"Server Actions must be async functions." being reported incorrectly

[chungweileong94]

Link to the code that reproduces this issue

https://github.com/chungweileong94/nextjs-server-action-async-func-bug

To Reproduce

1. Start the server, pnpm dev
2. Navigate to the homepage

Current vs. Expected behavior

Expected:

• No error

Current

• "Server Actions must be async functions." error will shows up, but incorrectly.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:05:14 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T8103
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 20.12.0
  npm: 10.5.0
  Yarn: 1.22.21
  pnpm: 8.15.7
Relevant Packages:
  next: 15.0.3-canary.6 // Latest available version is detected (15.0.3-canary.6).
  eslint-config-next: N/A
  react: 19.0.0-rc-7c8e5e7a-20241101
  react-dom: 19.0.0-rc-7c8e5e7a-20241101
  typescript: 5.3.3
Next.js Config:
  output: N/A

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

Not sure

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

next dev (local), next build (local)

Additional context

I'm an author of a small library called server-act, which the library is a simple server action builder by using the function builder pattern to construct a server action, like

const action = builder
  .input(...)
  .action(...);

However, since Next.js 15, Next.js failed to detect the async function/server action correctly. For instance:

export const action = builder
  .input(() => { ... }) // <-- Next.js keeps saying that this needs to be an async function, message: "Server Actions must be async functions."
  .action(...); // <-- this will always return an async function, aka server action

The same thing happens to one of the similar library as well, TheEdoRan/next-safe-action#288.