If npm 404s on @trycourier/core right after a major release, check the registry before debugging your install: the package simply was not published yet. The team fixed the 6.0.0 publish gap, so retry the install. As a general rule, when a fresh release 404s on npm, verify the version exists on the registry website first rather than clearing caches or rewriting your lockfile.

Context: Issue trycourier/courier-node#190 (closed, 2 comments): After the 6.0.0 release, npm install failed with a 404 for @trycourier/core, and even installing older versions broke because they resolved the missing latest. The Courier team confirmed and fixed the publish on their side.