Build vs Buy Is the Wrong Question. Staff Engineers Ask This Instead.

Every system design interview has a build vs buy question, and most candidates answer it like shoppers. "Buying is faster, building gives control." That answer is not wrong. It is also not an answer. It is a shrug with a tie on.

The staff-level answer starts somewhere else entirely: nobody cares what you built. They care what you refused to build.

Auth is never the differentiator. Neither is billing, email delivery, or file uploads. These are solved problems with vendors who have entire teams thinking about them while you sleep. Building them yourself is not engineering rigor. It is volunteering to own someone else's roadmap at 3am, forever.

The two questions that actually decide it:

1. Is this undifferentiated heavy lifting? If yes, buy it.
2. Does this make our product different? If no, buy it.

Both questions are about the product, not the technology, which is exactly why interviewers ask this. They are checking whether you can think like someone with a budget and a deadline instead of a hobbyist with opinions.

Now the part nobody mentions: the maintenance bill. Everyone prices the build. Nobody prices the five years after. The login flow you built in two sprints needs SSO next quarter, passkeys the quarter after that, and an audit trail when the enterprise deal lands. The vendor prices all of that into the contract. Your two sprints were just the down payment. An honest estimate of "build" is build cost plus maintenance forever, and forever is expensive.

Here is the decision rule I actually use, and the one that lands in interviews:

The build vs buy rule: buy anything where the failure mode is boring (auth, payments, email, CI). Build only what changes when your product wins. And wrap every bought capability behind your own thin interface, because the day the vendor raises prices or dies, you want the migration to be a one-week swap, not a rewrite.

That seam is the real senior move. Buying without a seam is outsourcing your architecture. Building everything is NIH with extra steps.

The question was never build or buy. The question is: what do we refuse to own?

Watch the 40-second version: Build vs Buy: How Staff Engineers Decide. This is question 8 of 60 in the Frontend System Design Interview Playbook.