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Agent skill

Characterisation Tests

by citypaul in citypaul/.dotfiles

A skill your agent uses when modifying existing code that lacks tests and you need to document its actual current behavior before making changes -- the legacy code dilemma where you need tests to…

Custom licenceAuto-check passedTesting & QA

Install Characterisation Tests

skills CLI
$ npx skills add citypaul/.dotfiles --skill characterisation-tests -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install citypaul/.dotfiles characterisation-tests --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/citypaul/.dotfiles.git skills-src && mkdir -p .claude/skills && cp -r skills-src/claude/.claude/skills/characterisation-tests .claude/skills/characterisation-tests && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
characterisation-tests
GitHub stars
740
Token cost
~3.6k tokens
SKILL.md length
1,614 words
Files
3
Skills in repo
44
Repo updated
First seen
Licence
Custom licence

At a glance

A skill your agent uses when modifying existing code that lacks tests and you need to document its actual current behavior before making changes -- the legacy code dilemma where you need tests to…

  • Works in 5 steps: Use a piece of code in a test harness → Write an assertion you know will fail… → Run the tests and let the failure tell… → …
  • Test-driving new behavior (see tdd)
  • SKILL.md covers Core Concept, When to Use, When NOT to Use and Naming and Identification, plus 7 more sections
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Characterisation Tests is an agent skill from citypaul/.dotfiles. Use when modifying existing code that lacks tests and you need to document its actual current behavior before making changes -- the legacy code dilemma where you need tests to refactor safely but the code was not written for testability. Specifically for understanding and pinning down what code currently does, not what it should do. Do NOT use for test-driving new behavior (see tdd), general test writing patterns (see testing), verifying test effectiveness (see mutation-testing), or making untestable code…

Its SKILL.md is about 3.6k tokens, which your agent loads only when the skill is triggered. The skill folder holds 3 other files (for example `resources/modern-tooling.md` and `resources/writing-process.md`).

It sits in Testing & QA, covering Legacy modernization, Test coverage and Test-driven development.

When your agent uses it

  • Test-driving new behavior (see tdd)
  • General test writing patterns (see testing)
  • Verifying test effectiveness (see mutation-testing)
  • Making untestable code testable (see finding-seams)

Example prompts

  • “/characterisation-tests”

Workflow steps

5 steps, taken from the first numbered list in SKILL.md.

  1. Use a piece of code in a test harness
  2. Write an assertion you know will fail (use a dummy value like "PLACEHOLDER")
  3. Run the tests and let the failure tell you the behavior -- the test runner shows the actual value
  4. Change the test so it expects the behavior the code actually produces
  5. Repeat -- let curiosity guide you; the code itself suggests what to test next

What it can do on your machine

Read from SKILL.md and the folder at commit cd4028d. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md (its code samples are typescript).

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    No URLs in SKILL.md.

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Characterisation Tests loads about 3.6k tokens when it runs. Until then it costs about 141 tokens; SKILL.md has 1,614 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~141
When it runs · the whole SKILL.md, loaded when a task matches
~3.6k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

Its licence (Custom licence) doesn't allow us to republish the file, so here is its outline and opening line. It has 1,614 words (~3,566 tokens).

“For making untestable code testable first, load the finding-seams skill. For test-driving new behavior, load the tdd skill. For general test patterns, load the testing skill. Use mutation-aware test-design rules while characterising. If the repository requires mutation testing, or the…”

— opening of SKILL.md by citypaul, Custom licence
name
characterisation-tests

Read the full SKILL.md on GitHub

Files

SKILL.md and 2 other files in claude/.claude/skills/characterisation-tests of citypaul/.dotfiles.

  • SKILL.md
  • resources/modern-tooling.md
  • resources/writing-process.md

Open the folder on GitHubat commit cd4028d

Compare with similar skills

Characterisation Tests next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Characterisation Tests compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Characterisation Tests this skillcitypaul/.dotfiles740—~3.6kAutomated safety check: PassCustom licence
Test Guidelinesgetsentry/sentry-dart873—~3.1kAutomated safety check: PassMIT
TDD Refactor Guardgustavscirulis/snapgrid1171 repos~1.8kAutomated safety check: NotesCustom licence
Test Driven Developmentaiskillstore/marketplace430—~1.9kAutomated safety check: PassNone
Test Coveragescott-fryxell/brayness125—~1.6kAutomated safety check: PassMIT
TDD ImplementFastLED/FastLED7.5k—~1kAutomated safety check: PassMIT

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Categories

Questions about Characterisation Tests

What does Characterisation Tests do?

A skill your agent uses when modifying existing code that lacks tests and you need to document its actual current behavior before making changes -- the legacy code dilemma where you need tests to…. dotfiles. Use when modifying existing code that lacks tests and you need to document its actual current behavior before making changes -- the legacy code dilemma where you need tests to refactor safely but the code was not written for testability.

When should I use Characterisation Tests?

Characterisation Tests fits situations like: test-driving new behavior (see tdd); general test writing patterns (see testing); verifying test effectiveness (see mutation-testing); making untestable code testable (see finding-seams).

How do I install Characterisation Tests in Claude Code?

Run `npx skills add citypaul/.dotfiles --skill characterisation-tests -a claude-code`. Or copy the skill folder (claude/.claude/skills/characterisation-tests in citypaul/.dotfiles) into .claude/skills/characterisation-tests in your project. Claude Code loads it when a task matches its description.

How do I install Characterisation Tests in Codex?

Run `npx skills add citypaul/.dotfiles --skill characterisation-tests -a codex`. Or copy the skill folder (claude/.claude/skills/characterisation-tests in citypaul/.dotfiles) into .agents/skills/characterisation-tests in your project. Codex loads it when a task matches its description.

Can I use Characterisation Tests in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add citypaul/.dotfiles --skill characterisation-tests -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/characterisation-tests, .gemini/skills/characterisation-tests, .github/skills/characterisation-tests and .opencode/skills/characterisation-tests in your project.

What does Characterisation Tests need to run?

SKILL.md names no scripts, command-line tools or credentials: Characterisation Tests is instructions for the agent only.

Does Characterisation Tests access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Characterisation Tests safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Characterisation Tests use?

Characterisation Tests has a licence file (the repository's licence) that doesn't match a standard licence. Read it on GitHub before reusing the skill.

How many tokens does Characterisation Tests use?

About 3.6k tokens (SKILL.md is roughly 14k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Characterisation Tests?

Skills that share tags, products or a category with Characterisation Tests: Test Guidelines (getsentry/sentry-dart, 873 stars), TDD Refactor Guard (gustavscirulis/snapgrid, 117 stars), Test Driven Development (aiskillstore/marketplace, 430 stars) and Test Coverage (scott-fryxell/brayness, 125 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Characterisation Tests?

citypaul (a GitHub user) maintains it in citypaul/.dotfiles, which has 740 GitHub stars. The repository holds 44 skills in this directory. The repository was last updated on October 9, 2026.

Source: citypaul/.dotfiles on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.