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: Use ultrathink for test strategy design and failure analysis. Shallow reasoning misses edge cases and produces brittle tests that pass today but break tomorrow.

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.

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

1. Table-driven tests MUST use named subtests -- every test case needs a name field passed to t.Run
2. Integration tests MUST use build tags (//go:build integration) to separate from unit tests
3. Tests MUST NOT depend on execution order -- each test MUST be independently runnable
4. Independent tests SHOULD use t.Parallel() when possible
5. NEVER test implementation details -- test observable behavior and public API contracts
6. Packages with goroutines SHOULD use goleak.VerifyTestMain in TestMain to detect goroutine leaks
7. Use testify as helpers, not a replacement for standard library
8. Mock interfaces, not concrete types
9. Keep unit tests fast (< 1ms), use build tags for integration tests
10. Run tests with race detection in CI
11. Include examples as executable documentation

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

Naming Conventions

func TestAdd(t *testing.T) { ... }              // function test
func TestMyStruct_MyMethod(t *testing.T) { ... } // method test
func BenchmarkAdd(b *testing.B) { ... }          // benchmark
func ExampleAdd() { ... }                        // example

Table-Driven Tests

Table-driven tests are the idiomatic Go way to test multiple scenarios. Always name each test case.

func TestCalculatePrice(t *testing.T) {
    tests := []struct {
        name     string
        quantity int
        unitPrice float64
        expected  float64
    }{
        {
            name:      "single item",
            quantity:  1,
            unitPrice: 10.0,
            expected:  10.0,
        },
        {
            name:      "bulk discount - 100 items",
            quantity:  100,
            unitPrice: 10.0,
            expected:  900.0, // 10% discount
        },
        {
            name:      "zero quantity",
            quantity:  0,
            unitPrice: 10.0,
            expected:  0.0,
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got := CalculatePrice(tt.quantity, tt.unitPrice)
            if got != tt.expected {
                t.Errorf("CalculatePrice(%d, %.2f) = %.2f, want %.2f",
                    tt.quantity, tt.unitPrice, got, tt.expected)
            }
        })
    }
}

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:

import (
    "testing"
    "go.uber.org/goleak"
)

func TestMain(m *testing.M) {
    goleak.VerifyTestMain(m)
}

To exclude specific goroutine stacks (for known leaks or library goroutines):

func TestMain(m *testing.M) {
    goleak.VerifyTestMain(m,
        goleak.IgnoreCurrent(),
    )
}

Or per-test:

func TestWorkerPool(t *testing.T) {
    defer goleak.VerifyNone(t)
    // ... test code ...
}

testing/synctest for Deterministic Goroutine Testing

Experimental: testing/synctest is not yet covered by Go's compatibility guarantee. Its API may change in future releases. For stable alternatives, use clockwork (see Mocking).

testing/synctest (Go 1.24+) provides deterministic time for concurrent code testing. 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)
• When race conditions need to be reproducible
• When tests are flaky due to timing issues

import (
    "testing"
    "time"
    "testing/synctest"
    "github.com/stretchr/testify/assert"
)

func TestChannelTimeout(t *testing.T) {
    synctest.Run(func(t *testing.T) {
        is := assert.New(t)

        ch := make(chan int, 1)
        go func() {
            time.Sleep(50 * time.Millisecond)
            ch <- 42
        }()

        select {
        case v := <-ch:
            is.Equal(42, v)
        case <-time.After(100 * time.Millisecond):
            t.Fatal("timeout occurred")
        }
    })
}

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:

func BenchmarkStringConcatenation(b *testing.B) {
    b.Run("plus-operator", func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            result := "a" + "b" + "c"
            _ = result
        }
    })

    b.Run("strings.Builder", func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            var builder strings.Builder
            builder.WriteString("a")
            builder.WriteString("b")
            builder.WriteString("c")
            _ = builder.String()
        }
    })
}

Benchmarks with different input sizes:

func BenchmarkFibonacci(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 i := 0; i < b.N; i++ {
                Fibonacci(size)
            }
        })
    }
}

Parallel Tests

Use t.Parallel() to run tests concurrently:

func TestParallelOperations(t *testing.T) {
    tests := []struct {
        name string
        data []byte
    }{
        {"small data", make([]byte, 1024)},
        {"medium data", make([]byte, 1024*1024)},
    }