What Is Spec-Driven Development? The Fix for Vibe Coding Drift
Spec-Driven Development (SDD) is the practice of writing a structured, versioned specification before an AI coding agent writes any code, so the agent implements against explicit goals and acceptance criteria instead of guessing at intent. It replaces the improvisational "prompt and pray" style with a repeatable loop — Specify, Plan, Tasks, Implement — where a human reviews at every step. It exists because AI made code cheap to produce and easy to get wrong.
Why Vibe Coding Drifts
Left to infer intent from a casual prompt, an agent fills the gaps with guesses: it invents an API, picks a data shape you did not want, handles the happy path and skips the edge case you cared about. Each guess is small; over a feature they accumulate into drift you can no longer untangle, because the intent was never written down anywhere the agent — or a colleague, or you in three months — could check against.
What SDD Does Instead
You write a spec describing what the system should do. From it you derive a plan (the technical how), which breaks into atomic tasks, which the agent implements one at a time against criteria written before any code existed. At each phase a human reviews and refines before proceeding — which is where misunderstandings get caught, at the cheapest possible moment.
The Core Trade-Off
Vibe coding optimises for speed; spec-driven development optimises for intent. Neither is always right — for a throwaway script, vibe away. The rule worth remembering is simple: if the intent matters beyond the next prompt, structure wins. That single line tells you when to reach for a spec and when to skip it.
Why It Caught On So Fast
SDD emerged in 2025 as a direct response to these failure modes, and within a year every major AI coding ecosystem had shipped its own implementation. The flavours differ, but the core move is identical everywhere: write a structured spec before invoking the agent, so it has explicit goals and constraints instead of vibes. When every vendor independently arrives at the same four phases, the pain they were solving was real.