facebook/react · error · Error

Server actions must be functions

Error message

Server actions must be functions

What it means

In the browser-build server writer for Parcel, an incoming action request goes through loadServerAction: resolve the id against the server manifest, preload the referenced bundles, then require the module and use the resolved binding as the action. The binding must be a function; a missing export name (undefined after a rename) or a non-function export (const, object, class) fails the typeof check and throws.

Source

Thrown at packages/react-server-dom-parcel/src/server/ReactFlightDOMServerBrowser.js:295

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 of a 'use server' file an async function
2. Rebuild the manifest/bundles after renaming exports so ids and export names match
3. Add a unit test that imports each 'use server' module and asserts typeof export === 'function' for every name the manifest references

Example fix

// before — manifest says 'saveNote' but the module exports something else
'use server';
export async function persistNote(note) { /* renamed */ }
export const noteSchema = {type: 'object'}; // non-function export gets referenced by stale id

// after — only async function exports, manifest rebuilt
'use server';
export async function saveNote(note) { /* ... */ }

// dev/CI check: every export of a 'use server' module must be a function
import * as actions from './note-actions';
for (const [name, value] of Object.entries(actions)) {
  if (typeof value !== 'function') {
    throw new Error(`note-actions exports non-function '${name}'`);
  }
}

export function isServerActionExport(mod, name) {
  const value = mod[name];
  return typeof value === 'function';
}

try {
  await loadServerAction(id);
} catch (e) {
  if (/Server actions must be functions/.test(e.message)) {
    // stale manifest or non-function export — rebuild and verify the named export exists
    return new Response('Action unavailable', {status: 500});
  }
  throw e;
}

When it happens

Trigger: A 'use server' module whose referenced export is not a function, or whose export name no longer exists because the manifest is stale after a rename — requireModule resolves the module but the named binding is undefined.

Common situations: Renaming an exported action without rebuilding the manifest; exporting constants or objects alongside actions in a 'use server' file; default-vs-named export confusion between builds.

Related errors

• Server actions must be functions
• Invalid server action: ${ref}
• No server callback has been registered. Call setServerCallba
• Attempted to load a Server Reference outside the hosted root
• Server Functions cannot be called during initial render. Thi

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/179eb76582d7649b. Report an issue: GitHub.