Cannot create first user with custom `basePath` set in dev mode

[jefferyto]

Describe the Bug

After following the reproduction steps and clicking on the "Create" button on the "Create first user" screen, Payload tries to send a POST request to http://localhost:3000/api/users/first-register (the default api route) instead of http://localhost:3000/payload/api/users/first-register (the api route for basePath set to "/payload"), leading to a 404 response.

Interestingly, this only occurs in dev mode. When running a production build, the correct api route for first-register is used and the user is created successfully.

Link to the code that reproduces this issue

https://github.com/jefferyto/payload-basepath-first-register

Reproduction Steps

1. Checkout the reproduction repo (note that basePath is set to "/payload" in next.config.mjs), npm install, npm run dev
2. Open http://localhost:3000/payload/admin in a browser
3. Fill in the "Create first user" form and click the "Create" button

Which area(s) are affected? (Select all that apply)

area: core

Environment Info

Binaries:
  Node: 22.21.1
  npm: 11.6.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  payload: 3.64.0
  next: 15.4.7
  @payloadcms/db-sqlite: 3.64.0
  @payloadcms/drizzle: 3.64.0
  @payloadcms/graphql: 3.64.0
  @payloadcms/next/utilities: 3.64.0
  @payloadcms/richtext-lexical: 3.64.0
  @payloadcms/translations: 3.64.0
  @payloadcms/ui/shared: 3.64.0
  react: 19.1.0
  react-dom: 19.1.0
Operating System:
  Platform: linux
  Arch: x64
  Version: #36-Ubuntu SMP PREEMPT_DYNAMIC Sat Oct 11 02:18:29 UTC 2025
  Available memory (MB): 7225
  Available CPU cores: 4

[dotnize]

For us Payload 3.64 works fine, but we are getting this issue on Payload 3.65 and above.

These versions work fine for us:

Binaries:
  Node: 25.2.1
  npm: N/A
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  payload: 3.64.0
  next: 15.5.7
  @payloadcms/db-postgres: 3.64.0
  @payloadcms/drizzle: 3.64.0
  @payloadcms/graphql: 3.64.0
  @payloadcms/next/utilities: 3.64.0
  @payloadcms/translations: 3.64.0
  @payloadcms/ui/shared: 3.64.0
  react: 19.2.1
  react-dom: 19.2.1
Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Enterprise
  Available memory (MB): 16135
  Available CPU cores: 8

[cweseloh]

Take a look at dev tools network tab and check which path is being used for the API on that page. I've had similar issues with the basepath being missing in the API URL in dev mode. However, Payload says it's not something they have control over.