babelrcRoots cross-package visibility boundary in Babel 7 monorepo

Answer

No, by default in Babel 7, @babel/core does not load a sibling package's .babelrc file for source files outside that package, even within a shared monorepo root, unless babelrcRoots is explicitly configured to include the sibling directory.

Confirmed Facts

• Babel resolves configuration by traversing ancestor directories up to the configured root, not sideways across sibling directories.
• babelrcRoots functions as an explicit allowlist defining which package directories are eligible to contain a configuration file for files within those packages.
• rootMode modifies how the nearest ancestor containing a config is treated as the root but does not establish per-package visibility boundaries.
• Configuration caching can mask boundary changes during incremental builds if the cache key does not incorporate changes to babelrcRoots or the .babelrc files themselves.

Likely Explanation

The default behavior enforces strict ancestor scoping. A .babelrc in packages/foo is only relevant to files within packages/foo (or descendants) unless the monorepo root is the effective root and includes packages/foo in its scope, or babelrcRoots is used to explicitly permit cross-package visibility.

Steps for Resolution

1. Ensure build-tool and @babel/core caches are cleared to verify default behavior without cache interference.
2. Compile a representative file from the target package (Package B) and inspect which configuration files are loaded via Babel's debug logging.
3. If cross-package inheritance is observed unexpectedly, explicitly define babelrcRoots to only include necessary package roots, thereby establishing the intended permission boundary.

To provide a definitive recommendation regarding configuration impact, please specify the exact installed @babel/core version and the working directory (cwd) used during the Babel invocation.