When users interact with a product, they see the surface: the interface, the animations, the data that appears in front of them. What they do not see is the layer underneath — the network of APIs that make the product work. The payment processor that handles the transaction. The mapping service that plots the route. The AI model that generates the response. The identity provider that authenticates the user. Modern software products are not monolithic systems. They are compositions — carefully assembled networks of capabilities, most of which are delivered by APIs built and maintained by someone else. The quality of those API choices shapes the product's capabilities, its reliability, and its cost structure as fundamentally as any internal engineering decision. For developers building products today, understanding which APIs are genuinely worth integrating — and which ones represent technical debt in disguise — is an increasingly important skill.…