Spec Driven Development: Build what you mean, not what you guess

Software is moving fast. AI coding tools can write whole features in minutes. That speed is great, but only if the code matches the idea in your head. Too often it does not. We ask an agent for a thing, it returns something that looks right, and then we spend hours fixing small gaps.

A growing answer to this problem is Spec Driven Development, or SDD. It puts a clear written specification at the center of the work. The spec tells everyone what to build and why. It also gives AI agents a firm guide so they stop guessing. In this post, we will break down SDD in plain words, look at GitHub’s Spec Kit that helps teams practice it, compare SDD with TDD and BDD, and give you a simple way to try it on your next feature.

What is Spec Driven Development

Spec Driven Development starts with a spec. Not a giant document that nobody reads, but a short, living file in your repo that says:

• who the user is

• what they need to do

• what success looks like

• any limits or rules that must be true

In SDD, the spec is the single source of truth. When something is unclear, you update the spec first, then you change the plan and the code. The spec evolves as the product evolves. It is versioned next to the code, so it stays close to reality.

The key idea is simple. Code serves the spec, not the other way around. Teams write the intent first, then let people and agents turn that intent into a plan, tasks, and code. GitHub describes this shift as making specifications executable rather than disposable notes.

Why SDD is catching on now

AI coding agents are powerful. They also need clear direction. Vague prompts lead to code that looks fine but misses the point. GitHub’s engineering blog calls out that agents work best when the instructions are unambiguous and structured.

SDD gives that structure. It reduces back and forth. It also lowers the stress of code review, because reviewers can compare changes to a shared spec. When the spec is right, the code has a much better chance of being right.

Meet GitHub Spec Kit

Spec Kit is an open source toolkit that turns SDD into a practical workflow. It works with common coding agents like Copilot, Claude Code, and Gemini CLI.

Spec Kit guides you through four simple stages:

1. Specify
   Write a clear product spec in simple language. Describe the user, goals, flows, and acceptance checks. This is the what and the why.

2. Plan
   Add technical context. Choose the stack, architecture, integrations, and guardrails like performance or security targets. This is the how at a high level.

3. Tasks
   Break the plan into small steps. Each step is reviewable and testable. Small steps make AI outputs easier to check and refine.

4. Implement
   Generate or write code for each task. Review diffs against the spec and the plan. Adjust the spec if something important changes, then continue.

Spec Kit also supports a project constitution. Think of it as a rules file. You can mandate a design system, accessibility standards, security rules, or performance budgets. When agents plan and code, those rules guide choices from the start. This helps larger teams keep quality and consistency without slowing down.

SDD vs TDD vs BDD in plain words

It helps to place SDD next to practices you may already use.

• TDD is tests first. You write a failing test, then code to make it pass. It is great for code quality and design at the unit level. It does not always capture the full product intent.

• BDD is behavior first. You write human readable scenarios that show how a user behaves. Teams then code to satisfy those scenarios. It is strong for shared understanding. It can still leave some technical choices open.

• SDD is specification first. You write a short spec that explains goals, rules, and success checks. You add a plan and small tasks. You then code. The spec is the anchor that keeps everyone aligned. You can still use TDD for each task and BDD for end-to-end checks. SDD sits above them as the source of intent.

In short, SDD tells you what and why. BDD checks behavior across the system. TDD locks in correctness at the code level. These practices work well together.

What teams like about SDD

Clear intent
When the spec is written early and updated often, people do not talk past each other. The spec is the reference in standups, planning, and reviews.

Better agent output
Agents thrive with solid context. With a spec, plan, and tasks, an agent can generate code that is closer to the goal on the first try.

Less rework
Thinking through flows and edge cases in the spec reduces late surprises. Fixing ideas on paper is cheaper than fixing code later.

Shared ownership
Design, product, engineering, and QA can read and edit the same doc. The barrier to give feedback is low because the spec is plain language.

Traceable decisions
Why did we do it this way? The answer is in the spec history. This helps with audits, onboarding, and knowledge transfer.

Guardrails that stick
A constitution makes rules real. If you require a design system or certain privacy rules, the plan and code can reflect that from the start.

Things to watch out for

Upfront time
A good spec takes an hour or two for a medium feature. That feels like a cost until you see the time it saves later.

Learning curve
The team must practice writing simple, useful specs. Avoid jargon. Aim for clarity.

Spec drift
If code changes but the spec does not, trust breaks. Make updating the spec part of the definition of done.

Right level of detail
Do not script every pixel. Capture intent and key constraints. Let the plan and tasks carry the detail.

Tool maturity
Spec Kit is new and still growing. Start small, test it, and share feedback with the community.

How to start with SDD

1. Pick a small project.

2. Write a one-page spec.

3. Add a technical plan.

4. Break the plan into tasks.

5. Build and review against the spec.

6. Update the spec when anything changes.

You do not have to rewrite your process. Just start treating specs as first-class parts of the project.

Closing thoughts

Spec Driven Development is about reducing guesswork. It helps people and AI work from the same source of truth. With a living spec and the right tools, teams can move faster and stay aligned. GitHub’s Spec Kit is a big step in that direction, giving developers a way to make specs practical rather than theoretical.

And at Beam, we see the same shift in how organizations build with AI. The best results come when intent is explicit and agents can act on clear, structured guidance. Spec Driven Development is part of that future. It is how teams will build faster, with fewer misunderstandings, and with AI that truly knows what you mean.