SDD Was the Start. Harness Engineering Is the Real Game.
Last year, the AI coding conversation had a clear hero: Spec-Driven Development (SDD). This year, people are talking about harness engineering instead. That looks like a trend. It is a signal that the bottleneck moved. SDD is about making intent explicit so an agent can start in the right direction. Harness engineering is about building the environment, constraints, feedback, and governance that keep the agent on track after the 50th or 100th step. If you have ever watched an agent do impressive work for 20 minutes and then slowly degrade into a mess, you already understand why the vocabulary changed. TL;DR SDD helps agents start correctly Harness engineering keeps them correct over time The bottleneck moved from generation to verification Long-running reliability is now the real problem The SDD moment: why it caught on Early “agentic coding” had a predictable failure mode. You’d say: “Add user auth,” or “Make a dashboard,” or “Fix onboarding.” The agent would produce something that looked plausible. It might even compile. Then you’d try to use it, and realize half the work was guesswork. ...