---
name: documentation-and-adrs
description:
  Records decisions and documentation. Use when making architectural decisions,
  changing public APIs, shipping features, or when you need to record context
  that future engineers and agents will need to understand the codebase.
---

# Documentation and ADRs

## Policy Capsule

- Document the **why**, not the what. Capture context, constraints, alternatives considered, and trade-offs — code already shows what was built.
- Write an ADR for any decision that would be expensive to reverse (framework choice, data model, auth strategy, API architecture, hosting platform).
- Store ADRs at `docs/decisions/` with sequential numbering (`ADR-001`, `ADR-002`, …) and the canonical sections: **Status, Date, Context, Decision, Alternatives Considered, Consequences**.
- Mark an ADR's status as `Accepted`, `Superseded by ADR-XXX`, or `Deprecated`. Never silently delete an ADR — supersede it.
- Do **not** document obvious code; do **not** restate what the code already says. Stale or redundant docs are worse than no docs.
- Comments explain **non-obvious intent** (the why). If a comment describes what the code does, refactor the code instead.
- Keep user-facing docs (README, API docs, changelog) updated as part of the change — out-of-date docs are bugs.
- Pair every public API change with a changelog entry that links the relevant Story/Epic and any superseding ADR.
- When you find yourself explaining the same thing repeatedly in chat, write it down — the explanation belongs in the project docs or an ADR.

## Overview

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 to Use

- Making a significant architectural decision
- Choosing between competing approaches
- Adding or changing a public API
- Shipping a feature that changes user-facing behavior
- Onboarding new team members (or agents) to the project
- When you find yourself explaining the same thing repeatedly

**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.

## Architecture Decision Records (ADRs)

ADRs capture the reasoning behind significant technical decisions. They're the
highest-value documentation you can write.

### When to Write an ADR

- Choosing a framework, library, or major dependency
- Designing a data model or database schema
- Selecting an authentication strategy
- Deciding on an API architecture (REST vs. GraphQL vs. tRPC)
- Choosing between build tools, hosting platforms, or infrastructure
- Any decision that would be expensive to reverse

### ADR Template

Store ADRs in `docs/decisions/` with sequential numbering:

```markdown
# 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)
```

### ADR Lifecycle

```text
PROPOSED → ACCEPTED → (SUPERSEDED or DEPRECATED)
```

- **Don't delete old ADRs.** They capture historical context.
- When a decision changes, write a new ADR that references and supersedes the
  old one.

## Inline Documentation

### When to Comment

Comment the _why_, not the _what_:

```typescript
// 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;
}
```

### When NOT to Comment

```typescript
// 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
```

### Document Known Gotchas

```typescript
/**
 * 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 {
  // ...
}
```

## API Documentation

For public APIs (REST, GraphQL, library interfaces):

### Inline with Types (Preferred for TypeScript)

```typescript
/**
 * 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> {
  // ...
}
```

### OpenAPI / Swagger for REST APIs

```yaml
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
```

## README Structure

Every project should have a README that covers:

```markdown
# 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.
```

## Changelog Maintenance

For shipped features:

```markdown
# 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)
```

## Documentation for Agents

Special consideration for AI agent context:

- **CLAUDE.md / rules files** — Document project conventions so agents follow
  them
- **Spec files** — Keep specs updated so agents build the right thing
- **ADRs** — Help agents understand why past decisions were made (prevents
  re-deciding)
- **Inline gotchas** — Prevent agents from falling into known traps

## Common Rationalizations

| 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. |

## Red Flags

- Architectural decisions with no written rationale
- Public APIs with no documentation or types
- README that doesn't explain how to run the project
- Commented-out code instead of deletion
- TODO comments that have been there for weeks
- No ADRs in a project with significant architectural choices
- Documentation that restates the code instead of explaining intent

## Verification

After documenting:

- [ ] ADRs exist for all significant architectural decisions
- [ ] README covers quick start, commands, and architecture overview
- [ ] API functions have parameter and return type documentation
- [ ] Known gotchas are documented inline where they matter
- [ ] No commented-out code remains
- [ ] Rules files (CLAUDE.md etc.) are current and accurate
