Go Testing Agent Skill

Go Testing

> Compatibility: Diff examples may use github.com/google/go-cmp.

Resource Routing

• scripts/gen-table-test.sh - Run when generating a table-driven test scaffold.
• assets/table-test-template.go - Use as a copyable table-test starting point.
• references/TABLE-DRIVEN-TESTS.md - Read when choosing table tests, subtests, or parallel test patterns.
• references/TEST-HELPERS.md - Read when writing helpers, fixtures, cleanup, or test doubles.
• references/TEST-ORGANIZATION.md - Read when structuring packages, black-box tests, or larger test suites.
• references/VALIDATION-APIS.md - Read when choosing t.Error, t.Fatal, cmp.Diff, or assertion style.
• references/INTEGRATION.md - Read when testing external services, HTTP handlers, databases, or long-running setup.

Quick Reference

| Pattern | Use When |

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

| t.Error | Default — report failure, keep running |

| t.Fatal | Setup failed or continuing is meaningless |

| cmp.Diff | Comparing structs, slices, maps, protos |

| Table-driven | Many cases share identical logic |

| Subtests | Need filtering, parallel execution, or naming |

| t.Helper() | Any test helper function (call as first statement) |

| t.Cleanup() | Teardown in helpers instead of defer |

Useful Test Failures

> Normative: Test failures must be diagnosable without reading the test

> source.

Every failure message must include: function name, inputs, actual (got), and

expected (want). Use the format YourFunc(%v) = %v, want %v.

// Good:
t.Errorf("Add(2, 3) = %d, want %d", got, 5)

// Bad: Missing function name and inputs
t.Errorf("got %d, want %d", got, 5)

Always print got before want: got %v, want %v — never reversed.

No Assertion Libraries

> Normative: Do not use assertion libraries. Use cmp.Diff for complex

> comparisons.

if diff := cmp.Diff(want, got); diff != "" {
    t.Errorf("GetPost() mismatch (-want +got):\n%s", diff)
}

For protocol buffers, add protocmp.Transform() as a cmp option. Always

include the direction key (-want +got) in diff messages. Avoid comparing

JSON/serialized output — compare semantically instead.

t.Error vs t.Fatal

> Normative: Use t.Error by default to report all failures in one run.

> Use t.Fatal only when continuing is impossible.

Choose t.Fatal when:

• Setup fails (DB connection, file load)
• The next assertion depends on the previous one succeeding (e.g., decode after

encode)

Never call t.Fatal/t.FailNow from a goroutine other than the test

goroutine — use t.Error instead.

Table-Driven Tests

> See assets/table-test-template.go when scaffolding a new table-driven test and need the canonical struct, loop, and subtest layout.

> Advisory: Use table-driven tests when many cases share identical logic.

Use table tests when: all cases run the same code path with no conditional

setup, mocking, or assertions. A single shouldErr bool is acceptable.

Don't use table tests when: cases need complex setup, conditional mocking,

or multiple branches — write separate test functions instead.

Key rules:

• Use field names when cases span many lines or have same-type adjacent fields
• Include inputs in failure messages — never identify rows by index

> Validation: After generating or modifying tests, run go test -run TestXxx -v to verify the tests compile and pass. Fix any compilation errors before proceeding.

Test Helpers

> Normative: Test helpers must call t.Helper() first and use t.Cleanup()

> for teardown.

func setupTestDB(t *testing.T) *sql.DB {
    t.Helper()
    db, err := sql.Open("sqlite3", ":memory:")
    if err != nil {
        t.Fatalf("Could not open database: %v", err)
    }
    t.Cleanup(func() { db.Close() })
    return db
}

Test Error Semantics

> Advisory: Test error semantics, not error message strings.

// Bad: Brittle string comparison
if err.Error() != "invalid input" { ... }

// Good: Semantic check
if !errors.Is(err, ErrInvalidInput) { ... }

For simple presence checks when specific semantics don't matter:

if gotErr := err != nil; gotErr != tt.wantErr {
    t.Errorf("f(%v) error = %v, want error presence = %t", tt.input, err, tt.wantErr)
}

Related Skills

• Error testing: See go-error-handling when testing error semantics with errors.Is/errors.As or sentinel errors
• Interface mocking: See go-interfaces when creating test doubles by implementing interfaces at the consumer side
• Naming test functions: See go-naming when naming test functions, subtests, or test helper utilities
• Linter integration: See go-linting when running linters alongside tests in CI or pre-commit hooks