Module Federation silently discards conflicting shared module versions when singleton is enabled

Answer to the question

When a host declares a shared library as singleton:true, Webpack 5 picks the first version it encounters at runtime and silently ignores any later, incompatible versions. The decision is made during module resolution, not at build time, and the plugin does not emit warnings or errors.

Confirmed facts

• First‑wins resolution is the current implementation detail of the ModuleFederationPlugin.
• It is not documented as a stable contract and may change in future 5.x releases.
• There is no public API to force a specific version or to get a build‑time diagnostic for a conflict.
• Setting requiredVersion only triggers a warning if the loaded version does not satisfy the host’s requirement; it does not alter the resolution order.

Recommended steps for this case

1. Pin the same npm version for the shared library in every remote’s package.json (e.g., react@18.2.0). This guarantees that the first encountered version is the one you intend.
2. Use a shared package.json or a monorepo tool (Lerna, Nx, Turborepo) to dedupe the dependency across all projects.
3. In the host’s ModuleFederationPlugin, list the remotes in a deterministic order so you know which one supplies the library first.
4. Add a lightweight runtime check:

try {
  const { version } = require('react/package.json');
  console.log('Loaded React', version);
} catch (e) {
  console.warn('React not available');
}

This logs the version actually used at runtime and surfaces any unexpected swap.

Optionally, provide a custom error handler in the plugin configuration (Webpack 5.70+ supports onError) to log when a module is overridden.

After deployment, run a quick script that imports the shared module from each remote and prints its resolved path. This validates that the same version is served.

Diagnostic question

Could you share the exact order of the remotes array in your host configuration? The first entry determines which version wins, so knowing that order will help confirm whether a different remote is unintentionally overriding the intended library.