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Persona: You are a Go engineer who treats tests as executable specifications. You write tests to constrain behavior, not to hit coverage targets.
Thinking mode: Reason as thoroughly as possible for test strategy design and failure analysis — shallow reasoning misses edge cases and produces brittle tests that pass today but break tomorrow. On Claude Code, use ultrathink to trigger extended thinking explicitly.
Orchestration mode: Fan out the three sub-agents described in Audit mode (unit quality and coverage gaps, integration isolation, goroutine/race issues) for auditing a large test suite, and merge their findings into one gap report. On Claude Code, use ultracode to opt into multi-agent orchestration explicitly.
Modes:
Write mode — generating new tests for existing or new code. Work sequentially through the code under test; use gotests to scaffold table-driven tests, then enrich with edge cases and error paths.
Review mode — reviewing a PR's test changes. Focus on the diff: check coverage of new behaviour, assertion quality, table-driven structure, and absence of flakiness patterns. Sequential.
Audit mode — auditing an existing test suite for gaps, flakiness, or bad patterns (order-dependent tests, missing t.Parallel(), implementation-detail coupling). Launch up to 3 parallel sub-agents split by concern: (1) unit test quality and coverage gaps, (2) integration test isolation and build tags, (3) goroutine leaks and race conditions.
Debug mode — a test is failing or flaky. Work sequentially: reproduce reliably, isolate the failing assertion, trace the root cause in production code or test setup.
Community default. A company skill that explicitly supersedes samber/cc-skills-golang@golang-testing skill takes precedence.
Dependencies:
gotests: go install github.com/cweill/gotests/gotests@latest
Go Testing Best Practices
This skill guides the creation of production-ready tests for Go applications. Follow these principles to write maintainable, fast, and reliable tests.
Best Practices Summary
Table-driven tests MUST use named subtests -- every test case needs a name field passed to t.Run
Integration tests MUST use build tags (//go:build integration) to separate from unit tests
Tests MUST NOT depend on execution order -- each test MUST be independently runnable
Independent tests SHOULD use t.Parallel() when possible
NEVER test implementation details -- test observable behavior and public API contracts
Packages with goroutines SHOULD use goleak.VerifyTestMain in TestMain to detect goroutine leaks
Use testify as helpers, not a replacement for standard library
Mock interfaces, not concrete types
Keep unit tests fast (< 1ms), use build tags for integration tests
Run tests with race detection in CI
Include examples as executable documentation
Test files MUST be named after the source file under test, not after the function or method being tested
Test functions SHOULD appear in the same order as the functions/methods they test in the source file
Test Structure and Organization
File Conventions
// package_test.go - tests in same package (white-box, access unexported)package mypackage
// mypackage_test.go - tests in test package (black-box, public API only)package mypackage_test
Name the test file after the source file it tests, not after the function or method under test. Go's convention is one test file per source file (foo.go -> foo_test.go), because tools (go test, coverage reports, IDE "jump to test" navigation, gotests) and reviewers all resolve tests by source file, not by symbol. A source file usually declares several functions/methods; splitting its tests by symbol name scatters them across many files and breaks that file-to-file mapping.
// ✓ Good — one test file per source file
helloworld.go -> helloworld_test.go // contains TestHelloWorld, TestAbcd, TestXyz, ...
// ✗ Bad — test file named after the function/method instead of the source file
helloworld.go -> abcd_test.go // wrong: should be helloworld_test.go
Exception: very large source files MAY be split into multiple _test.go files by concern (e.g. foo_test.go + foo_edgecases_test.go), but each split file's name MUST still be derived from the source file name, never from an individual function name. Prefer keeping a single _test.go file per source file even when it grows large — splitting adds navigation overhead and is rarely worth it; reach for the exception only when a single file becomes genuinely unwieldy to browse or review.
Within a test file, order test functions to match the order their tested functions/methods appear in the source file. A reader (human or agent) scrolling foo.go alongside foo_test.go can then find the matching test by position instead of searching; drift between the two orderings compounds every time either file grows.
Common Pitfall: Assert Scope Leaking into Subtests
Never create a testify assert/require instance in the parent test function and reuse it inside t.Run closures. assert.New(t) captures the exact *testing.T it was built with, so if that t belongs to the parent, every failure raised inside the subtest gets attributed to the parent test in go test output — the failing subtest itself still reports --- PASS, silently hiding which case broke. This happens whether or not the subtest calls t.Parallel().
// WRONG -- `is` is bound to the parent's tfuncTestCalculatePrice(t *testing.T) {
is := assert.New(t)
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
is.Equal(tt.expected, CalculatePrice(tt.quantity, tt.unitPrice)) // misattributed on failure
})
}
}
// RIGHT -- each subtest builds its own instance from its own tfuncTestCalculatePrice(t *testing.T) {
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
is := assert.New(t)
is.Equal(tt.expected, CalculatePrice(tt.quantity, tt.unitPrice))
})
}
}
Verify with a deliberately-broken case: if go test -v -run TestName shows --- FAIL: TestName but every --- PASS: TestName/subtest_name line still says PASS, the assert scope is leaking.
Unit Tests
Unit tests should be fast (< 1ms), isolated (no external dependencies), and deterministic.
Testing HTTP Handlers
Use httptest for handler tests with table-driven patterns. See HTTP Testing for examples with request/response bodies, query parameters, headers, and status code assertions.
Goroutine Leak Detection with goleak
Use go.uber.org/goleak to detect leaking goroutines, especially for concurrent code:
testing/synctest for Deterministic Goroutine Testing
testing/synctest (Go 1.25+) provides deterministic tests for goroutines, timers, deadlines, and context cancellation. Time advances only when all goroutines are blocked, making ordering predictable.
When to use synctest instead of real time:
Testing concurrent code with time-based operations (time.Sleep, time.After, time.Ticker)
Use synctest.Test in Go 1.25+ and Go 1.26+. Do not use the old Go 1.24 experimental synctest.Run API in Go 1.25+ or Go 1.26+ code. If a module explicitly targets Go 1.24 and opts into GOEXPERIMENT=synctest, use the old API only as a compatibility fallback.
Key differences in synctest:
time.Sleep advances synthetic time instantly when the goroutine blocks
time.After fires when synthetic time reaches the duration
All goroutines run to blocking points before time advances
Test execution is deterministic and repeatable
Test Timeouts
For tests that may hang, use a timeout helper that panics with caller location. See Helpers.
Benchmarks
→ See samber/cc-skills-golang@golang-benchmark skill for advanced benchmarking: b.Loop() (Go 1.24+), benchstat, profiling from benchmarks, and CI regression detection.
Write benchmarks to measure performance and detect regressions:
funcBenchmarkStringConcatenation(b *testing.B) {
b.Run("plus-operator", func(b *testing.B) {
for b.Loop() {
result := "a" + "b" + "c"
_ = result
}
})
b.Run("strings.Builder", func(b *testing.B) {
for b.Loop() {
var builder strings.Builder
builder.WriteString("a")
builder.WriteString("b")
builder.WriteString("c")
_ = builder.String()
}
})
}
Benchmarks with different input sizes:
funcBenchmarkFibonacci(b *testing.B) {
sizes := []int{10, 20, 30}
for _, size := range sizes {
b.Run(fmt.Sprintf("n=%d", size), func(b *testing.B) {
b.ReportAllocs()
for b.Loop() {
Fibonacci(size)
}
})
}
}
For Go 1.24+, new benchmarks should use b.Loop(). Use legacy b.N loops only when the module targets Go <1.24 or when preserving old benchmark code intentionally.
Go 1.26+: test artifacts
When a test, benchmark, or fuzz target needs to persist files for inspection, use ArtifactDir() instead of ad-hoc paths or repo-local output.
funcTestRenderGoldenArtifact(t *testing.T) {
dir := t.ArtifactDir()
out := filepath.Join(dir, "rendered.json")
if err := os.WriteFile(out, renderedBytes, 0o644); err != nil {
t.Fatal(err)
}
t.Logf("artifact written: %s", out)
}
Available on *testing.T, *testing.B, and *testing.F in Go 1.26+.
Parallel Tests
Use t.Parallel() to run tests concurrently:
funcTestParallelOperations(t *testing.T) {
tests := []struct {
name string
data []byte
}{
{"small data", make([]byte, 1024)},
{"medium data", make([]byte, 1024*1024)},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
t.Parallel()
is := assert.New(t)
result := Process(tt.data)
is.NotNil(result)
})
}
}
# Generate coverage file
go test -coverprofile=coverage.out ./...
# View coverage in HTML
go tool cover -html=coverage.out
# Coverage by function
go tool cover -func=coverage.out
# Total coverage percentage
go tool cover -func=coverage.out | grep total
Integration Tests
Use build tags to separate integration tests from unit tests:
//go:build integrationpackage mypackage
funcTestDatabaseIntegration(t *testing.T) {
db, err := sql.Open("postgres", os.Getenv("DATABASE_URL"))
if err != nil {
t.Fatal(err)
}
defer db.Close()
// Test real database operations
}
Run integration tests separately:
go test -tags=integration ./...
For Docker Compose fixtures, SQL schemas, and integration test suites, see Integration Testing.
Mocking
Mock interfaces, not concrete types. Define interfaces where consumed, then create mock implementations.
For mock patterns, test fixtures, and time mocking, see Mocking.
Enforce with Linters
Many test best practices are enforced automatically by linters: thelper, paralleltest, testifylint. See the samber/cc-skills-golang@golang-lint skill for configuration and usage.
Cross-References
-> See samber/cc-skills-golang@golang-stretchr-testify skill for detailed testify API (assert, require, mock, suite)
-> See samber/cc-skills-golang@golang-database skill (testing.md) for database integration test patterns
-> See samber/cc-skills-golang@golang-concurrency skill for goroutine leak detection with goleak
-> See samber/cc-skills-golang@golang-continuous-integration skill for CI test configuration and GitHub Actions workflows
-> See samber/cc-skills-golang@golang-lint skill for testifylint and paralleltest configuration
-> See samber/cc-skills-golang@golang-continuous-integration skill for automated AI-driven code review in CI using these guidelines
Quick Reference
go test ./... # all tests
go test -run TestName ./... # specific test by exact name
go test -run TestName/subtest ./... # subtests within a test
go test -run 'Test(Add|Sub)' ./... # multiple tests (regexp OR)
go test -run 'Test[A-Z]' ./... # tests starting with capital letter
go test -run 'TestUser.*' ./... # tests matching prefix
go test -run '.*Validation.*' ./... # tests containing substring
go test -run TestName/. ./... # all subtests of TestName
go test -run '/(unit|integration)' ./... # filter by subtest name
go test -race ./... # race detection
go test -cover ./... # coverage summary
go test -bench=. -benchmem ./... # benchmarks
go test -fuzz=FuzzName ./... # fuzzing
go test -tags=integration ./... # integration tests
