incremental-implementation – skill Viết lách & Nội dung cho agent AI

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• <support-root>/references/definition-of-done.md

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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.

Apply the project's actual available commands and commit policy; npm test/build/tsc/lint below are examples. A feature flag must guard backend permissions and effects as well as UI. Database rollback may require forward repair, data recovery and compatibility staging rather than a reversible down migration. Keep removal/replacement slices compatible so separated commits do not leave a broken intermediate state. Report skipped/unavailable checks; passing tests support bounded claims, not proof of correctness.

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 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
• [ ] 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

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

| 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. |

| "Let me run the build command again just to be sure" | After a successful run, repeating the same command adds nothing unless the code has changed since. Run it again after subsequent edits, not as reassurance. |

Red Flags

• More than 1

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