Why can I not convey user-appropriate messages via throws from server actions?

[craigmcc]

I am looking at converting an old CRA-based app to NextJS 15 + React 19. In my old app, I have a bunch of database mutations (now becoming Server Actions) that would complain with thrown errors if something was wrong that the Zod schema for something did not detect. In this particular case, it's on the signup page for my app, which wants to prevent users with duplicate email addresses from signing up.

In my old app, the functional logic that checks for this (to avoid a probably incomprehensible error message about "foreign key violation" or similar to a user) throws an error that says something simple: "That email address is already in use". Simple, clear, gives the prospective user an option to do something (because we are still on the same page.

In dev mode in NextJS, it does the same thing. My sign up form does a try-catch around the logic that sets up a new user, and the catch block causes the above message to be displayed.

In prod mode, that doesn't happen. Instead, I get:

which, basically, says, the actual error cause was recorded in a server log message. That's actually true.

But what in the heck am I supposed to do for the user? What I would like is an option to actually return the message in the "throws" clause, since in my app those are purposely user directed (I also log more details on the server side, but that's not an issue that the user cares about).

Can I please have an option that tells NextJS in production mode to pass through any messages from a thrown error? I am totally fine if this is not the default -- but I would really rather not have to reinvent the entire relationship between client and server just to work around this NextJS limitation.

[frantisek-heca]

you not catching the throws? and returning the message?

[craigmcc]

I am catching the message. The problem is that, in production mode, NextJS is swallows the real message I am returning (the “That email address is already in use” that my server action threw), and replaces it with what is in the big red box.

In dev mode, it displays what I actually returned. In prod mode, it logs the real message on the server, and returns to the client a totally useless message (NextJS is censoring the actual message on the thrown error, and returning a useless message instead).