golang-modernize
This Claude Code skill modernizes Go codebases by identifying and replacing outdated patterns, deprecated APIs, and older language idioms with current equivalents from Go 1.21 through 1.26. Use it when actively coding to receive targeted suggestions for the current file, when explicitly invoking modernization, or when Go version upgrades or deprecation warnings occur. Full-scan mode deploys five parallel agents to systematically audit deprecated packages, language features, standard library improvements, testing patterns, and tooling infrastructure.
git clone --depth 1 https://github.com/samber/cc-skills-golang /tmp/golang-modernize && cp -r /tmp/golang-modernize/skills/golang-modernize ~/.claude/skills/golang-modernizeSKILL.md
<!-- markdownlint-disable ol-prefix -->
**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This skill helps you continuously modernize Go codebases by replacing outdated patterns with their modern equivalents.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.
**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)
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.
### `.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](./references/versions.md).
## Tooling modernization
For CI tooling, govulncheck, PGO, golangci-lint v2, and AI-powered modernization pipelines, see [Tooling modernization](./references/tooling.md).
## Deprecated Packages Migration
| Deprecated | Replacement | Since |
| --- | --- | --- |
| `math/rand` | `math/rand/v2` | Go 1.22 |
| `crypto/elliptic`Golang benchmarking, profiling, and performance measurement. Use when writing, running, or comparing Go benchmarks, profiling hot paths with pprof, interpreting CPU/memory/trace profiles, analyzing results with benchstat, setting up CI benchmark regression detection, or investigating production performance with Prometheus runtime metrics. Also use when the developer needs deep analysis on a specific performance indicator - this skill provides the measurement methodology, while `samber/cc-skills-golang@golang-performance` provides the optimization patterns.
Golang CLI application development. Use when building, modifying, or reviewing a Go CLI tool — especially for command structure, flag handling, configuration layering, version embedding, exit codes, I/O patterns, signal handling, shell completion, argument validation, and CLI unit testing. Also triggers when code uses cobra, viper, or urfave/cli. For cobra-specific APIs → See `samber/cc-skills-golang@golang-spf13-cobra` skill; for viper configuration layering → See `samber/cc-skills-golang@golang-spf13-viper` skill.
Golang code style conventions — line length and breaking, variable declarations, control flow clarity, when comments help vs hurt. Use when writing or reviewing Go code, asking about style or clarity, or establishing project coding standards. Not for naming conventions (→ See `samber/cc-skills-golang@golang-naming` skill), linter configuration (→ See `samber/cc-skills-golang@golang-lint` skill), or doc comments (→ See `samber/cc-skills-golang@golang-documentation` skill).
Golang concurrency patterns. Use when writing or reviewing concurrent Go code involving goroutines, channels, select, locks, sync primitives, errgroup, singleflight, worker pools, or fan-out/fan-in pipelines. Also triggers when you detect goroutine leaks, race conditions, channel ownership issues, or need to choose between channels and mutexes.
Idiomatic context.Context usage in Golang — propagation through API boundaries, cancellation, timeouts and deadlines, request-scoped values, context.WithoutCancel for background work outliving requests. Apply when designing context propagation across layers, debugging leaked or unexpired contexts, choosing between context.Background/TODO/WithoutCancel, or storing values in context. Not for code that merely accepts ctx as first parameter.
CI/CD pipeline configuration using GitHub Actions for Golang projects — testing, linting, SAST, security scanning, code coverage, Dependabot, Renovate, GoReleaser, code review automation, and release pipelines. Use when setting up or improving Go project CI, configuring GitHub Actions workflows, adding linters or security scanners, automating dependency updates, or adding quality gates.
Golang data structures — slices (internals, capacity growth, preallocation, slices package), maps (internals, hash buckets, maps package), arrays, container/list/heap/ring, strings.Builder vs bytes.Buffer, generic collections, pointers (unsafe.Pointer, weak.Pointer), and copy semantics. Use when choosing or optimizing Go data structures, implementing generic containers, using container/ packages, unsafe or weak pointers, or questioning slice/map internals.
Comprehensive guide for Go database access — parameterized queries, struct scanning, NULLable columns, transactions, isolation levels, SELECT FOR UPDATE, connection pool, batch processing, context propagation, and migration tooling. Use when writing, reviewing, or debugging Golang code that interacts with PostgreSQL, MariaDB, MySQL, or SQLite; for database testing; or for questions about database/sql, sqlx, or pgx. Does NOT generate database schemas or migration SQL.