golang

Use fmt.Errorf wrapping when callers don't need to distinguish error kinds, sentinel errors (var ErrNotFound) for two to three distinguishable kinds, and custom error structs when you have many kinds or need attached context. Add Split-Brain Error Types when users need separate safe messages.

Implement slog.LogValuer on your structs using an allow-list that only exports safe fields, or define a Secret string type whose LogValue and String methods return [REDACTED]. This masks secrets in both slog output and fmt printing using only the standard library.

Start with a single repository Tx callback method, which covers most cases. Only adopt Unit of Work when a transaction must atomically span multiple repositories and neither aggregate redesign nor eventual consistency with async processing can avoid it.

This Skill recommends the standard library only: use t.Fatalf for preconditions where failure makes subsequent checks meaningless, and t.Errorf for validations so multiple failures surface at once. Write table-driven tests with t.Run from the start, even with a single case.

The Transactor pattern using context.WithValue loses type safety and makes transaction boundaries implicit and hard to trace. Prefer explicit passing via DBTX interfaces or Unit of Work callbacks so transaction scope is visible in function signatures.

Keep cobra as wiring only: put business logic in an internal app package, use a NewRootCmd factory instead of global command variables so each test gets an isolated command tree, and confine viper to a config package that returns a validated struct.