Ghost fallback chunks / import: false behaviour with large transitive dep trees — Re.Pack MF

[HimanshuNarang]

Environment

Re.Pack: @callstack/repack 4.4.1
webpack: 5.107.2
Platform: Android

Description

I've been working through the optimal shared config for a production React Native app with Module Federation and have a few questions based on testing against your example at repack-examples/module-federation/app1/webpack.config.mjs.

Observation 1 : Official example only shares react + react-native, The example only declares react and react-native as shared. In a real app with large SDKs (tracking, UI component libraries, animation, totalling ~80 MB
bundled), should these also be declared as shared singletons? If not, are two instances of a tracking SDK acceptable in a host+remote setup?

Observation 2 : eager: STANDALONE produces ~78 MB ghost fallback chunks for large packages
When I apply eager: STANDALONE (i.e. eager: false in APK builds) to a package with a large transitive dep tree (e.g. @shared/trackingsdk, whose tree totals ~78 MB), webpack generates ghost fallback chunks of ~78 MB.

This happens even after declaring all of the package's direct deps as shared — the
bloat comes from deeper transitive deps not in the shared scope. I found that eager: true eliminates the ghost chunks (code goes into the container instead), but multiplies APK size when adding more remotes (each container duplicates the ~80 MB).

Observation 3 : import: false doesn't suppress transitive bundling, I tried { singleton: true, eager: false, import: false } to avoid both the ghost chunk and the container copy — expecting webpack to generate only a
consume-shared stub. The container correctly shrank to ~50 KB, but the exposed module chunk (App_SRP_js.chunk.bundle) absorbed the full ~79 MB dep tree instead. The bytes moved files, not disappeared.

Questions:

1. Is import: false supported in Re.Pack's ModuleFederationPlugin? The behaviour differs from webpack 5's documented spec.
2. For packages with large transitive dep trees, what is the recommended approach to avoid both ghost chunks AND container bloat when the host always provides these deps?
3. The comment in your example says "to be figured out" on the eager: STANDALONE line : is there an updated recommendation for production setups?

Reproducible Demo

Base repo: callstack/repack-examples (module-federation example)
Branch to fork: main

Step 1 — Add moment as a test package moment is used as the probe, 392 KB bundled, easy to detect with grep, no native deps.

cd module-federation
yarn add moment

Step 2 — Modify host webpack config, In host/webpack.config.mjs, add moment to the shared config so the host provides it:

shared: {
react: { singleton: true, eager: true },
'react-native': { singleton: true, eager: true },
moment: { singleton: true, eager: true }, // ← add this
},

Step 3 — Modify app1 (remote) webpack config, In app1/webpack.config.mjs, add moment with import: false:

shared: {
react: { singleton: true, eager: STANDALONE },
'react-native': { singleton: true, eager: STANDALONE },
moment: { singleton: true, eager: false, import: false }, // ← add this
},

Step 4 — Import moment in the exposed component, In app1/src/App.tsx, add a direct import:

import moment from 'moment';
// use it anywhere so webpack doesn't tree-shake it
console.log(moment().format('YYYY'));

Step 5 — Build and inspect

Build host (provides moment to shared scope)

cd host && yarn bundle:ios # or android

Build remote

cd app1 && yarn bundle:ios

Step 6 — Check remote output

Should print 0 — moment must NOT be in the exposed chunk

grep -c "isMoment|_isUTC|proto.format" app1/build/ios/*.chunk.bundle

Check bundle sizes

ls -lh app1/build/ios/*.bundle

---

Expected result

grep output

0 ← moment is consumed from shared scope, not bundled

Exposed module chunk

app1_src_App_tsx.chunk.bundle ~50 KB (just App.tsx, no moment)

Actual result

grep output

14 ← moment IS in the exposed module chunk

Exposed module chunk

app1_src_App_tsx.chunk.bundle ~450 KB (App.tsx + full moment library)

With import: false, webpack 5 MF spec says no local fallback should be generated — the module is consumed exclusively from the shared scope. However, Re.Pack's ModuleFederationPlugin compiles the package into the exposed module chunk instead. The bytes change location but are never absent.