Document decisions, not just code. The most valuable documentation captures the why — the context, constraints, and trade-offs that led to a decision. Code shows what was built; documentation explains why it was built this way and what alternatives were considered. This context is essential for future humans and agents working in the codebase.
When NOT to use: Don't document obvious code. Don't add comments that restate what the code already says. Don't write docs for throwaway prototypes.
ADRs capture the reasoning behind significant technical decisions. They're the highest-value documentation you can write.
Store ADRs in docs/decisions/ with sequential numbering:
# ADR-001: Use PostgreSQL for primary database
## Status
Accepted | Superseded by ADR-XXX | Deprecated
## Date
2025-01-15
## Context
We need a primary database for the task management application. Key requirements:
- Relational data model (users, tasks, teams with relationships)
- ACID transactions for task state changes
- Support for full-text search on task content
- Managed hosting available (for small team, limited ops capacity)
## Decision
Use PostgreSQL with Prisma ORM.
## Alternatives Considered
### MongoDB
- Pros: Flexible schema, easy to start with
- Cons: Our data is inherently relational; would need to manage relationships manually
- Rejected: Relational data in a document store leads to complex joins or data duplication
### SQLite
- Pros: Zero configuration, embedded, fast for reads
- Cons: Limited concurrent write support, no managed hosting for production
- Rejected: Not suitable for multi-user web application in production
### MySQL
- Pros: Mature, widely supported
- Cons: PostgreSQL has better JSON support, full-text search, and ecosystem tooling
- Rejected: PostgreSQL is the better fit for our feature requirements
## Consequences
- Prisma provides type-safe database access and migration management
- We can use PostgreSQL's full-text search instead of adding Elasticsearch
- Team needs PostgreSQL knowledge (standard skill, low risk)
- Hosting on managed service (Supabase, Neon, or RDS)
PROPOSED → ACCEPTED → (SUPERSEDED or DEPRECATED)
Comment the why, not the what:
// BAD: Restates the code
// Increment counter by 1
counter += 1;
// GOOD: Explains non-obvious intent
// Rate limit uses a sliding window — reset counter at window boundary,
// not on a fixed schedule, to prevent burst attacks at window edges
if (now - windowStart > WINDOW_SIZE_MS) {
counter = 0;
windowStart = now;
}
// Don't comment self-explanatory code
function calculateTotal(items: CartItem[]): number {
return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
}
// Don't leave TODO comments for things you should just do now
// TODO: add error handling ← Just add it
// Don't leave commented-out code
// const oldImplementation = () => { ... } ← Delete it, git has history
/**
* IMPORTANT: This function must be called before the first render.
* If called after hydration, it causes a flash of unstyled content
* because the theme context isn't available during SSR.
*
* See ADR-003 for the full design rationale.
*/
export function initializeTheme(theme: Theme): void {
// ...
}
For public APIs (REST, GraphQL, library interfaces):
/**
* Creates a new task.
*
* @param input - Task creation data (title required, description optional)
* @returns The created task with server-generated ID and timestamps
* @throws {ValidationError} If title is empty or exceeds 200 characters
* @throws {AuthenticationError} If the user is not authenticated
*
* @example
* const task = await createTask({ title: 'Buy groceries' });
* console.log(task.id); // "task_abc123"
*/
export async function createTask(input: CreateTaskInput): Promise<Task> {
// ...
}
paths:
/api/tasks:
post:
summary: Create a task
requestBody:
required: true
content:
application/json:
schema:
$ref: '#/components/schemas/CreateTaskInput'
responses:
'201':
description: Task created
content:
application/json:
schema:
$ref: '#/components/schemas/Task'
'422':
description: Validation error
Every project should have a README that covers:
# Project Name
One-paragraph description of what this project does.
## Quick Start
1. Clone the repo
2. Install dependencies: `npm install`
3. Set up environment: `cp .env.example .env`
4. Run the dev server: `npm run dev`
## Commands
| Command | Description |
|---------|-------------|
| `npm run dev` | Start development server |
| `npm test` | Run tests |
| `npm run build` | Production build |
| `npm run lint` | Run linter |
## Architecture
Brief overview of the project structure and key design decisions.
Link to ADRs for details.
## Contributing
How to contribute, coding standards, PR process.
For shipped features:
# Changelog
## [1.2.0] - 2025-01-20
### Added
- Task sharing: users can share tasks with team members (#123)
- Email notifications for task assignments (#124)
### Fixed
- Duplicate tasks appearing when rapidly clicking create button (#125)
### Changed
- Task list now loads 50 items per page (was 20) for better UX (#126)
Special consideration for AI agent context:
| Rationalization | Reality |
|---|---|
| "The code is self-documenting" | Code shows what. It doesn't show why, what alternatives were rejected, or what constraints apply. |
| "We'll write docs when the API stabilizes" | APIs stabilize faster when you document them. The doc is the first test of the design. |
| "Nobody reads docs" | Agents do. Future engineers do. Your 3-months-later self does. |
| "ADRs are overhead" | A 10-minute ADR prevents a 2-hour debate about the same decision six months later. |
| "Comments get outdated" | Comments on why are stable. Comments on what get outdated — that's why you only write the former. |
After documenting: