---

name: go-modernize description: > Modernize Go code to use current language features and standard library additions. Covers generics, log/slog, errors.Join, slices/maps packages, range-over-func, and iterators introduced in Go 1.21-1.23+. Use when: "modernize", "update to modern Go", "use generics", "replace interface{}", "upgrade Go version", "slog", "errors.Join", "range over func", "iterators". Not for: general style (go-coding-standards), error philosophy (go-error-handling), logging architecture (go-observability). user-invocable: true license: MIT compatibility: Designed for Claude Code or similar AI coding agents working on Go projects. Requires the Go toolchain. gopls is optional, for the modernize analyzer. allowed-tools: Read Edit Write Glob Grep Bash(go:) Bash(gofmt:) Bash(golangci-lint:) Bash(gopls:) metadata: author: eduardo-sl version: "1.4.1"

Go Modernize

Go evolves. Code written for Go 1.16 should not look the same as code targeting Go 1.25+. Modernize incrementally — update go.mod, then adopt new patterns.

Never adopt a feature above the go directive in go.mod. Raise the directive deliberately, in its own commit, and only to a version the project's CI and deployment images actually run.

Detailed reference material, loaded on demand:

• references/generics.md — replacing interface{} with type parameters, constraints, generic containers, when NOT to use generics.
• references/stdlib-migrations.md — before/after examples for slog, errors.Join, slices/maps helpers, range-over-int, and iterators.

Read a reference file only when the summary below is not enough.

Modernization Procedure

1. Check the go directive in go.mod — it caps which features you can use.

2. Run the official modernizers first — they find and fix the mechanical migrations automatically:

   # Go 1.26+ — the modernizers now live in go fix
   go fix ./...
   
   # Go 1.25 and earlier
   go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...
   

   Both rewrite source in place. Commit before running, and review the diff. If neither command is available, apply the table below manually.

3. Scan the table below for the judgment-based migrations the analyzer does not cover (generics, iterators, logger replacement) and apply them case by case.

4. Run go build ./... and the test suite after each group of changes.

Feature Table by Go Version

Go Version	Feature	Action
1.13+	errors.Is, errors.As	Replace == error comparisons
1.13+	http.NewRequestWithContext	Replace http.NewRequest
1.16+	embed	Replace go-bindata / packr
1.18+	Generics	Replace interface{} utility functions
1.20+	errors.Join	Replace manual error accumulation
1.21+	log/slog	Replace log for structured logging
1.21+	slices, maps	Replace hand-written slice/map utilities
1.21+	min, max builtins	Replace math.Min/math.Max (float64-only)
1.22+	Range over int	Replace for i := 0; i < n; i++
1.23+	Range over func	Replace callback-based iteration
1.23+	unique.Make	Replace hand-rolled string interning
1.24+	for b.Loop()	Replace for range b.N in benchmarks
1.24+	t.Context()	Replace context.Background() in tests
1.24+	os.Root	Replace manual path-traversal checks
1.24+	go.mod tool directive	Replace the tools.go blank-import file
1.24+	runtime.AddCleanup	Replace runtime.SetFinalizer
1.25+	testing/synctest	Replace time.Sleep in concurrency tests
1.25+	sync.WaitGroup.Go	Replace wg.Add(1) + go func(){defer wg.Done()}
1.26+	errors.AsType	Replace errors.As with a declared target variable
1.26+	slog.NewMultiHandler	Replace hand-written fan-out handlers
1.26+	new(expr)	Replace a temp variable taken by address

Key Migrations at a Glance

Generics — type-safe utilities (Go 1.18+)

// ❌ Before — loses type safety
func Contains(slice []interface{}, target interface{}) bool { /* ... */ }

// ✅ After — type-safe generic
func Contains[T comparable](slice []T, target T) bool { /* ... */ }

Use generics for container types (Set[T], Result[T]) and utility functions. Do NOT use them where a single concrete type works, or as a substitute for interfaces in runtime polymorphism. Details and constraint patterns: references/generics.md.

Structured logging (Go 1.21+)

// ❌ Before
log.Printf("processing order %s for user %s", orderID, userID)

// ✅ After
slog.Info("processing order",
    slog.String("order_id", orderID),
    slog.String("user_id", userID),
)

Keep zap/zerolog only if you need their performance for high-throughput logging; for most services slog is sufficient.

errors.Join (Go 1.20+)

var errs []error
for _, item := range items {
    if err := validate(item); err != nil {
        errs = append(errs, err)
    }
}
if err := errors.Join(errs...); err != nil {
    return fmt.Errorf("validation: %w", err)
}

errors.Join preserves the chain — errors.Is/errors.As work on each joined error. Never accumulate error strings manually.

slices and maps helpers (Go 1.21+)

found := slices.Contains(items, target)          // not a manual loop
slices.SortFunc(users, func(a, b User) int {     // not sort.Slice
    return cmp.Compare(a.Name, b.Name)
})
keys := slices.Collect(maps.Keys(m))             // not a manual key loop
clone := maps.Clone(m)                           // not a manual copy loop

Range over int (Go 1.22+) and iterators (Go 1.23+)

for i := range n { process(i) }                  // not for i := 0; i < n; i++

for i, v := range slices.Backward(items) {       // stdlib iterators
    fmt.Printf("%d: %v\n", i, v)
}

Custom iter.Seq/iter.Seq2 iterators replace callback-based iteration — full worked example in references/stdlib-migrations.md.

Context-aware HTTP requests (Go 1.13+, often missed)

// ❌ Before — request without context
req, err := http.NewRequest(http.MethodGet, url, nil)

// ✅ After — context propagated
req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)

Concurrency and tests (Go 1.24-1.25)

// ❌ Before
var wg sync.WaitGroup
for _, job := range jobs {
    wg.Add(1)
    go func() {
        defer wg.Done()
        process(job)
    }()
}
wg.Wait()

// ✅ After — Go 1.25
var wg sync.WaitGroup
for _, job := range jobs {
    wg.Go(func() { process(job) })
}
wg.Wait()

wg.Go cannot be called after wg.Wait returns, which removes the classic "Add after Wait" race that the waitgroup vet analyzer (Go 1.25+) reports.

In tests, context.Background() becomes t.Context(), for range b.N becomes for b.Loop(), and time.Sleep-based concurrency tests become synctest.Test. → See the go-test-quality skill.

Error inspection (Go 1.26)

// ❌ Before — needs a declared target, and the pointer indirection is easy to get wrong
var pathErr *fs.PathError
if errors.As(err, &pathErr) {
    log.Println(pathErr.Path)
}

// ✅ After — Go 1.26
if pathErr, ok := errors.AsType[*fs.PathError](err); ok {
    log.Println(pathErr.Path)
}

errors.Is is unchanged. Only the As form gains a generic alternative.

Value interning and cleanup (Go 1.23-1.24)

// ✅ unique.Make deduplicates repeated values; Value() returns the canonical copy
h := unique.Make(hostname)          // unique.Handle[string], comparable, cheap
store[h] = conn                     // one string kept in memory, not one per entry

// ✅ AddCleanup replaces SetFinalizer: multiple cleanups, no resurrection,
//    and it works on objects that are part of a cycle
runtime.AddCleanup(obj, func(fd int) { syscall.Close(fd) }, obj.fd)

Reach for unique only where profiling shows duplicate values dominating the heap — a config parser reading millions of rows, not a request handler.

Verification Checklist

1. go.mod version matches the features used in the codebase
2. No interface{} where any or type parameters would be clearer
3. log/slog used instead of log.Printf for structured logging
4. errors.Join used instead of manual error string concatenation
5. slices.Contains, slices.SortFunc, maps.Clone replace hand-written loops
6. Range over int (for i := range n) used where applicable
7. http.NewRequestWithContext used instead of http.NewRequest
8. No sort.Slice — use slices.SortFunc with cmp.Compare
9. Generics used for type-safe containers and utilities, not overused for trivial cases
10. Third-party dependencies evaluated against stdlib alternatives added in recent Go versions
11. sync.WaitGroup.Go used instead of manual Add/Done pairs (Go 1.25+)
12. Tests use t.Context() and for b.Loop() (Go 1.24+)
13. tools.go replaced by the go.mod tool directive (Go 1.24+)