The React Server Writer cannot be used outside a react-serve

Error message

The React Server Writer cannot be used outside a react-server environment. You must configure Node.js using the `--conditions react-server` flag.

What it means

react-server-dom-esm/npm/server.js is the default-condition stub for the server subpath in the published package. The real server implementation only resolves when Node activates the 'react-server' export condition (package.json maps the subpath under that condition); without it, Node falls back to this stub, which throws with the exact remedy in the message: run Node with --conditions react-server.

Source

Thrown at packages/react-server-dom-esm/npm/server.js:3

'use strict';

throw new Error(
  'The React Server Writer cannot be used outside a react-server environment. ' +
    'You must configure Node.js using the `--conditions react-server` flag.'
);

View on GitHub (pinned to eafeac097b)

Solutions

Start Node with the condition: node --conditions react-server server.js
For bundlers, add 'react-server' to resolve.conditions in the server build only (webpack 5 resolve.conditions)
For tests, set NODE_OPTIONS='--conditions react-server' or map the subpath to the .react-server.js artifact in jest moduleNameMapper
Make sure only server code imports this subpath — client code belongs on react-server-dom-esm/client

Example fix

# before
$ node server.js # throws: ...configure Node.js using the `--conditions react-server` flag

# after
$ node --conditions react-server server.js

Defensive patterns

Strategy: validation

Validate before calling

// fail fast if the react-server condition is not active
if (!process.execArgv.some(a => a.includes('react-server'))) {
  throw new Error('Start this process with: node --conditions react-server');
}

Prevention

Bake --conditions react-server into the start script so the flag is never forgotten
Add 'react-server' to resolve.conditions only for server bundles
Keep server imports of this subpath out of client graphs (lint boundaries)

When it happens

Trigger: require/import 'react-server-dom-esm/server' (directly or transitively) in a plain `node app.js` process started without --conditions react-server; a client-side bundle accidentally importing the server entry; bundler or test-runner resolve configurations that drop the react-server condition.

Common situations: Running the RSC server without the documented Node flag; webpack/vite server builds missing 'react-server' in resolve.conditions; Jest resolving the stub because its module resolution ignores custom conditions.

Related errors

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/c9260d15e803aaf5. Report an issue: GitHub.