Refactoring code

Refactoring Code

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Refactoring changes the internal structure of code to make it easier to understand and cheaper to modify without changing its observable behavior. This skill teaches you how to think through a refactoring engagement — understand, diagnose, assess risk, transform incrementally, verify, and review — not just which transformations exist. The catalogs (smells, patterns, architecture, security, performance, testing) live in references/ and are loaded only when the step you're on needs them.

The two hats

Make the change easy, then make the easy change.

At any moment you wear exactly one hat: refactoring (structure changes, behavior identical) or adding functionality (behavior changes, structure stable). Never both in the same step. If a task needs both, refactor first with tests green, commit, then change behavior. Mixing them makes failures undiagnosable: when a test breaks you can't tell whether the structure or the behavior change caused it.

The contract

These rules are not preferences — they are what makes a change refactoring rather than rewriting:

1. Preserve observable behavior. Business logic, API contracts, database reads/writes and schemas, public interfaces, error semantics, side-effect ordering, and backward compatibility all stay identical — unless the user explicitly approves a breaking change. When in doubt whether something is observable, treat it as observable.
2. Never remove or weaken tests. Tests are the definition of current behavior. If a test blocks a refactoring because it asserts on internals, surface that and propose a replacement — don't delete it quietly. If tests are missing, recommend or add them first (see references/testing.md).
3. Smallest reversible steps. Compose large refactorings from many tiny green-to-green transformations. Ten small commits beat one big rewrite because a failure always points at the last step.
4. Separate analysis, planning, and transformation. Report what you found, propose what you'll do, then do it. Don't discover-and-rewrite in one motion — the user needs a decision point before code changes.
5. Match the codebase. Follow its naming, idioms, structure, and formatting. Refactoring toward your personal style is churn, not improvement.
6. Don't gold-plate. Improve what the task touches or the user asked about. Mention out-of-scope smells; don't fix them unprompted.

How to think: the workflow

Work through these steps in order. Each step names the reference file to read when you reach it — don't front-load them all.

Phase A — Understand (before judging anything)

1. Understand the system. What does this code do for its users? Trace entry points, data flow, and dependency direction. Read callers, not just the target file.
2. Understand responsibilities. For each class/module in scope: what is it for, who depends on it, what would change together? Misplaced responsibility is the root of most smells.

Phase B — Diagnose and plan

3. Detect smells. Name them precisely — "Feature Envy in Order.calculateShipping" beats "this looks messy". Read references/code-smells.md for the catalog with detection heuristics.
4. Estimate risk. Score each candidate change using the risk model below. Risk decides step size and verification depth, and sometimes decides not to refactor.
5. Choose the safest transformation that fixes the smell. Prefer the smallest pattern that resolves the problem; introduce design patterns only when the smell demands them, never speculatively. Read references/patterns.md for the pattern catalog and selection guidance. Write the plan as an ordered sequence of named, individually verifiable steps.

Phase C — Transform and verify

6. Refactor incrementally. One named transformation at a time: tests green → apply → tests green → checkpoint/commit. If tests go red, revert — don't debug a broken refactoring forward.
7. Validate behavior. Tests, type-checker, build, and — for anything user-facing — actually exercising the affected flow. Diff review: confirm every change is structural.

Phase D — Review and recommend

8. Evaluate architecture. Did dependency direction, cohesion, and boundaries improve? Read references/architecture.md when the engagement touches module boundaries or the user asks about design.
9. Review security. You've just read this code more closely than anyone in months — check the flagged areas in references/security.md. Report vulnerabilities; fixing one is a behavior change that needs the user's sign-off.
10. Review performance. Check for N+1 queries, hot-path allocations, blocking I/O per references/performance.md, and verify your own refactoring didn't regress performance.
11. Recommend future improvements. What's out of scope but worth doing next? Ordered by value/risk.

For small tasks ("extract this function") compress the phases but keep the order — you still understand before you cut, and you still verify after. For audits and large engagements, follow references/workflow.md for the detailed per-phase checklists and the full report template.

Risk model

Risk is the probability that a transformation silently changes behavior, times the blast radius if it does. Assess before every change:

Risk	Typical changes	Protocol
Low	Rename (tool-assisted), Extract Variable/Constant, Extract Method with no shared mutable state, Remove provably dead code, formatting	Batch several per commit; compiler/type-checker is often sufficient verification
Medium	Move Method/Field, Extract Class, signature changes, Replace Conditional with Polymorphism, Encapsulate Field/Collection	One per commit; run the full test suite; check all call sites
High	Anything touching concurrency, shared mutable state, side-effect ordering, reflection/serialization/ORM mappings, framework-magic conventions (DI containers, decorators, naming-based wiring), public APIs, SQL, caching, or code with no test coverage	Characterization tests first; smallest possible steps; consider Parallel Change (expand → migrate → contract); flag to the user before starting

Danger multipliers that upgrade any change one tier: dynamic dispatch or reflection can hide call sites from search; serialized/persisted formats can depend on field names and order; overridden methods mean callers you can't see; test coverage below the change's blast radius.

If no tests exist and none can be added, say so explicitly, restrict yourself to transformations the compiler/type-checker can verify, and keep steps microscopic.

Working in very large repositories

Don't try to read everything — scope deliberately:

• Measure blast radius before planning. Search for every usage of the symbols you'll touch (including string-based references: reflection, config files, templates, serialization). Fan-in count is your best single risk signal.
• Refactor along seams. Pick a boundary (one module, one vertical slice) where the change is complete and shippable by itself. Avoid plans that require touching 40 files atomically.
• Use Parallel Change for wide-reaching renames/signature changes: add the new form, migrate callers incrementally, remove the old form last. Each stage ships green.
• Sample, don't exhaust. For a repo-wide smell (e.g., duplicated validation logic), find representative instances, fix the pattern in one place, then report the remaining instances with locations rather than editing hundreds of files unprompted.

Quick smell → pattern map

The one-screen version for common cases; full catalog with heuristics in references/code-smells.md, mechanics in references/patterns.md.

Smell	First-choice refactorings
Long Method / Deep Nesting	Extract Method, Replace Nested If with Guard Clauses, Decompose Conditional
God Object / Long Class	Extract Class, Split Class, Extract Module
Duplicate Code	Merge Duplicate Logic, Extract Method/Function
Switch on type / repeated conditionals	Replace Conditional with Polymorphism, Extract Strategy, Introduce State
Primitive Obsession / Data Clumps	Replace Primitive with Object, Introduce Value Object
Magic Numbers / Strings	Extract Constant
Feature Envy	Move Method
Shotgun Surgery / Divergent Change	Move Method/Field to consolidate, Extract Class to separate
Tight Coupling / hidden new calls	Introduce Dependency Injection, Extract Interface, Introduce Facade/Adapter
Missing Encapsulation	Encapsulate Field, Encapsulate Collection
Dead Code / Speculative Generality	Remove Dead Code, Inline/Collapse the unused abstraction

Output

Scale the report to the engagement. A one-function extract needs: smells found (file:line), refactorings applied, confirmation tests stay green. A codebase audit or multi-step refactor uses the full report structure — Executive Summary, Architecture Review, Code Smells, Risk Assessment, Refactoring Plan, Incremental Changes, Security Review, Performance Review, Testing Impact, Complexity & Maintainability, Suggested Design Patterns, Future Improvements — templated in references/workflow.md.

Always cite locations as file:line, name every smell and pattern by its catalog name, and state explicitly whether behavior is preserved and how you verified it.

Routing Boundary

Use this skill when the user requests a behavior-preserving structural change to existing code: extract, simplify, reorganize, remove duplication, improve testability, or incrementally modernize.

Do NOT use this skill when the user wants diagnosis only (code-review, solid-review, or clean-architecture-review), a new feature, a behavior-changing bug fix (debugging), or only pattern selection (design-pattern-advisor).

Routing note: A review can recommend a refactor, but this skill wins only when the requested deliverable includes a safe structural implementation plan or change.

Optional Workflow Integration

This skill is fully standalone: it never requires another skill, a handoff, or workflow files. Workflow output is opt-in when the user requests persistent output or .ai-workflow/ already exists (unless the user opts out). Follow the packaged workflow contract.

Relevant handoff topics: architecture, code-quality, complexity, dependencies, duplication, maintainability, performance, security, testing.

When enabled, inspect only matching concise handoffs as optional leads, verify important claims in the repository and characterization tests, and avoid opening full artifacts unless evidence is needed. Complete this skill's normal refactoring report first; then save that specialized report to .ai-workflow/artifacts/refactoring-code.md, write the standardized concise handoff to .ai-workflow/handoffs/refactoring-code.json, and update only runs.refactoring-code in state.json while preserving other runs and unknown metadata. Missing, invalid, or irrelevant workflow data never blocks behavior-preserving refactoring.

Reference map

Read	When
references/workflow.md	Multi-step engagements, audits, or when you need the full report template and per-phase checklists
references/code-smells.md	Diagnosing — naming what's wrong and why it hurts
references/patterns.md	Planning/executing — mechanics, risk, and selection guidance for every supported transformation
references/architecture.md	Boundaries, layering, SOLID, or migration toward an architectural style
references/security.md	Step 9 of every engagement; mandatory when code touches input handling, auth, SQL, files, or serialization
references/performance.md	Step 10 of every engagement; mandatory when code touches loops over I/O, queries, rendering, or hot paths
references/testing.md	Before refactoring untested code; when recommending or generating tests