supercov – Coding skill for AI agents

Supercov

Overview

Supercov runs the project's own test command on the user's machine and reports line, branch and MC/DC coverage per test. It changes no source, tests or configuration. This skill uses that report to pick one untested piece of code at a time and write a focused test for it.

It works with JavaScript, TypeScript, Python, Ruby, Rust, Go, Java and Kotlin projects.

When to Use This Skill

• Use when the user asks to add or improve tests.
• Use when the user asks which code is untested, or asks to raise coverage.
• Use when the user asks which file to refactor first and has agreed to a quality check (see "Quality check").

Preconditions

• Node.js 22 or newer. If it is missing, tell the user. Do not install software without the user's approval.
• The project's test command, for example npm test, pytest or cargo test. Ask if it is not obvious from the repository.
• Always run the pinned version, npx [email protected]. The first run downloads that release from npm, so get the user's approval before it.

How It Works

Step 1: Measure the suite

Run the full test command under Supercov. Coverage is measured locally and sends nothing over the network.

npx [email protected] -- npm test

Step 2: Pick a target

List the untested code, pick one item, and read it in context.

npx [email protected] runs latest gaps --limit 5
npx [email protected] runs latest file src/session.js

Step 3: Write one focused test

Write one test for the chosen target. Change only test files unless the user asks otherwise.

Step 4: Rerun and report

Rerun the command from step 1. Report the test you added and the coverage before and after.

For anything not shown here, npx [email protected] docs <topic> prints the guide for that version. Topics: agent-loop, quality, supported-suites, troubleshooting.

Examples

Example 1: Add a test for untested code

npx [email protected] -- npm test
npx [email protected] runs latest gaps --limit 5
# add one test for the first gap, then:
npx [email protected] -- npm test

Example 2: Other languages

npx [email protected] -- pytest
npx [email protected] -- cargo test
npx [email protected] -- go test ./...
npx [email protected] -- ./gradlew test

Quality check

npx [email protected] quality ranks files by code smells, and npx [email protected] quality patch reports what a change introduced.

• These two commands send the project's source files to TypeSafe's API for analysis. Run them only when the user asked for a quality check, and tell the user before running that their source files will be sent to TypeSafe.
• They need the user's own TYPESAFE_API_KEY in the environment. Never ask the user to paste a key into the conversation, and never create, read or print one.
• If the key is missing, tell the user the check needs it, then answer from reading the code and say Supercov did not check it.

Best Practices

• ✅ Measure the full suite first, so the gaps reflect every test.
• ✅ Add one test per gap and rerun before picking the next.
• ✅ Report coverage before and after with the numbers Supercov printed.
• ❌ Don't change application code to raise coverage unless the user asks.
• ❌ Don't run the quality commands without the user's request and consent.

Limitations

• Coverage shows which code ran under a test. It does not show that the test checked the result.
• Supercov measures the suites listed in npx [email protected] docs supported-suites. Other runners are not measured.
• The test command runs as the project defines it. If the suite needs services, credentials or network access, those needs still apply.
• This skill does not replace environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, or safety boundaries are missing.

Security & Safety Notes

• The skill runs the project's own test command and writes test files in the user's repository. Run it only in a repository the user has authorized, and keep changes to test files unless the user asks otherwise.
• Supercov writes its run data to a .supercov directory in the project. It does not edit source, tests or configuration.
• npx [email protected] downloads a pinned release from the npm registry. Get the user's approval before the first run. Upgrading the pin needs a new review.
• Coverage commands make no network requests. Only the quality commands contact a third party, TypeSafe's API, and only with the user's consent and their own key.

Related Skills

• @test-driven-development - Use when the behavior does not exist yet and the test should come first.