Write Tests Skill | Agent Skills

Write Tests

Generate high-quality automated tests for a change or an untested file, in the framework the repo already uses, and run them until they pass. The skill is polyglot: it detects the language and framework per repo, matches the surrounding conventions, and closes the loop between implement and review in the co-dev workflow.

The point is not "emit a plausible test file" — Claude can already do that. The point is a repeatable, verified capability: detect the right framework, follow house style, enumerate the cases that matter (happy path, edges, error paths, boundaries), generate, execute, iterate to green, and report real coverage. A test that was never run is not done.

When to Use

After implementing a change, before opening a PR (pairs with work-issue).
To backfill coverage on an untested file, class, or module.
To add a regression test that proves a bug fix (pairs with verify-resolved-issues).
When the user asks to "write tests", "add specs", "cover this", or "test this before I ship".

Do not run it on pure config/docs changes with no runtime surface. If there is product code, there is something to test.

Core principle: match the repo, don't impose

Use the framework that is already in the repo. Detect it (Step 1) and follow it — do not introduce Vitest into a Jest repo, Pest into a PHPUnit-only repo, or Swift Testing into an XCTest file. The "2026 default" recommendations in the matrix below apply only when a repo has no tests yet and you must choose. When tests exist, the existing choice wins, full stop.

Then match local conventions: the existing spec/ vs test/ layout, file naming, factory/fixture style (FactoryBot, factory_boy, test data builders), shared helpers/setup, mocking library, and assertion idioms already in the code. New tests should read like the tests already there.

Step 1: Detect language and framework

Detect per target directory (monorepos may mix stacks — resolve the stack of the file(s) under test, not the repo root). Read the manifest first, then confirm against existing test files.

| Manifest present | Language | How to pick the framework | | --- | --- | --- | | Gemfile / *.gemspec | Ruby | grep rspec Gemfile* → RSpec (specs in spec/); else Minitest/Test::Unit (test/). Feature/system → Capybara. Fixtures → FactoryBot if in the Gemfile. | | composer.json | PHP | grep pestphp/pest → Pest; grep behat/behat → Behat (features/); else PHPUnit. Laravel → run via php artisan test. Mocks → Mockery. | | pyproject.toml / requirements*.txt / setup.py | Python | grep -ri pytest → pytest (tests/); else unittest. Mocks → unittest.mock; fixtures → factory_boy if present; property → hypothesis if present. | | package.json | JS/TS | grep '"vitest"' → Vitest; grep '"jest"' → Jest; grep react-native → Jest. E2E → Playwright (grep '@playwright') else Cypress if present. Component → Testing-Library. | | *.csproj / *.sln | C#/.NET | grep -ri xunit → xUnit; grep -ri nunit → NUnit; grep -ri mstest → MSTest. Assertions → FluentAssertions if referenced; mocks → Moq/NSubstitute. | | Package.swift / *.xcodeproj / *.xcworkspace | Swift | grep -rl "import Testing" → Swift Testing for unit; else XCTest. UI → XCUITest (always XCTest). | | build.gradle(.kts) / settings.gradle | Kotlin/Java | grep -ri kotest → Kotest; else JUnit 5 (or JUnit4 if that's what's wired). Mocks → MockK (Kotlin) / Mockito (Java). Android UI → composeTestRule (Compose) or Espresso (Views). Flows → Turbine. | | go.mod | Go | stdlib testing + table-driven by default; testify if grep testify go.mod (use require). Concurrency/time → testing/synctest. | | CMakeLists.txt / *.cpp / *.cc | C/C++ | grep -ri gtest\|googletest → GoogleTest+GoogleMock; grep -ri catch2 → Catch2; run via CTest if wired. | | Cargo.toml | Rust | built-in #[test] / cargo test; integration tests in tests/; property → proptest if present; mocks → mockall if present. |

If a repo has no tests at all, pick the 2026 default from the matrix and tell the user which you chose and why before generating.

Repos with more than one framework (same language)

Expect to detect multiple frameworks for one language — it's normal, and the skill supports it. Two distinct cases:

Different test layers → different frameworks (keep them separate). A single language often uses one tool per layer: Ruby with RSpec (unit) + Capybara (system) + Cucumber (BDD); JS/TS with Vitest/Jest (unit) + Playwright/Cypress (E2E); Swift with Swift Testing (unit) + XCUITest (UI); Kotlin with Kotest/MockK (domain/ViewModel) + JUnit4 composeTestRule (Compose UI); PHP with PHPUnit/Pest (unit/feature) + Behat (acceptance). Pick the framework for the layer you're writing at, and follow that layer's existing tests.
Two frameworks at the same layer (coexistence / mid-migration). PHPUnit and Pest, Jest and Vitest, NUnit and xUnit, Minitest and RSpec, XCTest and Swift Testing can all live in one repo — usually because a migration is in progress. Resolve this per directory/module, not repo-wide:
- Match the framework used by the nearest existing tests — the sibling test file, the same module/package, or the closest test directory to the code under test. A new spec next to RSpec specs is RSpec even if the repo also has Minitest elsewhere.
- If the target area has no nearby tests and both frameworks are plausible, ask the user which to use (offer the two, note which looks newer/more common) rather than guessing. State the counts you found, e.g. "42 PHPUnit files, 6 Pest files — which for this new test?"
- Never mix two frameworks in the same test file. One file, one framework.
- When the user has stated a migration direction (e.g. "we're moving to Vitest"), prefer the target framework for new files and say so.

Framework matrix (mid-2026 defaults for greenfield only)

Recommendations below are for new test suites where nothing exists yet. For existing suites, match what's there.

| Language | New-suite default (2026) | Also support / when | Mocking | Coverage | | --- | --- | --- | --- | --- | | Ruby | RSpec + FactoryBot; Capybara for system specs | Minitest for gems/stdlib-style repos | RSpec mocks, instance_double | SimpleCov | | PHP | Pest (runs on PHPUnit; coexists) | PHPUnit for PHPUnit-heavy repos; Behat for existing BDD | Mockery, Prophecy | --coverage (Xdebug/PCOV) | | Python | pytest (fixtures, parametrize) | unittest for stdlib-style; hypothesis for property | unittest.mock, pytest-mock | pytest --cov / coverage | | JS/TS | Vitest | Jest for React Native / legacy CJS / deep Jest-mock repos; Playwright for E2E | vi/jest mocks, MSW, nock | --coverage (v8/istanbul) | | C#/.NET | xUnit v3 + FluentAssertions | NUnit 4 for existing NUnit suites / rich data-driven | Moq, NSubstitute | dotnet test --collect:"XPlat Code Coverage" | | Swift | Swift Testing (@Test, #expect) for unit — Xcode 16+/Swift 6 | XCTest for existing; always XCTest for XCUITest UI + XCTMetric performance | protocol test doubles; Quick/Nimble if present | Xcode coverage / swift test --enable-code-coverage | | Kotlin/Java | JUnit 5 + Kotest assertions + MockK for domain/ViewModel | JUnit4 composeTestRule+testTag for Compose UI; Espresso for View UI; Robolectric for JVM Android | MockK / Mockito | JaCoCo / Kover | | Go | stdlib testing + table-driven | testify (require) selectively; testing/synctest for time/concurrency; -fuzz for parsers | interfaces (no lib needed); testify/mock or gomock if present | go test -cover (+ -race) | | C/C++ | GoogleTest (+GoogleMock) or Catch2 | CTest as the harness | GoogleMock / FFF | --coverage (gcov/llvm-cov) | | Rust | built-in #[test] / cargo test | proptest for property; integration tests in tests/ | mockall / trait fakes | cargo llvm-cov / tarpaulin |

Ecosystem notes that change the generated code:

Swift: never mix #expect/#require and XCTAssert* in the same test function. New unit tests → Swift Testing; leave UI (XCUIApplication) and performance tests in XCTest.
Go: prefer table-driven subtests with named rows; reach for require over assert so a failed precondition stops the case. Use testing/synctest instead of real time.Sleep for timing-dependent tests.
Kotlin: Kotest specs don't map onto Compose's @get:Rule, so split — Kotest+MockK for domain/ViewModel logic, JUnit4-style composeTestRule for Compose UI, and prefer Modifier.testTag(...) over matching exact text.
PHP/Pest & JS/Vitest & Swift/Testing: all coexist with the older framework in the same suite — migration is never required to start; new files can use the modern one only if the repo has already adopted it.

Step 2: Decide what to test

Scope. If invoked after a change (or from work-issue), test the diff: git diff --name-only (and git diff for the exact lines). If invoked on a file/module, target that. If backfilling, prioritize the highest-risk untested code (business logic, money/enum/state handling, parsers, auth, boundaries) over trivial getters.
Read the code under test. Understand inputs, outputs, side effects, error paths, and collaborators (what needs mocking vs. what can run real).
Find the seams. Identify dependencies to stub (network, DB, clock, filesystem, third-party SDKs) and what should stay real (pure logic, in-memory structures). Over-mocking produces tests that pass while the app breaks — mock only true external boundaries.

Step 3: Enumerate the cases (before writing)

List the cases you will cover, then generate. At minimum consider:

Happy path — the primary expected behavior.
Edge/boundaries — empty, zero, one, max, off-by-one, unicode, timezone, rounding, large inputs.
Error paths — invalid input, exceptions, failed dependencies, timeouts, permission denied.
State/branches — each meaningful branch and enum/state value the code distinguishes.
Regression — when fixing a bug: a test that fails on the old behavior and passes on the fix.

For a bug fix, write the regression test first, confirm it fails against the current (unfixed) behavior if possible, then confirm the fix makes it pass.

Step 4: Generate the tests

Place files where the repo expects them (spec/, test/, tests/, __tests__/, *_test.go, androidTest/ vs test/, Tests/ target, etc.) with the repo's naming convention.
Reuse existing helpers, factories, fixtures, base classes, and custom matchers — don't reinvent setup that already exists.
One behavior per test; descriptive names that state intent. Arrange–Act–Assert.
The test name is the documentation — do not write a comment above a test. A preamble explaining why the test exists, what it protects, or how the behavior relates to another module is rationale, not code: it belongs in the PR body or the ticket. If a test's purpose isn't obvious, rename the test or sharpen the assertion message; don't annotate it. Same inside the body: no # Arrange / // act labels and no comment restating the line under it. The rare comment that earns its place states something the code cannot — a magic fixture value's origin, a third-party quirk being reproduced — in one line.
Deterministic only: no real network/clock/random. Inject or fake them. Freeze time where the framework supports it (testing/synctest, Timecop, vi.useFakeTimers, etc.).
Match the assertion style already used in the suite.

Step 5: Run and iterate to green

Run the suite (narrowly — just the new/affected tests first), read failures, fix, repeat until green. Then measure coverage — Step 6 is mandatory; only its scope is negotiable, so run it on the changed files first.

| Language | Run (narrow → full) | Coverage | | --- | --- | --- | | Ruby (RSpec) | bundle exec rspec path/to/spec.rb → bundle exec rspec | SimpleCov (auto on run) | | Ruby (Minitest) | bundle exec rake test TEST=path → bundle exec rake test | SimpleCov | | PHP (Pest) | ./vendor/bin/pest --filter=Name → ./vendor/bin/pest | ./vendor/bin/pest --coverage | | PHP (PHPUnit) | ./vendor/bin/phpunit --filter Name → php artisan test | --coverage-text | | PHP (Behat) | ./vendor/bin/behat features/x.feature | n/a | | Python (pytest) | pytest path::test_name → pytest | pytest --cov=pkg | | Python (unittest) | python -m unittest module.Class.test | coverage run -m unittest discover && coverage report | | JS/TS (Vitest) | npx vitest run path → npx vitest run | npx vitest run --coverage | | JS/TS (Jest) | npx jest path -t "name" → npx jest | npx jest --coverage | | JS/TS (Playwright) | npx playwright test path | --reporter=html | | C#/.NET | dotnet test --filter Name → dotnet test | dotnet test --collect:"XPlat Code Coverage" | | Swift (SwiftPM) | swift test --filter Name → swift test | swift test --enable-code-coverage | | Swift (Xcode) | xcodebuild test -scheme S -destination '...' -only-testing:... | Xcode coverage report | | Kotlin/Android (unit) | ./gradlew :module:test --tests X → ./gradlew test | ./gradlew koverHtmlReport / JaCoCo | | Kotlin/Android (instrumented) | ./gradlew connectedAndroidTest (needs device/emulator) | — | | Go | go test -run TestName ./pkg → go test ./... (+-race) | go test -cover ./... | | C/C++ | ctest --test-dir build -R Name → ctest --test-dir build | --coverage + gcov/llvm-cov | | Rust | cargo test test_name → cargo test | cargo llvm-cov |

Detect the JS package manager from the lockfile (pnpm-lock.yaml → pnpm, yarn.lock → yarn, else npm); prefer the repo's test script (npm test) when it exists.

Instrumented mobile tests (Android connectedAndroidTest, iOS on a simulator) need a device/emulator. If none is available, generate the tests, state clearly that they were not executed for lack of a device, and run whatever JVM/unit-level tests you can.

Step 6: Check coverage

Coverage is part of this skill, not an afterthought. Measure it on the code under test and hold it to a floor.

Default targets: line ≥ 80% and branch ≥ 80% (or the language's nearest equivalent). Resolve the actual target in this precedence:

What the user asked for — an explicit number in the request wins.
What the repo already configures — an existing threshold (SimpleCov minimum_coverage, Jest/Vitest coverageThreshold, .coveragerc fail_under, coverlet threshold, JaCoCo/Kover rule, nyc check-coverage, gcovr --fail-under-*). Respect it, and never lower an existing higher bar.
The 80/80 default otherwise.

Procedure:

Run coverage scoped to the changed files/module (table below), reading both line and branch.
If below target, add tests for the uncovered lines and branches that matter — error paths, guard clauses, each branch/enum value — not trivial padding, and don't chase 100%. Re-run.
Report the actual line/branch percentages and whether the target was met.

| Language | Coverage command (line + branch) | Notes | | --- | --- | --- | | Ruby | SimpleCov: SimpleCov.enable_coverage :branch + minimum_coverage line: 80, branch: 80, then bundle exec rspec | branch coverage since SimpleCov 0.18 | | PHP | ./vendor/bin/pest --coverage --min=80 / phpunit --coverage-text | needs Xdebug or PCOV; path/branch coverage via Xdebug | | Python | pytest --cov=pkg --cov-branch --cov-fail-under=80 | --cov-branch enables branch metric | | JS/TS (Vitest) | vitest run --coverage + coverage.thresholds { lines: 80, branches: 80 } | v8/istanbul report branches | | JS/TS (Jest) | jest --coverage + coverageThreshold.global { lines: 80, branches: 80 } | | | C#/.NET | dotnet test --collect:"XPlat Code Coverage" + coverlet /p:Threshold=80 /p:ThresholdType=line,branch | coverlet reports line + branch | | Swift | swift test --enable-code-coverage → xcrun llvm-cov report (or Xcode "Gather coverage") | region/branch via llvm-cov; no built-in fail-under — parse the report | | Kotlin/Android | Kover koverVerify or JaCoCo rule with LINE + BRANCH counters minimum = 0.80 | JaCoCo counters: LINE, BRANCH | | Go | go test -covermode=atomic -coverprofile=c.out ./... && go tool cover -func=c.out | Go reports statement coverage — there is no built-in branch coverage | | C/C++ | build --coverage, run via CTest, then gcovr --fail-under-line 80 --fail-under-branch 80 (or llvm-cov report) | gcovr enforces line + branch | | Rust | cargo llvm-cov --fail-under-lines 80 | region/line based; branch coverage is limited |

Coverage caveats to state honestly: Go has no branch coverage — meet statement/line ≥ 80% and ensure every branch is exercised by a table-driven row. Swift and Rust report region/line coverage; treat "region" as the branch-equivalent and say so. Where a tool cannot measure branch coverage, report line coverage and note the branch metric is unavailable rather than implying it was met.

Step 7: Report

Summarize concisely:

Files added/changed and the framework used (name the framework per directory if more than one was involved).
What was covered (the case list from Step 3) and any deliberate gaps.
Test run result — actual pass/fail counts, not "should pass".
Coverage — actual line and branch percentages vs. the target (state the target and where it came from: user / repo config / 80-80 default). If a metric is unavailable for the language, say so.
Anything left for a human (e.g. instrumented tests not run, a flow that needs a device or live sandbox).

Integration with co-dev

work-issue → work-issue's Step 8 is a hard gate ("no change ships without tests"); it owns the policy (a behavior-altering change can't reach a PR without passing tests, bug fixes need a regression test). This skill is the how it satisfies that gate: framework detection, the case matrix, running to green, and the 80/80 coverage floor. After implementing an issue, generate specs for the diff and cover the issue's acceptance criteria as explicit cases before the PR.
verify-resolved-issues → when a fix is verified from code, generate a regression test that pins the fixed behavior so it can't silently revert.
run-linters → run linters on the generated test files too; they are source code and must pass the same gate.
code-review-deep → its tests phase flags coverage gaps; this skill closes them.
CI → when adding a new suite to a repo that had none, offer to wire a GitHub Actions job consistent with the repo's existing build.yml conventions.

Important Rules

Match the existing framework — resolved per directory/module, not repo-wide. A repo may legitimately hold several frameworks for one language (different layers, or a migration in progress). Match the framework of the nearest existing tests to the code under test; when two are plausible at the same layer and none is nearby, ask the user which to use rather than guessing. Never introduce a new framework into a repo that already has one unless the user asks, and never mix two frameworks in one file. Greenfield defaults apply only when no tests exist anywhere.
Run what you write. Every generated test must actually be executed and reported with real results. Never claim a test passes without running it. If it can't be run (no device, missing service), say so plainly.
Never fake green. Do not weaken assertions, add skip/xit/t.Skip, mark @Disabled, or delete a failing test to make the suite pass. A failing test means either the test or the code is wrong — fix the right one.
Don't change source to fit a wrong test. Only modify production code when the test exposed a real bug (and say so). Otherwise fix the test.
Mock only real boundaries. Network, DB, clock, filesystem, third-party SDKs — yes. Pure logic and in-memory collaborators — no. Over-mocking yields tests that pass while production breaks.
Deterministic and isolated. No real time, network, randomness, or shared mutable state between tests. Tests must pass run in any order and in parallel.
Never edit framework/config to bypass failures (jest.config, phpunit.xml, .rspec, coverage thresholds) to make a run go green. Same spirit as run-linters.
Hit the coverage floor. Default line ≥ 80% and branch ≥ 80% (or the language equivalent) unless the user specifies otherwise or the repo configures a different/higher bar — respect the repo's and never lower it. Report the real numbers; don't game the metric with assertion-free tests.
No narrative comments in test files. Default to none, hard cap of one line. A comment block above a test explaining its motivation, the convention it follows, or what a sibling integration does is rationale for a reviewer — put it in the PR body and let the test name carry the intent. Sweep before reporting: git diff -U0 -- <test paths> | grep -E '^\+[[:space:]]*(#|//|/\*|\*)' and delete what fails that test.
Assert behavior, not implementation. Prefer public API + observable output/testTag selectors over internal call counts and exact-text matching, so tests survive refactors.
Regression tests must actually catch the regression. For a bug fix, ensure the test fails on the old behavior before it passes on the new.
No silent scope cuts. If you skipped hard-to-test code (UI needing a device, external integration), list it in the report rather than pretending it's covered.

Agent Skills: Write Tests

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