`ERR_REQUIRE_ESM` when using `@netlify/blobs` inside Netlify Background Functions (CJS entry requires ESM dependency)

[marcosvrs]

When using @netlify/blobs in a Netlify Background Function that is otherwise fully ESM, the deployed background function fails with the following runtime error:

{
  "errorType": "Error",
  "errorMessage": "require() of ES Module /var/task/node_modules/@netlify/runtime-utils/dist/main.js from /var/task/node_modules/@netlify/blobs/dist/main.cjs not supported.\nInstead change the require of main.js in /var/task/node_modules/@netlify/blobs/dist/main.cjs to a dynamic import() which is available in all CommonJS modules.",
  "code": "ERR_REQUIRE_ESM",
  "stack": [
    "Error [ERR_REQUIRE_ESM]: require() of ES Module /var/task/node_modules/@netlify/runtime-utils/dist/main.js from /var/task/node_modules/@netlify/blobs/dist/main.cjs not supported.",
    "Instead change the require of main.js in /var/task/node_modules/@netlify/blobs/dist/main.cjs to a dynamic import() which is available in all CommonJS modules.",
    "    at TracingChannel.traceSync (node:diagnostics_channel:322:14)",
    "    at Object.<anonymous> (/var/task/node_modules/@netlify/blobs/dist/main.cjs:42:28)"
  ]
}

Steps to reproduce:

1. Create a Netlify Background Function with:

   import { getStore } from '@netlify/blobs';

2. Deploy with the default esbuild bundler.
3. At runtime, Netlify loads @netlify/blobs/dist/main.cjs, which contains:

   const runtimeUtils = require('@netlify/runtime-utils/dist/main.js')

   …but @netlify/runtime-utils is ESM-only. This triggers ERR_REQUIRE_ESM.

Expected behavior:

• Background Functions importing @netlify/blobs should work out of the box when using ESM.
• The CJS build of @netlify/blobs should not require() an ESM-only dependency.

Actual behavior:

• Deploy ends up using the CJS build of @netlify/blobs, which fails because it require()s an ESM-only file from @netlify/runtime-utils.

Environment:

• Netlify Functions runtime: Node 22
• Bundler: default
• @netlify/build: 35.1.2
• @netlify/plugin-nextjs: 5.12.1
• @netlify/blobs version: 10.0.7/10.0.8
• @netlify/runtime-utils version: 2.1.0
• Next.js: 14.2.31

Workarounds tested:

• Adding:

  [functions]
  external_node_modules = ["@netlify/blobs", "@netlify/runtime-utils"]

  to netlify.toml doesn't work.

• Replacing:

  import { getStore } from '@netlify/blobs';

  with:

  const { getStore } = await import('@netlify/blobs');

  doesn't work.

Request:

• Could the CJS build of @netlify/blobs be adjusted not to require() ESM-only modules?
• Alternatively, could @netlify/blobs mark @netlify/runtime-utils as ESM-only and ensure the correct entrypoint is chosen consistently?