What is the role of API-first design in a side-by-side extensibility strategy?

API-first design means defining an extension's API contract — its interface, data shapes, and behavior — before building the implementation behind it, treating the API itself as the primary design artifact rather than an incidental byproduct of whatever the implementation happens to expose.

flowchart LR
    A[Design API contract first] --> B[Review/agree with consumers before building]
    B --> C[Implement behind the agreed contract]
    C --> D[Consumers integrate against a stable, pre-agreed interface]

This matters significantly for side-by-side extensions specifically because they exist to be consumed by other systems — designing the contract upfront (and getting it reviewed by actual consumers before implementation begins) avoids the costly rework of changing a published API's shape after other teams have already built against it, and supports building multiple extensions against a consistent, well-thought-out interface style rather than each team inventing its own inconsistent conventions.