Yarn Berry PnP strict mode "Cannot find module" in CI

Yarn Berry Plug-n-Play enforces declared dependencies strictly. A package you import but never listed in package.json is blocked, even though a hoisted node_modules layout would have tolerated it.

What this error means

A build or test fails under Yarn PnP with "X tried to access Y, but it is not declared in your dependencies". It works under npm/yarn classic because hoisting made the undeclared package reachable.

node

Error: Your application tried to access lodash, but it isn't declared in
your dependencies; this makes the require call ambiguous and unsound.

Required package: lodash
Required by: my-app@workspace:.

Common causes

Undeclared (phantom) dependency

Code relied on a transitive package being hoisted into node_modules. PnP refuses to resolve packages you did not declare.

A dependency with its own undeclared imports

A third-party package imports something it never declared; PnP surfaces that latent bug.

How to fix it

Declare the dependency you import

Add the package directly to package.json so PnP can resolve it.

Terminal

yarn add lodash

Patch a dependency missing peer declarations

Use packageExtensions to add the missing dependency to a third-party package PnP rejects.

.yarnrc.yml

# .yarnrc.yml
packageExtensions:
  "buggy-pkg@*":
    dependencies:
      lodash: "*"

How to prevent it

Declare every package you import directly.
Use packageExtensions to fix third-party packages with missing deps.
Run PnP installs in CI early to catch phantom dependencies.