facebook/react · error · Error

No server callback has been registered. Call setServerCallba

Error message

No server callback has been registered. Call setServerCallback to register one.

What it means

react-server-dom-parcel's browser client ships no default transport for invoking Server Functions — the host framework supplies one via setServerCallback(fn). Every server reference decoded from a Flight payload routes through callCurrentServerCallback; if no callback was registered before the first invocation, it throws.

Source

Thrown at packages/react-server-dom-parcel/src/client/ReactFlightDOMClientBrowser.js:65

    qs.set('env', environmentName);
    return devServer + '/__parcel_source_map?' + qs.toString();
  }
  return null;
}

type CallServerCallback = <A, T>(id: string, args: A) => Promise<T>;

let callServer: CallServerCallback | null = null;
export function setServerCallback(fn: CallServerCallback) {
  callServer = fn;
}

function callCurrentServerCallback<A, T>(
  id: ServerReferenceId,
  args: A,
): Promise<T> {
  if (!callServer) {
    throw new Error(
      'No server callback has been registered. Call setServerCallback to register one.',
    );
  }
  return callServer(id, args);
}

export function createServerReference<A: Iterable<any>, T>(
  id: string,
  exportName: string,
): (...A) => Promise<T> {
  return createServerReferenceImpl(
    id + '#' + exportName,
    callCurrentServerCallback,
    undefined,
    findSourceMapURL,
    exportName,
  );
}

View on GitHub (pinned to eafeac097b)

Solutions

1. Call setServerCallback once at the very top of the client entry, before rendering anything that can invoke actions
2. Implement the callback as a POST of {id, args} that decodes the returned Flight payload using createFrom*
3. If an action can slip through early, queue or defer it until the callback is registered

Example fix

// before — no transport registered
import {createFromFetch} from 'react-server-dom-parcel/client';
// later: saveNote() throws

// after — bootstrap first
import {setServerCallback, createFromFetch} from 'react-server-dom-parcel/client';
setServerCallback(async (id, args) => {
  const payload = fetch('/server-endpoint', {
    method: 'POST',
    body: JSON.stringify({id, args}),
  });
  return createFromFetch(payload, window.__serverManifest);
});

import {setServerCallback} from 'react-server-dom-parcel/client';
let installed = false;
export function installServerCallback(fn) {
  setServerCallback(fn);
  installed = true;
}
export function canCallServer() {
  return installed;
}
// gate action call sites: if (!canCallServer()) queue or disable the action

try {
  await action(...args);
} catch (e) {
  if (/No server callback has been registered/.test(e.message)) {
    // bootstrap ordering bug — register the callback, then surface a retry prompt
  }
  throw e;
}

When it happens

Trigger: Calling any server function before setServerCallback(...) has run: an action invoked at module scope or during first render before the bootstrap executed; an SSR entry hitting an action; the bootstrap script missing entirely.

Common situations: Custom Parcel-based RSC frameworks wiring their own POST endpoint; script-ordering issues where the component bundle executes before the bootstrap; copying a minimal example and omitting the setup step.

Related errors

• Invalid server action: ${ref}
• Server actions must be functions
• Server actions must be functions
• 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/9548b47bd7618f05. Report an issue: GitHub.