working-with-legacy-code SKILL.md skillA
working-with-legacy-code from wondelai/skills is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add wondelai/skills/working-with-legacy-code@v1.2.0Pins v1.2.0 in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- wondelai/skills
- Path
working-with-legacy-code/SKILL.md- Versions recorded
- 3
- Latest version
- v1.2.0, committed Jun 30, 2026
- First version recorded
- Jun 11, 2026
- Size
- 19,511 bytes
- Repository stars
- 2,640
- License
- MIT
Safely change and test untested codebases using Feathers'' "Working Effectively with Legacy Code". Use when the user mentions "legacy code", "no tests", "untested codebase", "how do I test this", "seams", "characterization tests", "golden master", "sprout method", "afraid to change this code", "monster method", "dependency breaking", or "inherited a messy codebase". Also trigger when changing code without tests safely, getting a class under test when constructors, statics, or singletons block…
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# Working Effectively with Legacy Code A field manual for changing code that has no tests, distilled from Michael C. Feathers' *Working Effectively with Legacy Code*. Use it to get untestable classes into a harness, pin down current behavior with characterization tests, and make changes one safe, verifiable step at a time — without resorting to a rewrite. ## Core Principle **Legacy code is simply code without tests.** Not old code, not ugly code — untested code: without tests you cannot know whether a change preserves behavior, so every edit is a gamble. The craft is breaking dependencies just enough to get tests in place before changing anything — cover and modify, never edit and pray. ## Scoring **Goal: 10/10.** Rate changes to untested code 0-10 against the principles below. Report the current score and the specific steps needed to reach 10/10. - **9-10:** Change points covered by characterization tests before any edit; behavior changes and refactoring shipped as separate verified steps; dependencies broken with the least invasive technique - **7-8:** Tests at most change points, but occasional mixed refactor-plus-behavior commits or heavier dependency surgery than needed …
Read the whole file at its exact version.
How to install
mdr add wondelai/skills/working-with-legacy-code@sha256:4a621798f9a51ad1The line at the top of this page pins the label v1.2.0, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
[](https://markdownregistry.com/a/art_2xsvpc4shexqgmh5)
5 badge image fetches in 30 days. What the badge shows.
Versions
The registry has recorded 3 versions of this file, between Jun 11, 2026 and Jun 30, 2026, about one every 10 days.
| version | committed | commit | size | audit | |
|---|---|---|---|---|---|
| v1.2.0 latest | 2026-06-30 | 0dea03f | 19,511 B | A | view · diff |
| v1.1.0 | 2026-06-24 | 5356d12 | 19,735 B | A | view · diff |
| v1.0.0 | 2026-06-11 | 71ae229 | 19,688 B | A | view |
Audit of the latest version
- pass: Frontmatter block present
- pass: Frontmatter declares a name
- pass: Frontmatter declares a description
- pass: Size between 200 bytes and 200 KB (19511 bytes)
- pass: No zero-width or bidi control characters
- pass: No instruction hidden inside an HTML comment
- pass: No link to an exfiltration or paste host
- pass: No credential-shaped string
- pass: No instruction to send local credentials anywhere
- pass: No text hidden with inline styles
- pass: No prompt-injection phrasing
- pass: No curl or wget piped into a shell
- pass: No recursive delete of root, home or parent
- pass: No instruction to read or print local credentials
- pass: No base64 blob over 200 characters
- pass: No link to a raw IP address
- pass: No script tag
Source
wondelai/skills · 2,640 stars · license MIT · pushed 2026-11-02 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_2xsvpc4shexqgmh5 GET https://markdownregistry.com/api/v1/resolve?ref=wondelai/skills/working-with-legacy-code GET https://markdownregistry.com/api/v1/blob/4a621798f9a51ad1438f5b25b168615b7caeb540378c8f4d70ab733523135e21
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
Guides: the SKILL.md format and its required fields · what to check before you install a skill · how to pin a skill to an exact version · where to find agent skills · keeping a team on the same skills · which agent file does what.