go-defensive
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 mention "defensive programming." Does not cover error handling strategy (see go-error-handling).
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/cxuu/golang-skills --skill go-defensive- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads go-defensive automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
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:
Reviewing an API boundary?
├─ 1. Error handling → Return errors; don't panic (see go-error-handling)
├─ 2. Input validation → Copy slices/maps received from callers
├─ 3. Output safety → Copy slices/maps before returning to callers
├─ 4. Resource cleanup → Use defer for Close/Unlock/Cancel
├─ 5. Interface checks → Route compile-time assertions to go-interfaces
├─ 6. Time correctness → Use time.Time and time.Duration, not int/float
├─ 7. Enum safety → Start iota at 1 so zero-value is invalid
└─ 8. Crypto safety → crypto/rand for keys, never math/rand
Quick Reference
| Pattern | Rule | Details |
|---|---|---|
| Boundary copies | Copy slices/maps on receive and return | 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 |
| 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 |
| Panic/recover | Never expose panics across packages | PANIC-RECOVER.md |
| Mutable globals | Replace with dependency injection | Below |
Verify Interface Compliance
Route compile-time interface assertions to go-interfaces. 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.
// Receiving: copy incoming slice
d.trips = make([]Trip, len(trips))
copy(d.trips, trips)
// Returning: copy map before returning
result := make(map[string]int, len(s.counters))
for k, v := range s.counters { result[k] = v }
Defer to Clean Up
Use defer to clean up resources (files, locks). Avoids missed cleanup on multiple return paths.
p.Lock()
defer p.Unlock()
if p.count < 10 {
return p.count
}
p.count++
return p.count
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.
type User struct {
Name string `json:"name" yaml:"name"`
Email string `json:"email" yaml:"email"`
}
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.
const (
Add Operation = iota + 1 // Add=1, zero value = uninitialized
Subtract
Multiply
)
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.
type signer struct {
now func() time.Time // injected; tests replace with fixed time
}
func newSigner() *signer {
return &signer{now: time.Now}
}
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.
import "crypto/rand"
func Key() string { return rand.Text() }
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.
func safelyDo(work *Work) {
defer func() {
if err := recover(); err != nil {
log.Println("work failed:", err)
}
}()
do(work)
}
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.
var validID = regexp.MustCompile(`^[a-z][a-z0-9-]{0,62}$`)
var tmpl = template.Must(template.ParseFiles("index.html"))
Related Skills
- Error handling: See go-error-handling when choosing between returning errors and panicking, or wrapping errors at boundaries
- Concurrency safety: See go-concurrency when protecting shared state with mutexes, atomics, or channels
- Interface checks: See go-interfaces when adding compile-time interface satisfaction checks
- Data structure copying: See go-data-structures when working with slice/map internals or pointer aliasing
FAQ
What does the go-defensive skill do?
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 mention "defensive programming." Does not cover error handling strategy (see go-error-handling).
How do I install go-defensive?
Run npx skills add https://github.com/cxuu/golang-skills --skill go-defensive in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/go-defensive/ (personal) or .claude/skills/go-defensive/ (project).
Which AI agents can use go-defensive?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is go-defensive safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains go-defensive?
It is published by cxuu in cxuu/golang-skills. It has 1,224 recorded installs.
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