When should you build custom vs buy off the shelf?

Default to buy. Build only when the custom part IS your product's value. Everything else is glue — and you should buy glue, not build it.

Founders order developers to build custom authentication systems, spending 20-100 hours and thousands in salary, when Firebase Auth costs a few dollars per month for the same functionality. Same happens with payments, file storage, and notifications.

A useful framing from domain-driven design: the core domain is what makes your product unique — build this, spend 80% of effort here. The supporting domain is infrastructure that helps your core work — some custom work is fine. The generic domain is auth, payments, storage, email, notifications — buy these, always.

Most founders think "we should build it so we own it." But owning generic software means maintaining it forever. Every security update, every scaling issue, every bug becomes your problem. Buying means someone else owns that problem.

Find the Core identifies the one capability that creates competitive advantage — everything else is bought or deferred. Scope Shaping keeps the build focused on what fits inside the appetite. Consolidation warns that every service added is another failure point.

For every feature on your roadmap, ask: "Is this what makes our product special?" If yes, build. If no, buy.