A React 18 instrument that imports only react-dom@18.x has its JSX compiled against React 19 and fails to render #1801

A React 18 interactive instrument gets React 19's JSX runtime whenever it does not import react@18.x itself. resolveJsxImportSource decides which React the JSX compiles against by scanning the source for /runtime/v1/react@<version> only. An instrument that renders with /runtime/v1/react-dom@18.x/client and uses JSX does not need to import react@18.x: the automatic JSX runtime makes the import unnecessary for any component without hooks. So it falls through to the react@19.x default. React 19's JSX runtime stamps elements with Symbol.for('react.transitional.element'), which React 18's reconciler rejects. The task then fails at render with "Objects are not valid as a React child", which is exactly the failure this derivation exists to prevent (packages/instrument-bundler/AGENTS.md). For the same reason, a source that imports react-dom@18.x and react@19.x is not flagged as mixing majors.

Where

packages/instrument-bundler/src/build.ts:14-38:

const DEFAULT_REACT_PACKAGE = 'react@19.x';

const REACT_PACKAGE_PATTERN = /['"]\/runtime\/v1\/(react@[^'"/]+)/g;
// ...
  return `/runtime/v1/${packages.values().next().value ?? DEFAULT_REACT_PACKAGE}`;

Reproduce

1. Bundle an interactive instrument whose index.tsx imports only createRoot from /runtime/v1/react-dom@18.x/client.js, declares const App = () => <p>hi</p>;, and calls createRoot(document.body).render(<App />) in render.
2. Open it in apps/web.

Actual: the bundle imports /runtime/v1/react@19.x/jsx-runtime.js next to /runtime/v1/react-dom@18.x/client.js (verified on main), and rendering fails with React 18's "Objects are not valid as a React child" error.
Expected: the JSX compiles against /runtime/v1/react@18.x, and a source that imports react-dom of one major with react of another is a build error.

Tests

packages/instrument-bundler/src/__tests__/build.test.ts: it('should compile JSX against the react paired with an imported react-dom, so a react-dom 18 instrument without hooks still renders'). Add a fixture under src/__tests__/repositories/ (as the package's AGENTS.md asks) that imports only react-dom@18.x/client. Also add it('should reject react-dom and react of different majors in one instrument').

e2e: none needed beyond the fixture, since no built-in instrument uses React 18.

Suggested fix

Match both packages, /['"]\/runtime\/v1\/react(?:-dom)?@([^'"/]+)/g, collect the version, and build react@<version> from it. The existing "more than one version" error then also covers a mismatched react/react-dom pair.