name: legacy-testing
description: Michael Feathers–style legacy code change — treat code without automated tests as legacy, then follow identify change points, find test points, break dependencies, add tests (often characterization), and only then modify or refactor. Covers seams (object/link/preprocessing), sensing vs separation, fakes/mocks, sprout/wrap, pinch points, effect sketches, and dependency-breaking patterns. Composes with bugfix, TDD, and refactoring once a harness exists. Roadmap id: add-tests-to-legacy-code. Use when changing untested code, introducing a test seam, or adding coverage before a risky edit.
allowed-tools: Read, Edit, MultiEdit, Bash, Grep, Glob
Legacy code testing (Feathers-style)
Mission
Legacy code (Feathers): code without automated tests covering the behavior you need to change. Not “old” — unprotected. Without tests you cannot know whether a change helps or harms the system.
Goal: build a test harness and pin down behavior so later edits use normal bugfix, TDD, or refactoring skills safely.
Think: strap on sensors before opening the engine — you are not proving virtue first; you are documenting what happens today so the next move is reversible and observable.
Optimize for:
This skill does not replace ATDD for new capability definition; it often precedes confident refactoring.
Feathers’ change algorithm (five phases)
Work in order; do not skip “find test points” and “break dependencies” by writing giant integration tests that never fail for the right reason.
- Identify change points — where must the code differ after your goal?
- Find test points — where can you observe or assert behavior cheaply?
- Break dependencies — make the code callable / observable in a test harness (seams, fakes, parameterization).
- Write tests — usually characterization first; add bug-targeted tests if you already know wrong behavior.
- Modify and refactor — implement; then refactor with
skills/refactoring now that behavior is pinned.
Detail: references/feathers-change-algorithm.md.
Quick techniques (when time is short)
| Technique | Use when |
|---|
| Sprout Method / Class | New logic can live in a new unit; legacy calls into it (minimal surface change). |
| Wrap Method / Class | Add behavior before/after existing code (often decorator-like); existing core stays untouched at first. |
See references/sprout-and-wrap.md.
Core concepts (recap)
Integration with other discipline (Fowler, Beck, Farley, Cupac)
- Kent Beck — same tiny verified steps and revert instinct as TDD; legacy work just pays the debt to get to RED/GREEN/REFACTOR at useful seams.
- Martin Fowler — legacy session often ends in refactoring; until tests exist, many edits are seam-finding and characterization, not full catalog refactorings.
- Dave Farley — prefer small, mergable steps that keep CI honest; legacy work is not an excuse to turn off the pipeline for weeks — chunk into harness slices.
- Valentina Cupać — tests you add should target behavior you care about preserving, not implementation detail, or refactors will remain painful (
skills/refactoring/references/tests-and-design.md).
More: references/aligned-practices.md.
Language-agnostic execution
Shared delivery rules: docs/delivery-process.md (especially §8 harness, §2 verification, §3 change-surface) and docs/project-verification.md.
Inspect the repo for:
- test runner, module layout, DI patterns, build/link seams
- all verify steps the project defines — tests, lint, mutation, property-based, contract, component, integration; not only the test runner
- existing “golden” or snapshot tests (possibly abuse-prone — use thoughtfully)
Test strategy: Before adding tests, evaluate practices per test-strategy-selection.md.
Prefer narrowest tests that still encode the legacy contract you are about to touch.
Pre-harnessed: If automated checks already exist, are valid, and cover the change path → say so explicitly; do not run Feathers phases for theater. If harness is invalid (compile/type errors, wrong construction API) → fix harness before feature or large refactor work.
When this skill is the wrong tool
- Greenfield feature with strong suite → use
skills/tdd / skills/atdd
- Known bug with reproducible failure and existing coverage at seam →
skills/bugfix
- Pure structure change with good tests →
skills/refactoring only
Anti-patterns (summary)
- Changing production first, “we’ll backfill tests”
- Characterization tests that assert incidental output (stack traces, timestamps) → brittle noise
- Monster end-to-end only, no unit-level pinch points
- Mocks that encode desired design instead of current behavior during characterization
- One PR that “adds tests” by rewriting production semantics
Full list: references/anti-patterns.md.
Definition of done (typical slice)
- Change point and test point documented (short note in PR or team doc folder if used)
- Dependencies broken only as much as needed for the next test
- Characterization (and/or bug) tests green on baseline
- All applicable project verify steps for this scope run and passed (
docs/delivery-process.md §2; docs/project-verification.md)
- Change-surface complete if seams/factories/APIs changed (§3)
- Production change (if any) passes checks; refactor commits separate per
skills/refactoring
Checklists: checklists/before-change.md, checklists/one-intervention.md, checklists/session-done.md.
Output format
## Legacy harness / change: <goal>
### Baseline
- Legacy area: <module / classes / paths>
- Tests before: none | thin — noted risks
### Feathers sequence
1. Change points: <bullets>
2. Test points: <bullets>
3. Dependencies broken: <techniques + seams>
4. Tests added: <characterization | bug repro | both> — `<paths>`
5. Modify/refactor: <summary; hats clearly separated>
### Commits (suggested)
- test: characterization for …
- refactor: seam only … (optional)
- feat:/fix: … (after green net)
### Follow-up
- Next pinch point / deferred monster split
Additional resources
References
Examples
Checklists
Bibliography
Resource usage
Open references when seam choice, characterization scope, or dependency-breaking pattern is contentious.
