Use when hardening Go code at API boundaries — copying slices/maps, verifying interface compliance, using defer for cleanup, time.Time/time.Duration, or avoiding mutable globals. Also use when reviewing for robustness concerns like missing cleanup or unsafe crypto usage, even if the user doesn't men

Go Defensive Programming Patterns Compatibility: Crypto examples may use crypto/rand.Text , which requires Go 1.24+. Resource Routing references/BOUNDARY COPYING.md Read when copying slices/maps across API boundaries. references/GLOBAL STATE.md Read when introducing or removing package globals. references/MUST FUNCTIONS.md Read when deciding whether a panic on error helper is acceptable. references/PANIC RECOVER.md Read when evaluating panic, recover, or crash containment. references/TIME ENUMS TAGS.md Read when handling time types, enum zero values, or struct tags. Defensive Checklist Priority When hardening code at API boundaries, check in this order: Quick Reference Pattern Rule Details Boundary copies Copy slices/maps on receive and return [BOUNDARY COPYING.md](references/BOUNDARY COPYING.md) Defer cleanup defer f.Close() right after os.Open Below Interface check Compile time satisfaction assertion See go interfaces Time types time.Time / time.Duration , never raw int [TIME ENUMS TAGS.md](references/TIME ENUMS TAGS.md) Enum start iota + 1 so zero = invalid Below Crypto rand crypto/rand for keys, never math/rand Below Must functions Only at init; panic on failure [MUST FUNCTIONS.md](references/MUST FUNCTIONS.md) Panic/recover Never expose panics across packages [PANIC RECOVER.md](references/PANIC RECOVER.md) Mutable globals Replace with dependency injection Below Verify Interface Compliance Route compile time interface assertions to [go interfaces](../go interfaces/SKILL.md). Use this skill only to notice API boundary robustness risk; the interface skill owns when an assertion is appropriate and the exact assertion shape. Copy Slices and Maps at Boundaries Slices and maps contain pointers to underlying data. Copy at API boundaries to prevent unintended modifications. Defer to Clean Up Use defer to clean up resources (files, locks). Avoids missed cleanup on multiple return paths. Defer overhead is negligible. Place defer f.Close() immediately after os.Open for clarity. Arguments to deferred functions are evaluated when defer executes, not when the function runs. Multiple defers execute in LIFO order. Struct Field Tags Advisory : Always add explicit field tags to structs that are marshaled or unmarshaled. Field tags are a serialization contract — renaming a struct field without updating the tag silently breaks wire compatibility. Treat tags as part of the public API for any type that crosses a serialization boundary. Start Enums at One Start enums at non zero to distinguish uninitialized from valid values. Exception : When zero is the sensible default (e.g., LogToStdout = iota ). Time, Struct Tags, and Embedding Avoid Mutable Globals Inject dependencies instead of mutating package level variables. This makes code testable without global save/restore. Crypto Rand Do not use math/rand or math/rand/v2 to generate keys — this is a security concern . Time seeded generators have predictable output. For text output, use crypto/rand.Text directly, or encode random bytes with encoding/hex or encoding/base64 . Panic and Recover Use panic only for truly unrecoverable situations. Library functions should avoid panic. Key rules: Never expose panics across package boundaries — always convert to errors Acceptable to panic in init() if a library truly cannot set itself up Use recover to isolate panics in server goroutine handlers Must Functions Must functions panic on error — use them only during program initialization where failure means the program cannot run. Related Skills Error handling : See [go error handling](../go error handling/SKILL.md) when choosing between returning errors and panicking, or wrapping errors at boundaries Concurrency safety : See [go concurrency](../go concurrency/SKILL.md) when protecting shared state with mutexes, atomics, or channels Interface checks : See [go interfaces](../go interfaces/SKILL.md) when adding compile time interface satisfaction checks Data structure copying : See [go data structures](../go data structures/SKILL.md) when working with slice/map internals or pointer aliasing

go-defensive