Spec Driven Development: How does Agentic SDLC work?

While on a live stream a few days ago, I choose the old school way of developing software; which is planning the specs first and then hand coding it. Some of my viewers suggested using AI to build it, but I wanted to experience the satisfaction of creating and deploying a piece of software using your human brain and hands. Well, though I am still firm on my stance, I wanted to share how you can improve your current form of development with Spec Driven Development

What are Specs?

Specs or Specifications are the blueprint and rules for what a piece of software should do and how it should behave. Historically the Project Managers used to be incharge of creating the Specs doc which was meant for human developers but in 2026 the specs doc has evolved into a machine readable text files.

What are some popular Coding Paradigms?

1. Spec-Driven Development (SDD)

The Idea over here is to craft a hyper-precise blueprint in a machine readable format and then let the Human or AI coding agents pick the right part and generate the code for it. The way this works is a Human writes a starting point in form of API schema, JSON/YAML file or an OpenAPI blueprint(Try this one, it's crazy) which contains exact inputs, outputs, api req/res body schemas, validation rules, database models, etc. After this the AI or Human generates the required database, backend routes, frontend forms and unit tests.

Tooling that you can use: OpenAPI, Swagger(any md file with API Schema)

2. Domain-Driven Design (DDD)

This way of development is where the software structure must perfectly mirror the real-world business structure and language, bridging the gap between developers and non-technical business experts. The starting point is always a document with universal business concepts, terminology and business boundaries. So, Instead of thinking about tables and code, you group logic by business functions, such as for an e-commerce app is split into separate domains: Inventory, Billing and Shipping. The software components inside Billing cannot directly alter things inside Inventory without strict rules.

Tooling that you can use: Strategic mapping(https://sketchlab.webdevcody.com/)

3. Entity-Driven Development (EDD)

Dekho bhai, entity toh tumhe pata hona chahiye, an entity is an individual unit of data in a software system. Like "User", "Product", "Order" for an ecom app. Here the software is built around the core data objects(the "Entities") and how they link together. Engineers define the core entities, their properties, and who is allowed to access them. Once this foundational data graph is built, AI agents map out the entire application layer, database migrations, and basic API endpoints automatically. Build a skeleton and then let the coding agent build the muscles and skin.

Primary tooling: Graph Databases, Prisma, TypeORM (khud google search karke docs padh lena.)

How to Approach Spec-Driven Development

Yeh SDD sikhne ke liye bhai mindset change karna padega from "writing code to solve a problem" to "writing a blueprint that defines the solution." The absolute core principle of SDD is that the specs file is the "Source of Truth." If you need to change how the software works, you change the spec first, and let the coding agent update the code.

Let’s do this step by step in a way that even if you are a fresher, you can actually build something working.

Step 1: Pick your stack (keep it simple)

Do not over-engineer this part, just pick something that works well with tooling. For this tutorial:

• API Spec → OpenAPI (YAML)
• Backend → Node.js (or anything you like)
• Database → Prisma

That’s it, nothing fancy.

Step 2: Define your system using a spec (no coding yet)

Let’s build a simple “User Auth API” with just one endpoint: login.

Create a file:

auth.openapi.yaml

And write this:

openapi: 3.0.0
info:
title: Auth API
version: 1.0.0

paths:
/login:
post:
summary: Login user
requestBody:
required: true
content:
application/json:
schema:
type: object
required: [email, password]
properties:
email:
type: string
password:
type: string
minLength: 8
responses:
"200":
description: Successful login
content:
application/json:
schema:
type: object
properties:
token:
type: string
"401":
description: Invalid credentials

Now pause for a second and observe what you just did.

You have:

• Defined an endpoint
• Defined request validation
• Defined success response
• Defined error response

And you have not written a single line of backend code.

This is the core of SDD.

Step 3: Validate your spec before touching code

Treat this file like production code.

You can:

• Use a linter like Spectral
• Or just use AI

Example prompt:

Analyze this OpenAPI spec and tell me:
1. Missing edge cases
2. Security issues
3. Invalid assumptions

Fix issues here itself because once this spec is clean, your generated system will be clean.

Step 4: Visualize the flow (this helps beginners a lot)

Before generating anything, just understand what your system is doing.

This diagram is not optional, this is how you build clarity before implementation.

Step 5: Generate the backend skeleton

Now you let machines do the boring work.

You can use tools like OpenAPI Generator or just AI.

Example:

Generate a Node.js Express backend from this OpenAPI spec.
Include:
- Route setup
- Request validation middleware
- Clean folder structure

What you will get:

• Predefined routes
• Validation logic
• Typed request bodies

Now instead of writing everything, you are only filling in logic.

Step 6: Add your business logic only

Inside your generated route handler, you only write the actual logic.

Example:

export async function loginHandler(req, res) {
const { email, password } = req.body;

// fake check
if (email !== "[admin@test.com](mailto:admin@test.com)" || password !== "password123") {
return res.status(401).json({ error: "Invalid credentials" });
}

return res.status(200).json({
token: "dummy-jwt-token"
});
}

Notice something important here, you are not validating inputs here because the spec already defined it and your generated code already enforced it.

You are only writing logic.

Step 7: Add database schema using the same mindset

Now define your data model using Prisma.

Create:

schema.prisma

model User {
id       Int    @id @default(autoincrement())
email    String @unique
password String
}

Again, you are defining structure, not implementation.

Now your entire system looks like this:

Step 8: Make changes the correct way

Let’s say tomorrow you want to:

• Add username
• Change password rules

Do not touch the code first.

Update the spec:

minLength: 8 → minLength: 10

Then:

• Regenerate
• Adjust logic if needed

This keeps everything consistent.

Conclusion

Spec Driven Development is not some fancy trend that replaces developers, it is just a disciplined way of building software where you remove confusion before you start typing code.

If you look at most beginner projects, the problem is not that the code is bad, the problem is that the structure is unclear, APIs are inconsistent, and requirements keep changing mid way.

SDD fixes this by forcing you to slow down in the beginning and think properly, define your inputs, define your outputs, define your rules, and once that is done, the rest of the system becomes almost mechanical.

For freshers this is actually a huge advantage because you don’t need years of experience to write clean systems, you just need to be someone who can define things clearly and follow a structured approach.

Start with something small like a todo app or auth system, write the spec first, generate the backend, plug in your logic, and repeat this process a few times, and you will slowly realize that coding is no longer the bottleneck, clarity is.

Once you reach that point, you are no longer just writing code, you are designing systems.