facebook/react · error · Error

React Server Plugin: You must specify the isServer option as

Error message

React Server Plugin: You must specify the isServer option as a boolean.

What it means

ReactFlightWebpackPlugin validates its constructor options: options.isServer must be present and a boolean, otherwise construction throws. The plugin branches on this flag (client manifest generation vs server compiler), so an undefined/string value — usually a typo'd or missing key — fails fast at webpack config evaluation time.

Source

Thrown at packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js:74

type Options = {
  isServer: boolean,
  clientReferences?: ClientReferencePath | $ReadOnlyArray<ClientReferencePath>,
  chunkName?: string,
  clientManifestFilename?: string,
  serverConsumerManifestFilename?: string,
};

const PLUGIN_NAME = 'React Server Plugin';

export default class ReactFlightWebpackPlugin {
  clientReferences: $ReadOnlyArray<ClientReferencePath>;
  chunkName: string;
  clientManifestFilename: string;
  serverConsumerManifestFilename: string;

  constructor(options: Options) {
    if (!options || typeof options.isServer !== 'boolean') {
      throw new Error(
        PLUGIN_NAME + ': You must specify the isServer option as a boolean.',
      );
    }
    if (options.isServer) {
      throw new Error('TODO: Implement the server compiler.');
    }
    if (!options.clientReferences) {
      this.clientReferences = [
        {
          directory: '.',
          recursive: true,
          include: /\.(js|ts|jsx|tsx)$/,
        },
      ];
    } else if (
      typeof options.clientReferences === 'string' ||
      !isArray(options.clientReferences)
    ) {

View on GitHub (pinned to eafeac097b)

Solutions

1. Set isServer explicitly as a boolean: isServer: false for the client-manifest pass
2. Check for typos in the key and for accidental undefined (e.g. env ? false : undefined)
3. Validate options early in your webpack config so mistakes surface with your own message

Example fix

// before
new ReactFlightWebpackPlugin({clientReferences: [...]})

// after
new ReactFlightWebpackPlugin({isServer: false, clientReferences: [...]})

// Validate before constructing
function validatePluginOptions(options) {
  if (!options || typeof options.isServer !== 'boolean') {
    throw new Error('ReactFlightWebpackPlugin requires options.isServer: boolean');
  }
  return options;
}
new ReactFlightWebpackPlugin(validatePluginOptions(cfg));

function areValidPluginOptions(o) {
  return o != null && typeof o.isServer === 'boolean';
}

When it happens

Trigger: new ReactFlightWebpackPlugin({}) with no isServer; passing isServer: 'false' (string) or a variable that is undefined; spreading an options object that lacks the key.

Common situations: Copying example configs that omit the option | Config refactors that rename or conditionally delete the key | Version upgrades where the option became required

Related errors

• TODO: Implement the server compiler.
• Could not find the module "${modulePath}" in the React Clien
• The module "${modulePath}" is marked as an async ESM module
• Use react-server-dom-webpack/client instead.
• Use react-server-dom-webpack/client instead.

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/b86f2e09ea6f1dbc. Report an issue: GitHub.