---
name: incremental-implementation
description:
  Delivers changes incrementally. Use when implementing any feature or change
  that touches more than one file. Use 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

## Policy Capsule

- Deliver in thin vertical slices: **Implement → Test → Verify → Commit → Next slice**. Never write more than ~100 lines before testing.
- Each slice MUST leave the system in a working, testable state — build green, existing tests pass, type-check and lint clean.
- Each commit changes one logical thing; never mix refactors with features or formatting with behavior.
- Apply Simplicity First: write the naive, obviously-correct version. Three similar lines beats a premature abstraction; do not build for hypothetical future requirements.
- Enforce Scope Discipline: touch only what the task requires. No drive-by cleanups, import re-orderings, comment deletions, or syntax modernizations in files you're "just reading" — note them instead.
- Gate incomplete work behind a feature flag rather than holding a long-lived branch; default new behaviour to safe / opt-in.
- Keep increments rollback-friendly: prefer additive changes, ship DB migrations with rollbacks, and never delete + replace in the same commit.
- Prefer vertical slices that deliver end-to-end functionality; use contract-first slicing only when backend/frontend must develop in parallel, and risk-first slicing when the riskiest piece dominates the plan.
- Verify the increment checklist before moving on: tests pass, build succeeds, types check, lint clean, change committed with a descriptive message.

## 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

```text
┌──────────────────────────────────────┐
│                                      │
│   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:

```text
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:

```text
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:

```text
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?

```text
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:

```text
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:

```typescript
// 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:

```typescript
// 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 it in the same commit —
  separate them

## Working with Agents

When directing an agent to implement incrementally:

```text
"Let's implement Task 3 from the plan.

Start with just the database schema change and the API endpoint.
Don't touch the UI yet — we'll do that in the next increment.

After implementing, run `npm test` and `npm run build` to verify
nothing is broken."
```

Be explicit about what's in scope and what's NOT in scope for each increment.

## Increment Checklist

After each increment, verify:

- [ ] The change does one thing and does it completely
- [ ] All existing tests still pass (`npm test`)
- [ ] The build succeeds (`npm run build`)
- [ ] Type checking passes (`npx tsc --noEmit`)
- [ ] Linting passes (`npm run lint`)
- [ ] The new functionality works as expected
- [ ] The change is committed with a descriptive message

## Common Rationalizations

| Rationalization                                    | Reality                                                                                           |
| -------------------------------------------------- | ------------------------------------------------------------------------------------------------- |
| "I'll test it all at the end"                      | Bugs compound. A bug in Slice 1 makes Slices 2-5 wrong. Test each slice.                          |
| "It's faster to do it all at once"                 | It _feels_ faster until something breaks and you can't find which of 500 changed lines caused it. |
| "These changes are too small to commit separately" | Small commits are free. Large commits hide bugs and make rollbacks painful.                       |
| "I'll add the feature flag later"                  | If the feature isn't complete, it shouldn't be user-visible. Add the flag now.                    |
| "This refactor is small enough to include"         | Refactors mixed with features make both harder to review and debug. Separate them.                |

## Red Flags

- More than 100 lines of code written without running tests
- Multiple unrelated changes in a single increment
- "Let me just quickly add this too" scope expansion
- Skipping the test/verify step to move faster
- Build or tests broken between increments
- Large uncommitted changes accumulating
- Building abstractions before the third use case demands it
- Touching files outside the task scope "while I'm here"
- Creating new utility files for one-time operations

## Verification

After completing all increments for a task:

- [ ] Each increment was individually tested and committed
- [ ] The full test suite passes
- [ ] The build is clean
- [ ] The feature works end-to-end as specified
- [ ] No uncommitted changes remain
