Spec Driven Development: Beyond the first feature with API Design First

In my previous article, I shared my learnings experimenting with “API Design First with GitHub Spec Kit and Specmatic MCP” with OpenAPI specifications serving as API contracts.

In this follow-up, I take things a step further to test how the setup holds up beyond the first feature. The focus here is on two key aspects.

Going beyond the First Feature

Ensuring the system can evolve without introducing regression or drift, which is especially important when relying on coding agents that tend to optimise for speed but can easily introduce breaking changes.

Parallel Execution with Subagents

As I started building multiple features, I wanted to parallelise development to move faster. This mirrors real-world Contract-Driven Development. Once API contract is agreed upon, backend and frontend can proceed independently.