Incremental Implementation

Overview

A large change written in one pass fails in one piece: nothing can be run until everything exists, the first error could be anywhere in a thousand lines, and the reviewer gets a diff too big to read. The alternative is to cut the work into steps that each do one thing, prove it with a command, and leave the project in a state that could be merged.

This skill covers how to find the project's checks, how to cut a change into steps using the established patterns (vertical slice, expand–migrate–contract, branch by abstraction, feature flag), what to do after every step, and how to get back to a working state when a step goes wrong.

Instructions

1. Establish the baseline

1. Find the commands that define "working". The CI workflow is the authoritative list; otherwise read package.json scripts, Makefile, justfile, pyproject.toml or the README. Typically: build, type check, lint, tests.
2. Run them before touching anything, with a time limit so a hanging test cannot stall the loop (timeout 300 npm test). Record the result. A check that already fails is reported to the user and excluded from "green" by agreement, not fixed silently.
3. Look at the working tree. If it holds uncommitted work, ask before building on top of it.
4. Ask what the code cannot tell you: commit after every step or leave commits to the user; work on a new branch or the current one; whether unfinished work may reach the main branch behind a flag; whether a database or public API is involved and who else consumes it.

2. Cut the change into steps