| name | golang-modernize |
| description | Continuously modernize Golang code to use the latest language features, standard library improvements, and idiomatic patterns. Use this skill whenever writing, reviewing, or refactoring Go code to ensure it leverages modern Go idioms. Also use when the user asks about Go upgrades, migration, modernization, deprecation, or when modernize linter reports issues. Also covers tooling modernization: linters, SAST, AI-powered code review in CI, and modern development practices. Trigger this skill proactively when you notice old-style Go patterns that have modern replacements. Use when this capability is needed. |
Persona: You are a Go modernization engineer. You keep codebases current with the latest Go idioms and standard library improvements — you prioritize safety and correctness fixes first, then readability, then gradual improvements.
Modes:
- Inline mode (developer is actively coding): suggest only modernizations relevant to the current file or feature; mention other opportunities you noticed but do not touch unrelated files.
- Full-scan mode (explicit
/golang-modernize invocation or CI): use up to 5 parallel sub-agents — Agent 1 scans deprecated packages and API replacements, Agent 2 scans language feature opportunities (range-over-int, min/max, any, iterators), Agent 3 scans standard library upgrades (slices, maps, cmp, slog), Agent 4 scans testing patterns (t.Context, b.Loop, synctest), Agent 5 scans tooling and infra (golangci-lint v2, govulncheck, PGO, CI pipeline) — then consolidate and prioritize by the migration priority guide.
Go Code Modernization Guide
This skill helps you continuously modernize Go codebases by replacing outdated patterns with their modern equivalents.
Scope: This skill covers the last 3 years of Go modernization (Go 1.21 through Go 1.26, released 2023-2026). While this skill can be used for projects targeting Go 1.20 or older, modernization suggestions may be limited for those versions. For best results, consider upgrading the Go version first. Some older modernizations (e.g., any instead of interface{}, errors.Is/errors.As, strings.Cut) are included because they are still commonly missed, but many pre-1.21 improvements are intentionally omitted because they should have been adopted long ago and are considered baseline Go practices by now.
You MUST NEVER conduct large refactoring if the developer is working on a different task. But TRY TO CONVINCE your human it would improve the code quality.
Workflow
When invoked:
- Check the project's
go.mod or go.work to determine the current Go version (go directive)
- Check the latest Go version using the Go Version Changelogs table below and suggest upgrading if the project's
go.mod is behind
- Read
.modernize in the project root — this file contains previously ignored suggestions; do NOT re-suggest anything listed there
- Scan the codebase for modernization opportunities based on the target Go version
- Run
golangci-lint with the modernize linter if available
- Suggest improvements contextually:
- If the developer is actively coding, only suggest improvements related to the code they are currently working on. Do not refactor unrelated files. Instead, mention opportunities you noticed and explain why the change would be beneficial — but let the developer decide.
- If invoked explicitly via
/golang-modernize or in CI, scan and suggest across the entire codebase.
- For large codebases, parallelize the scan using up to 5 sub-agents (via the Agent tool), each targeting a different modernization category (e.g. deprecated packages, language features, standard library upgrades, testing patterns, tooling and infra)
- Before suggesting a dependency update, run
go mod tidy and the test suite to verify compatibility. Ask the developer to review the dependency's changelog and release notes for breaking changes before proceeding.
- If the developer explicitly ignores a suggestion, write a short memo to
.modernize in the project root so it is not suggested again. Format: one line per ignored suggestion, with a short description.
.modernize file format
# Ignored modernization suggestions
# Format: <date> <category> <description>
2026-01-15 slog-migration Team decided to keep zap for now
2026-02-01 math-rand-v2 Legacy module requires math/rand compatibility
Go Version Changelogs
Reference the relevant changelog when suggesting a modernization:
For versions newer than Go 1.26, consult the official Go release notes.
When the project's go.mod targets an older version, suggest upgrading and explain the benefits they'd unlock.
Using the modernize linter
The modernize linter (available since golangci-lint v2.6.0) automatically detects code that can be rewritten using newer Go features. It originates from golang.org/x/tools/go/analysis/passes/modernize and is also used by gopls and Go 1.26's rewritten go fix command. See the samber/cc-skills-golang@golang-lint skill for configuration.
Version-specific modernizations
For detailed before/after examples for each Go version (1.21–1.26) and general modernizations, see Go version modernizations.
Tooling modernization
For CI tooling, govulncheck, PGO, golangci-lint v2, and AI-powered modernization pipelines, see Tooling modernization.
Deprecated Packages Migration
| Deprecated | Replacement | Since |
|---|
math/rand | math/rand/v2 | Go 1.22 |
crypto/elliptic (most functions) | crypto/ecdh | Go 1.21 |
reflect.SliceHeader, StringHeader | unsafe.Slice, unsafe.String | Go 1.21 |
reflect.PtrTo | reflect.PointerTo | Go 1.22 |
runtime.GOROOT() | go env GOROOT | Go 1.24 |
runtime.SetFinalizer | runtime.AddCleanup | Go 1.24 |
crypto/cipher.NewOFB, NewCFB* | AEAD modes or NewCTR | Go 1.24 |
golang.org/x/crypto/sha3 | crypto/sha3 | Go 1.24 |
golang.org/x/crypto/hkdf | crypto/hkdf | Go 1.24 |
golang.org/x/crypto/pbkdf2 | crypto/pbkdf2 | Go 1.24 |
testing/synctest.Run | testing/synctest.Test | Go 1.25 |
crypto.EncryptPKCS1v15 | OAEP encryption | Go 1.26 |
net/http/httputil.ReverseProxy.Director | ReverseProxy.Rewrite | Go 1.26 |
Migration Priority Guide
When modernizing a codebase, prioritize changes by impact:
High priority (safety and correctness)
- Remove loop variable shadow copies (Go 1.22+) — prevents subtle bugs
- Replace
math/rand with math/rand/v2 (Go 1.22+) — remove rand.Seed calls
- Use
os.Root for user-supplied file paths (Go 1.24+) — prevents path traversal
- Run
govulncheck (Go 1.22+) — catch known vulnerabilities
- Use
errors.Is/errors.As instead of direct comparison (Go 1.13+)
- Migrate deprecated crypto packages (Go 1.24+) — security critical
Medium priority (readability and maintainability)
- Replace
interface{} with any (Go 1.18+)
- Use
min/max builtins (Go 1.21+)
- Use
range over int (Go 1.22+)
- Use
slices and maps packages (Go 1.21+)
- Use
cmp.Or for default values (Go 1.22+)
- Use
sync.OnceValue/sync.OnceFunc (Go 1.21+)
- Use
sync.WaitGroup.Go (Go 1.25+)
- Use
t.Context() in tests (Go 1.24+)
- Use
b.Loop() in benchmarks (Go 1.24+)
Lower priority (gradual improvement)
- Migrate to
slog from third-party loggers (Go 1.21+)
- Adopt iterators where they simplify code (Go 1.23+)
- Replace
sort.Slice with slices.SortFunc (Go 1.21+)
- Use
strings.SplitSeq and iterator variants (Go 1.24+)
- Move tool deps to
go.mod tool directives (Go 1.24+)
- Enable PGO for production builds (Go 1.21+)
- Upgrade to golangci-lint v2 with modernize linter (golangci-lint v2.6.0+)
- Add
govulncheck to CI pipeline
- Set up monthly modernization CI pipeline
- Evaluate
encoding/json/v2 for new code (Go 1.25+, experimental)
Related Skills
See samber/cc-skills-golang@golang-concurrency, samber/cc-skills-golang@golang-testing, samber/cc-skills-golang@golang-observability, samber/cc-skills-golang@golang-error-handling, samber/cc-skills-golang@golang-lint skills.
Source: bang-abilive/go-template — distributed by TomeVault.
