Cross-posted from pheidi.training. This is the dev cut — same decisions, more code and auth detail.
Most build-vs-buy advice is stale. For most of my career, "don't roll your own X" was just true: engineering time was the bottleneck, so you rented anything that wasn't your core product. In 2026 the bottleneck moved. A focused feature that was a week of typing is now an afternoon of prompting and reviewing. So I started re-running the calculation, and I keep landing on build.
Some context: I'm the only person on Pheidi, an adaptive running-training app. The stack is ASP.NET Core + Blazor, C#, Azure SQL. You hand it a race, a schedule, and your history; it generates and continuously re-plans around your real life. No team, no contractors. Every architectural call gets filtered through "can one person own this forever?"
Let me start with the number that reframes the whole post: my entire AI bill — the assistant I pair with plus the tokens my agents spend — is roughly $100/month. The SaaS stack it displaced (feedback, email automation, analytics, an AI-features layer, plus a couple of stragglers) would each, individually, cost about that same $100. Bundle six of them and you're at 3–4× my total spend, indefinitely, with the price ratcheting up at every renewal.
