writing-go

name	writing-go
description	Apply modern Go syntax guidelines based on project's Go version. Use when user ask for modern Go code guidelines.

Modern Go Guidelines

Detected Go Version

!grep -rh "^go " --include="go.mod" . 2>/dev/null | cut -d' ' -f2 | sort | uniq -c | sort -nr | head -1 | xargs | cut -d' ' -f2 | grep . || echo unknown

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?" → [1.23] / [1.24] / [1.25] / [1.26]

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• Never use outdated patterns when a modern alternative is available

---

Validating Go Code

After writing or editing any .go file, run these against the changed package(s). All must pass; fix the code, do not suppress diagnostics.

gofmt -s -w <changed-files>
go vet ./<changed-pkg>/...
staticcheck ./<changed-pkg>/...   # if installed
go build ./...
go test ./<changed-pkg>/...
go fix ./...

If the repo's AGENTS.md specifies required env vars, directories to skip, or extra checks, follow those instead.

---

Features by Go Version

Go 1.24+

• t.Context() not context.WithCancel(context.Background()) in tests. ALWAYS use t.Context() when a test function needs a context.

Before:

func TestFoo(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()
    result := doSomething(ctx)
}

After:

func TestFoo(t *testing.T) {
    ctx := t.Context()
    result := doSomething(ctx)
}

• omitzero not omitempty in JSON struct tags. ALWAYS use omitzero for time.Duration, time.Time, structs, slices, maps.

Before:

type Config struct {
    Timeout time.Duration `json:"timeout,omitempty"` // doesn't work for Duration!
}

After:

type Config struct {
    Timeout time.Duration `json:"timeout,omitzero"`
}

• b.Loop() not for i := 0; i < b.N; i++ in benchmarks. ALWAYS use b.Loop() for the main loop in benchmark functions.

Before:

func BenchmarkFoo(b *testing.B) {
    for i := 0; i < b.N; i++ {
        doWork()
    }
}

After:

func BenchmarkFoo(b *testing.B) {
    for b.Loop() {
        doWork()
    }
}

• strings.SplitSeq not strings.Split when iterating. ALWAYS use SplitSeq/FieldsSeq when iterating over split results in a for-range loop.

Before:

for _, part := range strings.Split(s, ",") {
    process(part)
}

After:

for part := range strings.SplitSeq(s, ",") {
    process(part)
}

Also: strings.FieldsSeq, bytes.SplitSeq, bytes.FieldsSeq.

Go 1.25+

• wg.Go(fn) not wg.Add(1) + go func() { defer wg.Done(); ... }(). ALWAYS use wg.Go() when spawning goroutines with sync.WaitGroup.

Before:

var wg sync.WaitGroup
for _, item := range items {
    wg.Add(1)
    go func() {
        defer wg.Done()
        process(item)
    }()
}
wg.Wait()

After:

var wg sync.WaitGroup
for _, item := range items {
    wg.Go(func() {
        process(item)
    })
}
wg.Wait()

Go 1.26+

• new(val) not x := val; &x — returns pointer to any value. Go 1.26 extends new() to accept expressions, not just types. Type is inferred: new(0) → *int, new("s") → *string, new(T{}) → *T. DO NOT use x := val; &x pattern — always use new(val) directly. DO NOT use redundant casts like new(int(0)) — just write new(0). Common use case: struct fields with pointer types.

Before:

timeout := 30
debug := true
cfg := Config{
    Timeout: &timeout,
    Debug:   &debug,
}

After:

cfg := Config{
    Timeout: new(30),   // *int
    Debug:   new(true), // *bool
}

• errors.AsType[T](err) not errors.As(err, &target). ALWAYS use errors.AsType when checking if error matches a specific type.

Before:

var pathErr *os.PathError
if errors.As(err, &pathErr) {
    handle(pathErr)
}

After:

if pathErr, ok := errors.AsType[*os.PathError](err); ok {
    handle(pathErr)
}