facebook/react · critical · Error

458

458

Error message

Currently React only supports one RSC renderer at a time.

What it means

Starting an RSC request installs ReactSharedInternals.A as the Flight async dispatcher, and React permits only one RSC dispatcher per loaded React copy — it is kept installed permanently for the process. The guard fires when the slot already holds a dispatcher that is not the Flight DefaultAsyncDispatcher, which is the signature of two different RSC runtime copies (or an incompatible dispatcher) sharing one realm.

Solutions

Run npm ls react react-server react-server-dom-webpack (or yarn why) and remove duplicates via overrides/resolutions.
Import the RSC server APIs from a single module instance — one entry point, one chunk graph.
Align react, react-server-dom-* and framework versions so exactly one copy resolves everywhere.

Example fix

// before — npm ls react shows 19.0.0 and 19.1.0 in the same server bundle

// after — package.json forces one copy
"overrides": { "react": "19.1.0", "react-dom": "19.1.0" }

Defensive patterns

Strategy: validation

Validate before calling

// Boot-time check: the react resolved from app code and from the RSC runtime must match.
import {createRequire} from 'module';
const require = createRequire(import.meta.url);
const appReact = require.resolve('react');
const rscReact = require.resolve('react', {paths: [require.resolve('react-server-dom-webpack/server')]});
if (appReact !== rscReact) {
  throw new Error(`Two react copies: ${appReact} vs ${rscReact} — dedupe before starting.`);
}

Prevention

Add `npm ls react` (fail on duplicates) to CI.
Use bundler aliases or package overrides to force one react instance.
Never import react-server-dom-* both directly and through a framework wrapper in the same process.

When it happens

Trigger: Loading two copies of react-server/react-server-dom-* (one via require, one via a bundled ESM build) and calling render()/prerender() from both; npm hoisting mismatches where react resolves to different versions inside and outside the server bundle; mixing a framework's compiled RSC runtime with direct react-server-dom-webpack imports.

Common situations: Monorepos or bundlers that vendor react-server separately per entry point; frameworks (Next.js, Waku) plus direct Flight imports in the same server; upgrading react without upgrading react-server-dom-* in lockstep.

Related errors

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/07981db33662eb41. Report an issue: GitHub.

Appendix: source

Thrown at packages/react-server/src/ReactFlightServer.js:702

  this: $FlowFixMe,
  type: 20 | 21,
  model: ReactClientValue,
  bundlerConfig: ClientManifest,
  onError: void | ((error: mixed) => ?string),
  onAllReady: () => void,
  onFatalError: (error: mixed) => void,
  identifierPrefix?: string,
  temporaryReferences: void | TemporaryReferenceSet,
  debugStartTime: void | number, // Profiling-only
  environmentName: void | string | (() => string), // DEV-only
  filterStackFrame: void | ((url: string, functionName: string) => boolean), // DEV-only
  keepDebugAlive: boolean, // DEV-only
) {
  if (
    ReactSharedInternals.A !== null &&
    ReactSharedInternals.A !== DefaultAsyncDispatcher
  ) {
    throw new Error(
      'Currently React only supports one RSC renderer at a time.',
    );
  }
  ReactSharedInternals.A = DefaultAsyncDispatcher;
  if (__DEV__) {
    // Unlike Fizz or Fiber, we don't reset this and just keep it on permanently.
    // This lets it act more like the AsyncDispatcher so that we can get the
    // stack asynchronously too.
    ReactSharedInternals.getCurrentStack = getCurrentStackInDEV;
  }

  const abortSet: Set<Task> = new Set();
  const pingedTasks: Array<Task> = [];
  const cleanupQueue: Array<string | bigint> = [];
  if (enableTaint) {
    TaintRegistryPendingRequests.add(cleanupQueue);
  }
  const hints = createHints();

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