/incremental-implementation

name: incremental-implementation
description: Delivers changes incrementally in thin, verifiable slices. Use when implementing any feature or change that touches more than one file, or when picking up the next task from a plan. Use when rolling a change out behind a feature flag, when you're about to write a large amount of code at once, or when a task feels too big to land in one step.

Incremental Implementation

Overview

Build in thin vertical slices — implement one piece, test it, verify it, then expand. Avoid implementing an entire feature in one pass. Each increment should leave the system in a working, testable state. This is the execution discipline that makes large features manageable.

When to Use

• Implementing any multi-file change
• Building a new feature from a task breakdown
• Refactoring existing code
• Any time you're tempted to write more than ~100 lines before testing

**When NOT to use:** Single-file, single-function changes where the scope is already minimal.

The Increment Cycle

┌──────────────────────────────────────┐
│                                      │
│   Implement ──→ Test ──→ Verify ──┐  │
│       ▲                           │  │
│       └───── Commit ◄─────────────┘  │
│              │                       │
│              ▼                       │
│          Next slice                  │
│                                      │
└──────────────────────────────────────┘

For each slice:

1. **Implement** the smallest complete piece of functionality 2. **Test** — run the test suite (or write a test if none exists) 3. **Verify** — confirm the slice works as expected (tests pass, build succeeds, manual check) 4. **Commit** -- save your progress with a descriptive message (see `git-workflow-and-versioning` for atomic commit guidance) 5. **Move to the next slice** — carry forward, don't restart

Slicing Strategies

Vertical Slices (Preferred)

Build one complete path through the stack:

Slice 1: Create a task (DB + API + basic UI)
    → Tests pass, user can create a task via the UI

Slice 2: List tasks (query + API + UI)
    → Tests pass, user can see their tasks

Slice 3: Edit a task (update + API + UI)
    → Tests pass, user can modify tasks

Slice 4: Delete a task (delete + API + UI + confirmation)
    → Tests pass, full CRUD complete

Each slice delivers working end-to-end functionality.

Contract-First Slicing

When backend and frontend need to develop in parallel:

Slice 0: Define the API contract (types, interfaces, OpenAPI spec)
Slice 1a: Implement backend against the contract + API tests
Slice 1b: Implement frontend against mock data matching the contract
Slice 2: Integrate and test end-to-end

Risk-First Slicing

Tackle the riskiest or most uncertain piece first:

Slice 1: Prove the WebSocket connection works (highest risk)
Slice 2: Build real-time task updates on the proven connection
Slice 3: Add offline support and reconnection

If Slice 1 fails, you discover it before investing in Slices 2 and 3.

Implementation Rules

Rule 0: Simplicity First

Before writing any code, ask: "What is the simplest thing that could work?"

After writing code, review it against these checks:

• Can this be done in fewer lines?
• Are these abstractions earning their complexity?
• Would a staff engineer look at this and say "why didn't you just..."?
• Am I building for hypothetical future requirements, or the current task?

SIMPLICITY CHECK:
✗ Generic EventBus with middleware pipeline for one notification
✓ Simple function call

✗ Abstract factory pattern for two similar components
✓ Two straightforward components with shared utilities

✗ Config-driven form builder for three forms
✓ Three form components

Three similar lines of code is better than a premature abstraction. Implement the naive, obviously-correct version first. Optimize only after correctness is proven with tests.

Rule 0.5: Scope Discipline

Touch only what the task requires.

Do NOT:

• "Clean up" code adjacent to your change
• Refactor imports in files you're not modifying
• Remove comments you don't fully understand
• Add features not in the spec because they "seem useful"
• Modernize syntax in files you're only reading

If you notice something worth improving outside your task scope, note it — don't fix it:

NOTICED BUT NOT TOUCHING:
- src/utils/format.ts has an unused import (unrelated to this task)
- The auth middleware could use better error messages (separate task)
→ Want me to create tasks for these?

Rule 1: One Thing at a Time

Each increment changes one logical thing. Don't mix concerns:

**Bad:** One commit that adds a new component, refactors an existing one, and updates the build config.

**Good:** Three separate commits — one for each change.

Rule 2: Keep It Compilable

After each increment, the project must build and existing tests must pass. Don't leave the codebase in a broken state between slices.

Rule 3: Feature Flags for Incomplete Features

If a feature isn't ready for users but you need to merge increments:

// Feature flag for work-in-progress
const ENABLE_TASK_SHARING = process.env.FEATURE_TASK_SHARING === 'true';

if (ENABLE_TASK_SHARING) {
  // New sharing UI
}

This lets you merge small increments to the main branch without exposing incomplete work.

Rule 4: Safe Defaults

New code should default to safe, conservative behavior:

// Safe: disabled by default, opt-in
export function createTask(data: TaskInput, options?: { notify?: boolean }) {
  const shouldNotify = options?.notify ?? false;
  // ...
}

Rule 5: Rollback-Friendly

Each increment should be independently revertable:

• Additive changes (new files, new functions) are easy to revert
• Modifications to existing code should be minimal and focused
• Database migrations should have corresponding rollback migrations
• Avoid deleting something in one commit and replacing i