Vibe coding vs spec-driven development — the real difference isn't speed

Six months ago I rolled my eyes at "spec-driven development." It sounded like PRDs with a new name — the kind of process fetish that slows teams down and makes engineers quit. Vibe coding sounded way more fun: open Cursor, describe what you want, iterate. Ship fast.

I was half right. Vibe coding IS faster — right up until it isn't.

This is what I learned running both approaches in parallel on real features, and what actually separates them in practice.

Both approaches start with the same inputs

A vibe-coding session starts with a goal in your head and a chat window. A spec-driven session starts with a goal in your head and a spec document. The AI agents downstream — Claude Code, Cursor, Codex — don't care which one they get. They just read text and write code.

So the real question is: where do you do the thinking?

Vibe coding says: think while the model is generating. Tweak. Undo. Try again. Let the iteration loop be the thinking.

Spec-driven says: think before the model runs. Write down what you want. Then let the model execute without interruption.

On the surface, vibe coding looks faster because there's no upfront cost. No document. No meeting. Just start. But the comparison gets interesting when you run both on the same feature.

Day 1: vibe coding is 3x faster

Watch a good vibe coder open Cursor on a fresh feature. They'll be pushing working code in 20 minutes — the AI writes the scaffolding, they review, nudge, nudge again. Meanwhile, a spec-first person is still writing the spec.

If you only measured "time to first working code," vibe coding wins every time. It's not close.

This is why so much AI coding content on X and YouTube makes vibe coding look magical. The demo always ends at the moment something is rendering in localhost. Nobody ships the follow-up video.

Day 3: the gap closes

Day three is when the vibe-coded feature meets reality. The new visitor tries to sign in and gets a blank page. Two users with the same Google email collide. Someone refreshes during the OAuth callback and ends up in a weird state. The dashboard crashes on an empty list.

Each of these is a spec question that was never asked. The vibe coder starts patching:

• "Add an empty state to the dashboard."
• "Handle the case where Google returns no email."
• "Fix the OAuth callback race condition."

Each patch is fast individually. But now the spec is being written after the code, one bug at a time, and every patch risks breaking something else the AI wrote and nobody reviewed.

The spec-first person is still slower on the clock — but their spec included "Two providers returning the same email resolves to one account" and "OAuth callback handles replayed codes." So their agent built those cases up front, and day three is when their feature actually works.

Day 14: the real divergence

Here's the part nobody talks about.

By day 14, the vibe-coded feature has been patched so many times that the codebase looks like Ship of Theseus. Files are hundreds of lines. Functions do three unrelated things. The "why" behind half the code is lost — the AI made a call, the human said "yeah sure," and now it's load-bearing.

When someone (human or AI) tries to add a new feature that touches this area, the model can't reason about it well. It doesn't know which behavior is intentional and which is a patch. You get AI slop compounding on AI slop.

Meanwhile, the spec-driven feature has a document that says what it should do. When you add the next feature, you update the spec first. The AI reads the updated spec and writes code that fits. The codebase stays coherent because there's an authoritative source of truth outside the code.

This is what actually separates the two approaches. Not speed on day 1. Coherence on day 100.

When vibe coding actually wins

I'm not going to pretend spec-driven development is universally better. Vibe coding wins when:

• You're exploring. You don't know what you're building yet. You want to see three versions and pick one.
• The stakes are low. A throwaway prototype, a weekend hack, something you'll delete.
• You are the only user. Personal scripts where "works on my machine" is the success criterion.
• The scope is tiny. A single function, a one-file CLI, a static landing page.

For these cases, writing a spec first is bureaucratic overhead. The spec would be longer than the code.

When spec-driven wins (and it's most of the time)

Spec-driven wins when:

• Other people will read the code. Teammates, AI agents in future sessions, future you.
• The feature interacts with other features. Auth, payments, data migrations, anything with upstream/downstream coupling.
• You're going to edit this code more than once. Which is almost everything you build on purpose.
• Failure has a cost. A data loss, a security hole, a broken checkout.
• You're working with AI coding agents. Because agents compound context loss. Each session starts fresh unless you give them a durable document to reload from.

The last point is the one most people miss. AI coding tools don't remember yesterday. They can re-read your repo, but they can't re-derive why you made the design choices you made. A spec is the agent's persistent memory between sessions.

The hybrid that actually works

The best teams I've watched use a hybrid:

1. Vibe code the first rough version to figure out what the feature even is. Quick, exploratory, disposable.
2. Extract a spec from the working prototype — here's the happy path, here are the edge cases I discovered, here are the failure modes I ran into.
3. Throw away the prototype and rebuild from the spec using an agent, with the spec as the prompt.

Step 3 sounds wasteful. It's not. The second build is faster than the first because the thinking is done, and the result is a codebase that actually matches the spec, not a patch pile that mostly matches reality.

This hybrid is what ClearSpec was built for. You vibe-code in whichever tool you like, then paste your findings into a structured spec that AI agents can execute cleanly. The spec is the output of the messy exploration, and the input to the clean build.

What to do tomorrow

If you're currently pure vibe coding, try this on your next feature:

1. Spend your first 15 minutes writing a one-page spec with: goal, user stories, acceptance criteria, edge cases, failure states, out-of-scope.
2. Hand it to Claude Code or Cursor.
3. Do nothing else until the agent says it's done.
4. Review the output against the acceptance criteria, not against your gut.

You'll probably finish slower than a vibe session on day 1. You'll finish dramatically faster on day 3. And the code will still work on day 14.

That's the trade you're actually making. Speed today versus speed next week.

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Related: How to write a spec for Claude Code (that it actually follows) has the template I use and a walkthrough of OAuth done badly vs done well.

ClearSpec is a tool that generates structured specs from a plain-English description. Free to start.