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Next Server Actions don't work with next.js version 15.4 #1611
Description
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
- Enter a value between 8 and 12.
- 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
Activity
luchillo17 commentedon Jul 16, 2025
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
issuesinissueObject).crutchcorn commentedon Jan 30, 2026
Can you retest with:
I believe we have a fix for many (if not all) Form SSR behaviors
crutchcorn commentedon Feb 13, 2026
Should be fixed in 1.28.1