Setting basePath breaks Media collection image previews

[felixkpunkt]

Describe the Bug

Setting a basePath in the next.config.mjs seems to break the image previews in the admin area. The request URL to fetch the image does not contain the basePath.

Link to the code that reproduces this issue

https://github.com/felixkpunkt/payload-basePath-issue

Reproduction Steps

1. npx create-payload-app
2. set a custom basePath in the next.config.mjs
3. create a Media & save

Which area(s) are affected?

Not sure, area: core

Environment Info

Binaries:
  Node: 24.12.0
  npm: 11.6.2
  Yarn: N/A
  pnpm: 10.27.0
Relevant Packages:
  payload: 3.70.0
  next: 15.4.10
  @payloadcms/db-sqlite: 3.70.0
  @payloadcms/drizzle: 3.70.0
  @payloadcms/graphql: 3.70.0
  @payloadcms/next/utilities: 3.70.0
  @payloadcms/richtext-lexical: 3.70.0
  @payloadcms/translations: 3.70.0
  @payloadcms/ui/shared: 3.70.0
  react: 19.2.1
  react-dom: 19.2.1
Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Thu Jun  5 18:30:46 UTC 2025
  Available memory (MB): 15953
  Available CPU cores: 16

[felixkpunkt]

I did some digging and found a likely cause here: https://github.com/payloadcms/payload/blob/main/packages/payload/src/utilities/formatAdminURL.ts#L45

formatAdminURL reads directly from process.env.NEXT_BASE_PATH, while in my case args.basePath is also an empty string. As a result, the computed admin URLs for fetching the media miss the correct prefix.

When I explicitly set NEXT_BASE_PATH to match my configured basePath, the image previews in the admin panel start working again.

Interestingly, I was not able to reproduce this issue when running Payload directly from the monorepo source. The problem only occurs in a project created via npx create-payload-app, which suggests a difference between the workspace/dev setup and the published packages or production build behavior.

I initially wanted to fix this myself, but I’m not yet deep enough into the Payload core / Next integration setup to be fully confident about the correct fix. I’m therefore sharing my findings here in case they help point in the right direction.