Next Server Actions don't work with next.js version 15.4

[sbaechler]

Describe the bug

After updating to 15.4-canary I keep getting a server error when I post a form. The error seems to happen when parsing the return value from the server action. It is thrown by "server-action-reducer" which is a next.js module. Apparently the mimetype form/multipart is not allowed. (My form is not multipart)

When I downgrade to 15.3 the error disappears.

Your minimal, reproducible example

https://stackblitz.com/edit/tanstack-form-c425rd6a?file=package.json

Steps to reproduce

1. Enter a value between 8 and 12.
2. Hit Submit

Expected behavior

Submit should work with server action validation.

How often does this bug happen?

Every time

Screenshots or Videos

Platform

• OS: Mac OS
• Browser: Brave
• Node: 24

TanStack Form adapter

react-form

TanStack Form version

1.14.0

TypeScript version

No response

Additional context

No response

[luchillo17]

Did you change the stackblitz repro? I'm not seeing the same error than in the screenshot, the error I see is due to the expected shape of the errors from the server being different from the top level form errors loop (no issues in issueObject).