seikaikyo/golang-modernize

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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.

Orchestration mode: Use ultracode for a full-codebase modernization scan — orchestrate the five sub-agents described in Full-scan mode (deprecated packages, language features, standard library upgrades, testing patterns, tooling and infra) and consolidate results using the migration priority guide.

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. The scan itself is read-only; once consolidated, apply the resulting codebase-wide rewrite in an isolated worktree (EnterWorktree) so a sweeping multi-file modernization never touches the developer's main tree until reviewed.

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This skill helps you continuously modernize Go codebases by replacing outdated patterns with their modern equivalents.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.

Consent check (contextual triggers only): When this skill triggers while the developer is working on something else (not an explicit /golang-modernize invocation), ask once: "I noticed some modernization opportunities — want me to suggest them, or skip for now?" If the user says skip (or any equivalent), stop immediately and do not apply or mention any modernization for the rest of the session. Do not ask again in the current session.

Workflow

When invoked:

1. Check the project's go.mod or go.work to determine the current Go version (go directive)
2. Check the latest Go version using the Go Version Changelogs table below and suggest upgrading if the project's go.mod is behind
3. Read .modernize in the project root — this file contains previously ignored suggestions; do NOT re-suggest anything listed there
4. Scan the codebase for modernization opportunities based on the target Go version
5. Run golangci-lint with the modernize linter if available
6. 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.
7. 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). Once scanning is done and changes are ready to apply, do so in an isolated worktree (EnterWorktree) — a codebase-wide modernization sweep touches many files at once, and isolation keeps the main tree safe to abandon or review before merging.
8. 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.
9. 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.

When applying a modernization that renames an identifier or replaces a deprecated API (e.g. reflect.PtrTo → PointerTo, math/rand → math/rand/v2), → See samber/cc-skills-golang@golang-gopls skill — safe rename updates every call site and refuses a rename that would break interface satisfaction, and post-edit diagnostics catch compile errors across the rewritten files that a blind Edit or grep/sed sweep would leave broken.

.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:

| Version | Release | Changelog | | ------- | ------------- | --------------------------- | | Go 1.21 | August 2023 | https://go.dev/doc/go1.21 | | Go 1.22 | February 2024 | https://go.dev/doc/go1.22 | | Go 1.23 | August 2024 | https://go.dev/doc/go1.23 | | Go 1.24 | February 2025 | https://go.dev/doc/go1.24 | | Go 1.25 | August 2025 | https://go.dev/doc/go1.25 | | Go 1.26 | February 2026 | https://go.dev/doc/go1.26 |

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; gopls and Go 1.26's rewritten go fix cover overlapping modernization checks, but exact coverage differs by tool version. 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/rsa.EncryptPKCS1v15 for new encryption use | RSA-OAEP (rsa.EncryptOAEP / rsa.EncryptOAEPWithOptions) or HPKE/KEM design | 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)

1. Remove loop variable shadow copies (Go 1.22+) — prevents subtle bugs
2. Replace math/rand with math/rand/v2 (Go 1.22+) — remove rand.Seed calls
3. Use os.Root for user-supplied file paths (Go 1.24+) — prevents path traversal
4. Run govulncheck (Go 1.22+) — catch known vulnerabilities
5. Use errors.Is/errors.As instead of direct comparison (Go 1.13+)
6. Migrate deprecated crypto packages (Go 1.24+) — security critical

Medium priority (readability and maintainability)

7. Replace interface{} with any (Go 1.18+)
8. Use min/max builtins (Go 1.21+)
9. Use range over int (Go 1.22+)
10. Use slices and maps packages (Go 1.21+)
11. Use cmp.Or for default values (Go 1.22+)
12. Use sync.OnceValue/sync.OnceFunc (Go 1.21+)
13. Use sync.WaitGroup.Go (Go 1.25+)
14. Use t.Context() in tests (Go 1.24+)
15. Use b.Loop() in benchmarks (Go 1.24+)

Lower priority (gradual improvement)

16. Migrate to slog from third-party loggers (Go 1.21+)
17. Adopt iterators where they simplify code (Go 1.23+)
18. Replace sort.Slice with slices.SortFunc (Go 1.21+)
19. Use strings.SplitSeq and iterator variants (Go 1.24+)
20. Move tool deps to go.mod tool directives (Go 1.24+)
21. Enable PGO for production builds (Go 1.21+)
22. Upgrade to golangci-lint v2 with modernize linter (golangci-lint v2.6.0+)
23. Add govulncheck to CI pipeline
24. Set up monthly modernization CI pipeline
25. Evaluate encoding/json/v2 only when the project explicitly opts into GOEXPERIMENT=jsonv2 (Go 1.25+, experimental)
26. Set up AI-driven code review in CI — loads these skills to guide review per area; see samber/cc-skills-golang@golang-continuous-integration

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, samber/cc-skills-golang@golang-continuous-integration skills.

• → See samber/cc-skills-golang@golang-refactoring skill for staging a large modernization sweep as small human-reviewed PRs instead of one big worktree sweep.