The first time an API breaks a partner's integration, it's usually not because anyone made a bad decision. It's because nobody made a decision at all. The API grew field by field, endpoint by endpoint, until a change that seemed small on one side turned out to be load-bearing on the other.
Why a breaking change costs more than it looks like
When your API has one consumer (your own frontend), you can change it and your frontend at the same time, and nobody outside your team notices. The moment a second consumer exists, a mobile app, a partner integration, an internal tool built by another team, that's no longer true. Every one of them is depending on the current shape of your API without you necessarily knowing it.
A field that gets renamed, a response structure that changes, an endpoint that starts requiring a parameter it didn't before: any of these can silently break something you don't control and can't fix on your own schedule. The cost isn't the code change. It's the coordination, the support tickets, and the trust it costs with whoever was depending on you.
The common approaches, in plain terms
URL versioning (/v1/orders, /v2/orders) is the most visible and the easiest for consumers to understand. You keep the old version running while new consumers move to the new one, and retire the old version on your own timeline. It's a blunt tool, and that's often exactly what you want: nothing ambiguous about which version is being called.
Header-based versioning (a client sends API-Version: 2026-08-01 or similar) keeps URLs clean and lets you version more granularly, but it's easier for consumers to get wrong, since the version isn't visible in the request itself unless someone goes looking for it.
Additive-only changes avoid the need for a version bump at all in many cases. Adding a new optional field is almost always safe. Removing a field, changing its type, or changing what a status code means, is not. A lot of teams get most of the benefit of "versioning" just by adopting a strict rule: never remove or repurpose something that's already shipped, only add.
When a small team should actually bother
If you have one consumer and it's your own frontend, formal versioning is probably premature. Ship additive changes, coordinate breaking ones directly, and don't build infrastructure for a problem you don't have yet.
The moment a second real consumer shows up, especially one outside your team, it's worth deciding on an approach before you need it, not after the first incident. That doesn't mean over-engineering a versioning scheme on day one. It means having a rule (URL versioning is the easiest default for most small teams) and sticking to it once more than one party depends on your API.
Getting the backend and API layer right early is mostly about making these decisions on purpose instead of by accident. The API itself doesn't need to be clever. It needs to be predictable to whoever is building on top of it.