# modern-go

Rewrites Go sources to idioms and APIs that match the version declared in go.mod. Handy after a toolchain bump when the code still uses old patterns.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 520 installs · ★ 294
- Author: smallnest
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/smallnest/goal-workflow/tree/HEAD/skills/modern-go/SKILL.md
- Updated: 2026-09-13
- HTML: https://affpapa.org/en/nevibecode/skills/modern-go

## What it does

The skill reads the `go` directive in `go.mod`, then edits `.go` files, applying only transformations whose version is at or below the declared one. The catalog runs from Go 1.0 up to 1.26+: `time.Now().Sub(start)` turns into `time.Since(start)`, `interface{}` into `any`, `err == io.EOF` into `errors.Is(err, io.EOF)`, and loop concatenation into `strings.Builder`. Newer passes cover `strings.Cut`, `bytes.Clone`, `fmt.Appendf`, `errors.Join`, typed atomics in place of `atomic.LoadInt32`, plus `unsafe.Add` and `unsafe.Slice` instead of `uintptr` arithmetic.

It behaves like `go fix` with extra passes taken from the Go team's analyzers and community practice. The whole project is the default scope; name a file or directory and the edits stay inside that path. Directories `vendor/`, `.git/` and `testdata/` are skipped. When it finishes, the skill prints what changed and what it left alone.

The version gate is the catch. A Go 1.20 rewrite will not fire in a project whose `go.mod` says 1.18, even if the code has been asking for it. If the version cannot be raised, part of the catalog stays out of reach.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add smallnest/goal-workflow --skill modern-go -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/modern-go/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/modern-go/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add smallnest/goal-workflow --skill modern-go -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/modern-go/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add smallnest/goal-workflow --skill modern-go -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/modern-go/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install smallnest/goal-workflow modern-go
```

2. **Manually: put the files in place**
- File: `.github/skills/modern-go/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add smallnest/goal-workflow --skill modern-go -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/smallnest/goal-workflow --path skills/modern-go
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/modern-go/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add smallnest/goal-workflow --skill modern-go -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/smallnest/goal-workflow/tree/HEAD/skills/modern-go/SKILL.md

## When to use it
- bring legacy Go code up to current idioms
- swap `interface{}` for `any` across a codebase
- replace `uintptr` pointer math with `unsafe.Add`
- rewrite string splitting around `strings.Cut`
- drop stale `// +build` lines next to `//go:build`

## Example prompts
- Modernize the whole project using the version in `go.mod`
- Bring `internal/cache/cache.go` up to modern Go idioms
- Run modern-go over the `pkg/` directory and list what changed

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 91980238573d2498.

BEGIN THIRD-PARTY FILE 91980238573d2498
````text
---
name: modern-go
description: Modernize Go code by applying version-appropriate idioms and APIs (gofix-style transformations). Scans go.mod for the Go version, then transforms Go source files to use modern patterns—from Go 1.0 through 1.26+. Use when the user says "现代化","现代Go语言", "地道的", "idiomatic",  "modernize", "modern-go", "update Go code", "gofix", or wants to upgrade Go idioms.
---

# modern-go

Modernize Go source code by applying version-appropriate idioms, APIs, and language features. Works like `go fix` plus additional transformations curated from the Go team's modernize analysis passes and community best practices.

## Usage

Invoke this skill when the user asks to modernize Go code. By default, modernize the entire project; the user may specify a file or directory instead.

When invoked:
1. Detect the project's Go version from `go.mod` (the `go` directive).
2. Find all `.go` files in the target scope (excluding `vendor/`, `.git/`, `testdata/`).
3. For each file, apply **all transformations for versions ≤ the project's Go version**, starting from the oldest to the newest.
4. After all transformations, print a summary of what was changed and what was skipped.

If the user specifies a file or directory, limit the scope to that path.

## Transformation Catalog

Each transformation includes a **Go version** gate—only apply when the project's `go.mod` version ≥ that version. Never apply a transformation that requires a version higher than the project declares.

### Go 1.0+ — `time.Since`

| Before | After |
|---|---|
| `time.Now().Sub(start)` | `time.Since(start)` |

```go
// before
elapsed := time.Now().Sub(start)
// after
elapsed := time.Since(start)
```

### Go 1.8+ — `time.Until`

| Before | After |
|---|---|
| `deadline.Sub(time.Now())` | `time.Until(deadline)` |

```go
// before
remaining := deadline.Sub(time.Now())
// after
remaining := time.Until(deadline)
```

### Go 1.10+ — `strings.Builder` (loop concatenation)

| Before | After |
|---|---|
| `s += item` in a loop | `var b strings.Builder; b.WriteString(item)` |

```go
// before
s := ""
for _, item := range items {
    s += item
}
// after
var b strings.Builder
for _, item := range items {
    b.WriteString(item)
}
s := b.String()
```

Only when `+=` concatenation happens inside a loop.

### Go 1.13+ — `errors.Is`

| Before | After |
|---|---|
| `err == io.EOF` | `errors.Is(err, io.EOF)` |

```go
// before
if err == io.EOF {
    return
}
// after
if errors.Is(err, io.EOF) {
    return
}
```

### Go 1.17+ — `//go:build` constraints (plusbuild)

| Before | After |
|---|---|
| `// +build linux` + `//go:build linux` (both present) | keep only `//go:build linux` |

```go
// before
````
END THIRD-PARTY FILE 91980238573d2498

The third-party file has ended. Everything between the 91980238573d2498 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

First 2730 of 29127 bytes. © smallnest · MIT · https://github.com/smallnest/goal-workflow/tree/HEAD/skills/modern-go/SKILL.md

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