TL;DR: Here's the math that pushed me off EC2: a t3. micro instance runs you roughly $8-10/month whether it serves zero requests or ten thousand. 📖 Reading time: ~30 min What's in this article The Problem: Your Flask App Works Locally, Now What? How Lambda Actually Runs Your Flask App (Understand This First) Prerequisites and Local Setup Step 1 — Build a Minimal Flask App Worth Deploying Step 2 — Package Dependencies Correctly (Where Most Tutorials Fail) Step 3 — Write the SAM Template (template.yaml) Step 4 — Deploy and Test Gotchas I Hit That Aren't in the AWS Docs The Problem: Your Flask App Works Locally, Now What? Here's the math that pushed me off EC2: a t3.micro instance runs you roughly $8-10/month whether it serves zero requests or ten thousand. Lambda charges $0.20 per million invocations plus compute time billed in 1ms increments. If your API gets sporadic traffic — a webhook receiver, an internal tool, a side project that spikes once a week — you're essentially paying an idle tax on EC2.…