by the default ESM loader. On Windows, absolute paths must be valid file:// URLs. Received protocol 'c:'

This occurs because the retrieveCompiledConfig() function in cli/commands/utils.ts can dynamically import a local file using a raw absolute Windows path (C:\...) rather than a proper file:// URI on the line const config = await import(configPath).then((mod) => mod.default);.

This is potentially an issue only arising in monorepos, as I have not seen it happen elsewhere. The monorepo structure hoists @opennext/cloudflare to the node_modules in the project root.

This bug is also intermittent. The first time running the preview command can run successfully.

Steps to reproduce

1. Create a turbo repo: npx create-turbo@latest
2. CD into apps and run npm create cloudflare@latest -- my-next-app --framework=next --platform=workers
3. Run opennextjs-cloudflare preview
4. Observe the error

Expected behavior

System is able to build and run correctly

@opennextjs/cloudflare version

1.6.1

Wrangler version

4.25.1

next info output

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 10 Pro     
  Available memory (MB): 65365
  Available CPU cores: 16     
Binaries:
  Node: 22.17.0
  npm: 11.4.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.3.0
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.3
Next.js Config:
  output: N/A

Additional context

This can be temporarily patched for Windows users if you are running into this bug. A quick manual patch:

1. Locate the utils.ts file located in root\node_modules\@opennextjs\cloudflare\dist\cli\commands
2. Add import { pathToFileURL } from "url"; to the top of the file
3. Replace const config = await import(configPath).then((mod) => mod.default); with const config = await import(pathToFileURL(configPath).href).then((mod) => mod.default);

If this patch works successfully, you can automate the patching using patch-package so that you don't have to re-do this every time you re-install dependencies (or upgrade versions).

1. In your monorepo root, run npm i patch-package --save-dev
2. In your monorepo root, create the file : patches/@opennextjs+cloudflare+1.6.1.patch. The 1.6.1 denotes the version of the package where you must adjust accordingly, or leave as it is if you are using 1.6.1. Patches is a new folder you must create if it does not already exist.
3. Inside @opennextjs+cloudflare+1.6.1.patch file, insert the following:

diff --git a/node_modules/@opennextjs/cloudflare/dist/cli/commands/utils.js b/node_modules/@opennextjs/cloudflare/dist/cli/commands/utils.js
index 562ba80..d232437 100644
--- a/node_modules/@opennextjs/cloudflare/dist/cli/commands/utils.js
+++ b/node_modules/@opennextjs/cloudflare/dist/cli/commands/utils.js
@@ -7,6 +7,7 @@ import { printHeader, showWarningOnWindows } from "@opennextjs/aws/build/utils.j
 import logger from "@opennextjs/aws/logger.js";
 import { unstable_readConfig } from "wrangler";
 import { createOpenNextConfigIfNotExistent, ensureCloudflareConfig } from "../build/utils/index.js";
+import { pathToFileURL } from "url";
 export const nextAppDir = process.cwd();
 /**
  * Print headers and warnings for the CLI.
@@ -39,7 +40,8 @@ export async function retrieveCompiledConfig() {
         logger.error("Could not find compiled Open Next config, did you run the build command?");
         process.exit(1);
     }
-    const config = await import(configPath).then((mod) => mod.default);
+    // const config = await import(configPath).then((mod) => mod.default);
+    const config = await import(pathToFileURL(configPath).href).then((mod) => mod.default);
     ensureCloudflareConfig(config);
     return { config };
 }

4. Add "postinstall": "patch-package" to your root package.json scripts. This will automatically apply after running installs.

[srini-abhiram]

Thanks, the temp patch worked for me

[keshabmanni]

I'm also getting this error while running in preview.

[vicb]

Note that windows is not supported.
You should use WSL instead.

There should be a warning when you build.

[cording12]

I wish I could, but WSL introduces other problems for me (not related to opennext) - this is actually the path of least resistance if you can believe it.

As an update to others who tried the patch, you might find CI/CD pipelines in deployments can attempt to run it which will create a build failure; it isn't required for that though. I changed my postinstall script to the following which allows for it running in local dev, just not in prod:

"postinstall": "node -e \"if (process.env.NODE_ENV !== 'production' && process.env.CI !== 'true') { require('child_process').execSync('patch-package', {stdio: 'inherit'}) }\"",