Silent runtime failures after switching ClojureScript build to :advanced optimizations mid-migration

Migration setup

I am migrating a small ClojureScript frontend (a handful of namespaces, Reagent UI) from an older build setup to shadow-cljs, with the goal of zero downtime for users. The dev side is fine: the watch build with :none optimizations recompiles changed namespaces and the page updates without a restart.

The failure condition

The problem appears only after I produce a release build with :advanced optimizations. The minified bundle throws runtime errors when calling into third-party JavaScript libraries, and I understand this is the classic symptom of an incomplete externs file: property names get renamed by the Closure Compiler because they were not declared. The dev build never shows these errors, so they surface only late in the migration.

The unresolved trade-off

I can either keep the fast :none build with hot reloading during the migration window, or switch to :advanced early to catch externs gaps — but that slows iteration and I am unsure how to generate or validate externs for the libraries I use. I also have state in plain atoms outside component lifecycles that hot reloading will not preserve.

How can I detect missing externs before a full release build? Is there a practical way to run :advanced checks in CI without giving up the fast dev loop? And what is the recommended approach for preserving atom-based state across hot reloads during the cutover?