Vibe Coding vs Spec-Driven Development

Vibe coding and spec-driven development both use AI to write most of the code. The difference is whether you decided what to build before you built it. Vibe coding means you prompt, look at the result, prompt again, and steer by feel. Spec-driven means you write down what the thing is first, then generate against it. Vibe coding is faster for the first hour and slower for every hour after. Spec-driven is the opposite. If you are building anything you have to maintain, spec-driven wins, and it is not close.

I use both. That surprises people. Here is when each one is the right call.

What is vibe coding good for?

Throwaway work. Prototypes you will never ship. A quick script to reshape a CSV. A one-off page to test if a layout feels right. When the code has no future, there is nothing to maintain, so there is nothing for a spec to protect. Vibe away. Prompt loosely, accept what comes back, move on.

Vibe coding is also good for exploration. Sometimes you do not know what you want until you see three wrong versions of it. Generating fast and cheap is a legitimate way to find the shape of a problem. The mistake is not vibe coding. The mistake is keeping the vibe-coded result and building on top of it as if it were designed.

Where does vibe coding fall apart?

At the second feature. The first feature works because there is nothing for it to conflict with. The second feature has to fit against the first, and the first was never designed, so there is nothing to fit against. Now the model is guessing about code that was itself a guess. Guesses stacked on guesses. This is how vibe-coded projects hit a wall around week two, where every new change breaks two old ones and nobody can say why.

The failure is not visible early. That is what makes it dangerous. The demo looks great. Then you try to extend it and discover there is no there there. I wrote about that exact cliff in demo to shippable. Vibe coding gets you a great demo and a terrible foundation.

What does spec-driven development do differently?

It moves the deciding to the front. Before the model generates, you write down the entities, the routes, the states, the definition of done. Then the model builds against that, and every future feature builds against the same spec. The second feature fits the first because both fit the spec. Nothing is guessing about a guess.

This costs you time up front. Ten minutes, maybe twenty, writing a spec instead of prompting. You get that time back the first time you add a feature and nothing breaks. I laid out why I order it this way in write the spec before the prompt. The short version: the spec is the cheapest place to be wrong, so that is where you do your being wrong.

Is spec-driven just slower vibe coding?

No, and this is the part people miss. Spec-driven is not vibe coding with paperwork. The generate step is exactly as fast. You still let the model write most of the code. The only difference is that the model is aiming at a target you set instead of a target it invented. Same speed, better aim.

The whole thing runs as a loop: spec, generate, verify against the spec, correct, repeat. I broke that down in how the build loop works. Vibe coding is that loop with the spec and verify steps deleted. Which is to say it is not a loop at all. It is a straight line that eventually runs off the road.

So which should you use?

Vibe code when the output is disposable. Spec-drive when the output has to live. The test is simple: will anyone, including future you, have to change this code again? If yes, spec first. If no, vibe freely.

Most real products fail this test toward "spec first," because most real products get changed constantly. That is why I build spec-driven by default and treat vibe coding as a tool for scouting, not for building. I run the disciplined version inside Bootspring, where the spec drives the generation instead of living in a separate doc nobody reads. The tooling makes spec-driven as fast to start as vibe coding, which removes the only real reason people reach for the vibe in the first place.

Fast and collapses, or fast and continues. Pick the second one for anything that matters.