facebook/react · error · Error

Server actions must be functions

Error message

Server actions must be functions

What it means

loadServerAction() resolves a server action id through the parcel server manifest, preloads its chunk, then requires it. Files marked 'use server' must export only functions, so when requireModule(reference) returns anything other than a function (undefined binding, object, class), the returned promise rejects with this error.

Source

Thrown at packages/react-server-dom-parcel/src/server/ReactFlightDOMServerNode.js:803

export function decodeAction<T>(body: FormData): Promise<() => T> | null {
  return decodeActionImpl(body, serverManifest);
}

export function decodeFormState<S>(
  actionResult: S,
  body: FormData,
): Promise<ReactFormState<S, ServerReferenceId> | null> {
  return decodeFormStateImpl(actionResult, body, serverManifest);
}

export function loadServerAction<F: (...any[]) => any>(id: string): Promise<F> {
  const reference = resolveServerReference<any>(serverManifest, id);
  return Promise.resolve(reference)
    .then(() => preloadModule(reference))
    .then(() => {
      const fn = requireModule(reference);
      if (typeof fn !== 'function') {
        throw new Error('Server actions must be functions');
      }
      return fn;
    });
}

View on GitHub (pinned to eafeac097b)

Solutions

1. Make every export in the 'use server' file an async function
2. Move constants/types/classes to a plain server-side module without 'use server' and import them from the action file
3. Clear the bundler cache and rebuild so the server manifest maps each action id to a live export

Example fix

// before ('use server' file)
'use server';
export const LIMITS = {max: 10}; // non-function export
export async function addItem(item) {...}

// after
// limits.js (no 'use server'): export const LIMITS = {max: 10};
'use server';
export async function addItem(item) {...}

try {
  const fn = await loadServerAction(id);
} catch (e) {
  logger.error(`Server action ${id} failed to load: ${e.message}`);
  throw e;
}

When it happens

Trigger: A 'use server' module exporting a non-function (export const LIMITS = {...}, export default someObject, a class), or a manifest id mapping to a renamed/tree-shaken export so the required binding resolves to undefined.

Common situations: Adding constants or config objects to a 'use server' file; mixing default/class exports into action modules; stale server manifest after renaming exports while the bundler cache persists.

Related errors

• Invalid server action: ${ref}
• No server callback has been registered. Call setServerCallba
• Server actions must be functions
• Server actions must be functions
• Client Functions cannot be passed directly to Server Functio

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/f532f691b12baa30. Report an issue: GitHub.