Most tutorials for spec-driven development (SDD) assume you are starting with a blank canvas. In practice, at least for professional developers, you are rarely starting a greenfield project — you have an existing code base, possibly developed over years, and now you want to use agentic engineering to accelerate your development.
A spec-driven approach is still a good option, because even if you don't have a formalised spec for the rest of your product, you do want one for new features. This article walks through applying SDD to an existing project, using OpenSpec as the framework.
OpenSpec is light-weight, and uses spec changes (deltas) that are merged into a central spec repository. Because of this delta-based approach, you can drive a change to an existing system even if no central spec repository exists yet — making it a good fit for existing code bases.