Incremental Implementation

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 ──┐        │
│       ▲                           │        │
│       └───── Propose commit ◄─────┘        │
│              │                             │
│              ▼                             │
│          Next slice                        │
│                                            │
└────────────────────────────────────────────┘

For each slice:

1. Implement the smallest complete piece of functionality
2. Test — every slice gets a unit test, not just slices that "need" one; run the full suite alongside it
3. Verify — confirm the slice works as expected (unit tests pass, build succeeds, manual check)
4. Propose a commit — draft a descriptive message and tell the user the slice is ready (see git-workflow-and-versioning for atomic commit guidance). Never run git commit yourself — leave the actual commit to the user unless they've explicitly asked you to commit in this session.
5. Move to the next slice — carry forward, don't restart, and don't wait for the commit to happen first

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 proposed commit that adds a new component, refactors an existing one, and updates the build config.

Good: Three separately proposed commits — one for each change (the user runs them, or asks you to).

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 it in the same commit — separate them

Working with Agents

When directing an agent to implement incrementally:

"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
• A unit test was added for this increment's new logic
• 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
• A descriptive commit message has been proposed for the change (do not run git commit yourself — leave it to the user unless they've explicitly asked you to commit)

Note: Run each verification command after a change that could affect it. After a successful run, don't repeat the same command unless the code has changed since — re-running on unchanged code adds no information.

Common Rationalizations

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
• Running the same build/test command twice in a row without any intervening code change
• Running git commit yourself without the user explicitly asking you to in that session

Verification

After completing all increments for a task:

• Each increment was individually tested (a unit test was added, not just an existing-suite check) and left with a proposed commit message
• The full test suite passes
• The build is clean
• The feature works end-to-end as specified
• No unreviewed changes remain — the user has a summary and a proposed commit message for each increment; committing is theirs to do (or explicitly delegated to you)

See Also

• references/coding-patterns.md — structural patterns to apply while implementing each slice (clear main path, external systems behind a boundary, unrepresentable invalid states, decisions separated from actions, useful errors)
• review-findings.md at the project's external output location (see references/external-output-paths.md) — check it before starting a task, if it exists; it's a running log of patterns code review has already flagged in this project, and repeating one is avoidable