mosaic: Skill for Claude Code

Modern Go Guidelines

Detected Go Version

Run sh .agents/skills/use-modern-go/scripts/project-go-version.sh. Pass a module directory when the task targets a nested module.

The script prints the module's go directive or unknown if it cannot find one. Do not infer the target version from the installed toolchain.

How to Use This Skill

DO NOT search for go.mod files or try to detect the version yourself. Use ONLY the version shown above.

If version detected (not "unknown"):

• Say: "This project is using Go X.XX, so I'll stick to modern Go best practices and freely use language features up to and including this version. If you'd prefer a different target version, just let me know."
• Do NOT list features, do NOT ask for confirmation

If version is "unknown":

• Say: "Could not detect Go version in this repository"
• Use AskUserQuestion: "Which Go version should I target?" → [1.24] / [1.25] / [1.26] / [1.27]

When writing Go code, use ALL features from this document up to the target version:

• Prefer modern built-ins and packages (slices, maps, cmp) over legacy patterns
• Never use features from newer Go versions than the target
• Never use outdated patterns when a modern alternative is available

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Features by Go Version

Go 1.0+

• time.Since: time.Since(start) instead of time.Now().Sub(start)

Go 1.8+

• time.Until: time.Until(deadline) instead of deadline.Sub(time.Now())

Go 1.13+

• errors.Is: errors.Is(err, target) instead of err == target (works with wrapped errors)

Go 1.18+

• any: Use any instead of interface{}
• bytes.Cut: before, after, found := bytes.Cut(b, sep) instead of Index+slice
• strings.Cut: before, after, found := strings.Cut(s, sep)

Go 1.19+

• fmt.Appendf: buf = fmt.Appendf(buf, "x=%d", x) instead of []byte(fmt.Sprintf(...))
• atomic.Bool/atomic.Int64/atomic.Pointer[T]: Type-safe atomics instead of atomic.StoreInt32

var flag atomic.Bool
flag.Store(true)
if flag.Load() { ... }

var ptr atomic.Pointer[Config]
ptr.Store(cfg)