working-effectively-with-legacy-code SKILL.md skillA
working-effectively-with-legacy-code from ciembor/agent-rules-books is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add ciembor/agent-rules-books/working-effectively-with-legacy-code@git:20260909.802595fPins git:20260909.802595f in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- ciembor/agent-rules-books
- Path
working-effectively-with-legacy-code/SKILL.md- Versions recorded
- 1
- Latest version
- git:20260909.802595f, committed Sep 9, 2026
- Size
- 803 bytes
- Repository stars
- 3,214
- License
- MIT
Apply Michael Feathers-inspired legacy-code rules when changing hard-to-test code safely with characterization tests, seams, sprout methods, or wrap methods.
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 Skill Use this skill when a task involves risky legacy changes, missing tests, characterization tests, seams, dependency breaking, sprout method, wrap method, or incremental risk reduction. Before making design or code decisions, read and apply [working-effectively-with-legacy-code.mini.md](working-effectively-with-legacy-code.mini.md). Use [working-effectively-with-legacy-code.md](working-effectively-with-legacy-code.md) only as a deeper reference when the mini rules are not enough for the current legacy-code tradeoff.
Read the whole file at its exact version.
How to install
mdr add ciembor/agent-rules-books/working-effectively-with-legacy-code@sha256:02bdaf78eb4c8eb2The line at the top of this page pins the label git:20260909.802595f, 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_gs2q2jlmxeptopsc)
3 badge image fetches in 30 days. What the badge shows.
Versions
The registry has recorded 1 version of this file, committed on Sep 9, 2026.
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 (803 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
ciembor/agent-rules-books · 3,214 stars · license MIT · pushed 2026-09-10 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_gs2q2jlmxeptopsc GET https://markdownregistry.com/api/v1/resolve?ref=ciembor/agent-rules-books/working-effectively-with-legacy-code GET https://markdownregistry.com/api/v1/blob/02bdaf78eb4c8eb28eda674d8d31e31d7ec3c272bd928935daeb6c0869038daa
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.