Testing Go Agent Skill

Testing Go

You test behavior at the package boundary, not private internals. A Go test that breaks because you renamed an unexported field was testing the wrong thing. Target Go 1.25 (released 2025-08-12) unless the module's go directive says otherwise — several APIs below only exist in recent versions.

Default stack, in order of reach:

• The standard testing package. It is enough for ~90% of tests.
• testify/require (and assert) only where stdlib comparisons get noisy — deep-equal on big structs, repeated if err != nil { t.Fatal }. Not as a reflex.
• Hand-written fakes over any mock generator. A 12-line fake beats gomock for code you own.

Run go test ./... early and often. Add -race for anything that touches a goroutine. The test suite is your fastest feedback loop — keep it fast and deterministic so you actually run it.

What am I testing? Pick the tool

| Situation | Reach for |

|---|---|

| Pure logic, many input/output cases | Table-driven test + t.Run subtests |

| An HTTP client or http.Handler | net/http/httptest (server or recorder) |

| Goroutines, timers, timeouts, context deadlines | testing/synctest + -race |

| "Is it fast / did my change regress?" | testing.B.Loop + benchstat |

| "Does it crash or misbehave on weird input?" | func FuzzXxx(f *testing.F) |

| A dependency you own (DB, payment client, clock) | A hand-written interface fake |

If you find yourself reaching past the standard library for the first row, stop — table-driven tests need nothing else.

Table-driven tests

The canonical shape: a slice of anonymous structs with a name, run as named subtests.

func TestReverse(t *testing.T) {
	tests := []struct {
		name string
		in   string
		want string
	}{
		{name: "empty", in: "", want: ""},
		{name: "ascii", in: "abc", want: "cba"},
		{name: "unicode", in: "héllo", want: "olléh"},
	}
	for _, tc := range tests {
		t.Run(tc.name, func(t *testing.T) {
			got := Reverse(tc.in)
			if got != tc.want {
				t.Errorf("Reverse(%q) = %q, want %q", tc.in, got, tc.want)
			}
		})
	}
}

• Name every case. t.Run(tc.name, ...) gives you go test -run TestReverse/unicode to target one case, and a readable failure path instead of "index 2".
• Use t.Errorf, not t.Fatalf, inside the loop unless the rest of the case is meaningless after the failure. Fatalf stops the whole subtest; Errorf lets the other cases still run.
• One table per behavior, not per function. If two functions share inputs and expectations, one table. If a case needs three extra fields only it uses, that case wants its own test.

Bad — copy-pasted near-identical functions:

func TestAddOne(t *testing.T)  { if Add(1) != 2 { t.Fail() } }
func TestAddTwo(t *testing.T)  { if Add(2) != 3 { t.Fail() } }
func TestAddZero(t *testing.T) { if Add(0) != 1 { t.Fail() } }

Good — one table, each case named, failures localized (see the TestReverse shape above).

Parallel & isolation

for _, tc := range tests {
	t.Run(tc.name, func(t *testing.T) {
		t.Parallel() // this subtest runs concurrently with its siblings
		got := Process(tc.in)
		if got != tc.want {
			t.Errorf("Process(%q) = %v, want %v", tc.in, got, tc.want)
		}
	})
}

• Do not write tc := tc before t.Run. Go 1.22+ gives each loop iteration its own variable scope, so the historical copy is dead ceremony. Only modules whose go.mod declares go 1.21 or older still need it — and you should bump the directive instead.
• t.Cleanup(fn) over defer for teardown that must run even when a helper calls t.Fatal. Cleanups run LIFO after the test, and they nest correctly through helpers where a defer in main would not.
• t.Helper() as the first line of any assertion helper, so failures point at the caller's line, not inside the helper.
• t.TempDir() for filesystem tests — auto-removed, unique per test, parallel-safe. Never hardcode /tmp/mytest.
• t.Context() (Go 1.24+) gives a context cancelled when the test and its cleanups finish — pass it to anything taking a context.Context instead of context.Background().
• t.Setenv and T.Chdir forbid t.Parallel(). They mutate process-global state; the test panics if you call them in a parallel test. Keep env/cwd-mutating tests serial.

Fakes over mocks

Define the interface where you consume it (the test's package), keep it tiny, and hand-roll a fake with func fields you set per test.

// Charger is what the service needs — declared at the consumer, not the vendor.
type Charger interface {
	Charge(ctx context.Context, cents int64) (string, error)
}

type fakeCharger struct {
	charge func(ctx context.Context, cents int64) (string, error)
}

func (f fakeCharger) Charge(ctx context.Context, cents int64) (string, error) {
	return f.charge(ctx, cents)
}

func TestCheckout_chargesOnce(t *testing.T) {
	var calls int
	f := fakeCharger{charge: func(_ context.Context, c int64) (string, error) {
		calls++
		return "txn_123", nil
	}}
	if err := Checkout(t.Context(), f, 999); err != nil {
		t.Fatalf("Checkout: %v", err)
	}
	if calls != 1 {
		t.Errorf("Charge called %d times, want 1", calls)
	}
}

This is faster to read than a generated mock and never couples your test to call order. Reach for go.uber.org/mock (gomock) or testify/mock only when a team already mandates them or the interface is large and call-sequence assertions are the point — templates and the decision in references/mocks-and-fakes.md. For HTTP dependencies use net/http/httptest (server, recorder, or a fake RoundTripper) instead of a fake — same reference.

Concurrency & time (synctest)

testing/synctest graduated to GA in Go 1.25. It runs your code in an isolated "bubble" with a fake clock that jumps forward instantly the moment every goroutine in the bubble is durably blocked — so a test of a 30-second timeout finishes in microseconds, deterministically.

func TestTimeout(t *testing.T) {
	synctest.Test(t, func(t *testing.T) {
		ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
		defer cancel()

		done := make(chan error, 1)
		go func() { done <- slowOperation(ctx) }()

		synctest.Wait() // wait until the goroutine is durably blocked
		select {
		case err := <-done:
			if !errors.Is(err, context.DeadlineExceeded) {
				t.Errorf("got %v, want DeadlineExceeded", err)
			}
		case <-time.After(time.Minute): // fake time — never really waits
			t.Fatal("operation did not time out")
		}
	})
}

• Always run concurrent code with -race (go test -race ./...). The race detector finds data races that pass silently otherwise.
• synctest.Wait() blocks the bubble until all other bubble goroutines are durably blocked, letting you assert at a known point.
• Use synctest.Test(t, ...), not the old synctest.Run — Run is deprecated as of Go 1.26.
• Never use time.Sleep to "let the goroutine finish." That is the flaky-test factory. Full bubble semantics, durable-block definition, and goroutine-leak detection live in references/synctest-and-concurrency.md.

Coverage

go test -cover ./...                                   # quick per-package %
go test -coverprofile=cover.out ./...                  # write a profile
go tool cover -html=cover.out                          # open annotated source
go test -coverpkg=./... -coverprofile=cover.out ./...  # cross-package coverage
go build -cover -o ./bin/app .                         # coverage for integration binaries (1.20+)

A coverage percentage is a smoke alarm, not a goal. 100% line coverage with no assertions on the values is worthless; 70% on the branches that actually carry logic is fine. Use -coverpkg=./... when your tests live in a separate package and exercise code across the module, or the number undercounts. Profiles, integration-binary coverage, and reading the HTML report are in references/coverage-and-benchmarks.md.

Mutation: does the suite actually notice?

Coverage tells you which code ran; it cannot tell you whether any test would have noticed if that code were wrong. A table-driven test whose cases all assert err == nil and nothing else raises coverage without detecting anything. Mutation testing plants bugs on purpose — flip a < to <=, drop an early return, replace a returned value with the zero value — and a mutant the suite still passes is a test that asserts nothing.

This is not fuzzing. Fuzzing hunts inputs your code mishandles; mutation hunts tests that would not catch a bug you already know about. They find different things and neither substitutes for the other.

Go has no mature mutation tool — go-mutesting and gremlins exist but neither is a dependable default — so this is the manual procedure, and that makes the discipline stricter here, not looser:

• Pick the changed logic. Script the run and commit the script (e.g. tools/mutants.go or a shell driver); do not hand-edit N times, because you will need to rerun it and a restore mistake is silent.
• One mutant at a time: flip a comparison, off-by-one a bound or slice index, delete one branch, swap &&/||, return the zero value. go test ./... must fail for every one.
• Restore, confirm with git diff that nothing else moved, and rerun the suite green.
• Report as N/N killed, naming the mutants.

The runner must prove it executed each mutant. A hand-rolled driver that can report a kill it never ran only ever *inflates* the score — so that defect can never surface as a red run, and no failing test will ever tell you about it. Assert the mutated file changed on disk (compare hashes before and after) and that the test command actually exited non-zero for the reason you expect, rather than trusting the loop.

A survivor is not automatically a failure. Some mutants are semantically equivalent and cannot be killed; classify those with the reason instead of writing a test that asserts non-behaviour. Scale the whole layer to risk (P7): reach for it where a bug is expensive — money, auth, data loss, concurrency, a public API — not on every change.

Benchmarks

Use for b.Loop() (Go 1.24+), not the legacy for i := 0; i < b.N; i++. b.Loop manages the timer itself (setup before the loop and teardown after are excluded automatically), runs the body the right number of times, and the compiler is taught not to dead-code-eliminate calls inside it.

func BenchmarkHash(b *testing.B) {
	data := bytes.Repeat([]byte("x"), 1024) // setup excluded from timing
	b.ReportAllocs()
	for b.Loop() {
		_ = Hash(data)
	}
}

Bad — legacy loop with manual timer juggling and a sink to fool the optimizer:

func BenchmarkHash(b *testing.B) {
	data := bytes.Repeat([]byte("x"), 1024)
	b.ResetTimer()
	var sink uint64
	for i := 0; i < b.N; i++ {
		sink = Hash(data)
	}
	_ = sink
}

• b.ReportAllocs() always — allocation count regresses long before wall-clock does, and it is the first thing to optimize.
• Compare runs with benchstat, never eyeball two numbers — a single run's noise is larger than most real changes. Install and workflow in references/coverage-and-benchmarks.md.
• The sink/runtime.KeepAlive dance is only needed in pre-1.24 modules; inside b.Loop() it is redundant.

Fuzzing

Built in since Go 1.18. Worth it for parsers, decoders, and anything that ingests untrusted input — places where a malformed byte sequence can panic or corrupt state.

func FuzzParse(f *testing.F) {
	f.Add("key=value")        // seed corpus
	f.Add("")
	f.Fuzz(func(t *testing.T, s string) {
		got, err := Parse(s)
		if err != nil {
			return // rejecting bad input is fine; crashing is not
		}
		if out := got.String(); out != s {
			t.Errorf("round-trip: Parse(%q).String() = %q", s, out)
		}
	})
}

Run it with go test -fuzz=FuzzParse -fuzztime=30s. Without -fuzz the seed corpus runs as ordinary cases on every go test, so seeds are also free regression tests.

Anti-patterns

| Anti-pattern | Why it bites | Do instead |

|---|---|---|

| Testing unexported internals via export_test.go for everything | Tests rename-break on every refactor; you're testing structure, not behavior | Test through the public API; reserve export_test.go for genuinely untestable seams |

| Asserting on log output to check a code path ran | Couples tests to message wording; brittle and slow | Assert on returned values or fake-recorded calls |

| time.Sleep to synchronize goroutines | Flaky on loaded CI, slow always | testing/synctest + synctest.Wait(), or a channel |

| One giant TestEverything func | First failure hides the rest; no targetable subtests | Table-driven with named t.Run subtests |

| Chasing 100% coverage | Inflates with assertion-free tests; false confidence | Cover the branches that carry logic; read the profile |

| tc := tc before t.Run in a go 1.22+ module | Dead ceremony; signals copied-from-old-blog code | Delete it; bump the go directive if older |

| gomock/testify/mock to fake an interface you own | Ceremony + coupling to call order for a 12-line job | Hand-rolled func-field fake |

| Benchmark without b.ReportAllocs() | Misses allocation regressions, the cheapest win | Add b.ReportAllocs(); compare with benchstat |

| t.Parallel() in a test calling t.Setenv/T.Chdir | Panics — they mutate process-global state | Keep env/cwd tests serial |

Verify & references

Run scripts/verify.sh [dir] to check emitted tests: it runs gofmt -l, go vet ./..., and go test ./... -count=1 (with -race when supported) inside the first Go module it finds, and no-ops cleanly when there is none.

• references/synctest-and-concurrency.md — bubble model, durable-block definition, fake clock, synctest.Wait, -race, goroutine-leak detection, the GOEXPERIMENT→GA migration.
• references/mocks-and-fakes.md — fakes vs testify/mock vs gomock decision and minimal templates; httptest server/recorder/RoundTripper recipes.
• references/coverage-and-benchmarks.md — coverage profiles, -coverpkg, integration go build -cover, benchstat install and workflow, -cpuprofile/-memprofile.