Incremental Implementation Skill

Incremental Implementation

Overview

Build in thin vertical slices. Implement one piece, test it, verify it, then expand. Each increment MUST leave the system in a working, testable state. Slicing this way keeps large features tractable.

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:

Implement the smallest complete piece of functionality.
Test: run the test suite, or write a test if none exists.
Verify: confirm the slice works: tests pass, build succeeds, manual check holds.
Commit: save progress with a descriptive message; keep the commit atomic (one logical change).
Move to the next slice: carry forward, never 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

The worked slice sequence (Slice 0 contract, 1a backend, 1b frontend, 2 integration) lives in references/slicing-strategies.md — read it when backend and frontend develop in parallel against a shared API contract.

Risk-First Slicing

The worked slice sequence for de-risking the most uncertain piece first lives in references/slicing-strategies.md — read it when one piece of the task carries outsized uncertainty or risk that the rest depends on.

Implementation Rules

Rule 0: Simplicity First

Before writing code, ask: what is the simplest thing that could work?

After writing code, check it against:

Can this be done in fewer lines?
Are these abstractions earning their complexity?
Would a staff engineer ask "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 beat 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 absent from the spec because they "seem useful"
Modernize syntax in files you're only reading

When you notice something worth improving outside scope, record it. Do not 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. NEVER mix concerns.

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

Good: three separate commits, one per change.

Rule 2: Keep It Compilable

After each increment the project MUST build and existing tests MUST pass. NEVER leave the codebase broken between slices.

Rule 3: Feature Flags for Incomplete Features

When a feature isn't ready for users but you need to merge increments, gate it behind a flag so partial work stays dark:

The worked TypeScript and Python examples live in references/feature-flags.md — read it when a feature must merge to main before it is user-ready; skip it when nothing partial is being merged.

Rule 4: Safe Defaults

New code MUST default to conservative behavior. Opt in to side effects, never opt out:

// TypeScript — disabled by default, caller opts in
export function createTask(data: TaskInput, options?: { notify?: boolean }) {
  const shouldNotify = options?.notify ?? false;
  // ...
}

# Python — disabled by default, caller opts in
def create_task(data, *, notify: bool = False):
    should_notify = notify
    # ...

Rule 5: Rollback-Friendly

Each increment MUST be independently revertable:

Additive changes (new files, new functions) revert cleanly.
Modifications to existing code stay minimal and focused.
Database migrations carry a corresponding rollback migration.
NEVER delete something and replace it in the same commit. Separate the two.

Working with Agents

When directing an agent to implement incrementally, scope each increment explicitly:

"Implement Task 3 from the plan.

Start with the database schema change and the API endpoint only.
Leave the UI for the next increment.

After implementing, run the stack's test and build gates to confirm
nothing is broken."

State what is in scope and what is NOT in scope for each increment.

Increment Checklist

After each increment, verify. Command examples span ecosystems. Run whichever your stack uses:

[ ] The change does one thing and does it completely.
[ ] All existing tests still pass (npm test / pytest / go test ./... / cargo test).
[ ] The build succeeds (npm run build / python -m build / go build ./... / cargo build).
[ ] Type checking passes (tsc --noEmit / mypy . / cargo check).
[ ] Linting passes (npm run lint / ruff check / golangci-lint run).
[ ] 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, do not 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 MUST NOT 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 changed. Re-run after later edits, not as reassurance. |

Red Flags

More than 100 lines written without running tests
"Let me just quickly add this too" scope expansion
Skipping the test/verify step to move faster
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.

Agent Skills: Incremental Implementation

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