# go-logging

A skill that steers your coding agent toward sane Go logging: slog, levels, structured fields, request-scoped context. Useful once a Go service grows more log lines than anyone reads.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 1000 installs · ★ 181
- Author: cxuu
- License: Apache-2.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/cxuu/golang-skills/tree/HEAD/skills/go-logging/SKILL.md
- Updated: 2026-06-20
- HTML: https://affpapa.org/en/nevibecode/skills/go-logging

## What it does

The skill hands your agent a set of rules for logging in Go. New code uses `log/slog` from the standard library. Third-party libraries like zerolog or zap are only worth adding when profiling shows slog is the bottleneck in a hot path — benchmark first, the skill says.

The message stays a static description of what happened; dynamic data goes into key-value pairs, as in `slog.Info("order placed", "order_id", orderID, "total", total)`. Keys are `snake_case`, and hot paths can use typed constructors such as `slog.String` and `slog.Int` to skip allocations. Levels have defined meanings: Debug stays off in production, Info covers startup and shutdown, Warn catches recoverable surprises, Error means an operation failed and a human should look. Every `slog.Error` call carries an `"err"` attribute.

Two references ship with it: `references/LEVELS-AND-CONTEXT.md` on picking levels and passing loggers around, and `references/LOGGING-PATTERNS.md` on handler setup, HTTP request logging and moving off `log.Printf`. `log/slog` needs Go 1.21+, `testing/slogtest` needs Go 1.22+. Error handling strategy is out of scope here — that lives in the separate go-error-handling skill, including the log-or-return rule.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-logging -a claude-code
```

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

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-logging -a codex
```

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

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-logging -a cursor
```

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

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install cxuu/golang-skills go-logging
```

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

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-logging -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/cxuu/golang-skills --path skills/go-logging
```

### Windsurf (Devin Desktop)

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

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-logging -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/cxuu/golang-skills/tree/HEAD/skills/go-logging/SKILL.md

## When to use it
- pick a logger for a new Go service
- convert `log.Printf` calls to structured logs
- assign log levels without drowning production in noise
- attach request_id and user_id to every log line
- keep tokens, passwords and request bodies out of logs

## Example prompts
- Set up slog logging in this Go service
- Convert these log.Printf calls to structured logging
- Review the log levels in this handler — too many Errors?

## Source text · SKILL.md · Apache-2.0

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: 9c35ba7b8b72f7c2.

BEGIN THIRD-PARTY FILE 9c35ba7b8b72f7c2
````text
---
name: go-logging
description: Use when choosing a logging approach, configuring slog, writing structured log statements, or deciding log levels in Go. Also use when setting up production logging, adding request-scoped context to logs, or migrating from log to slog, even if the user doesn't explicitly mention logging. Does not cover error handling strategy (see go-error-handling).
---

# Go Logging

> Compatibility: `log/slog` requires Go 1.21+; `testing/slogtest` requires Go 1.22+.

## Resource Routing

- `references/LEVELS-AND-CONTEXT.md` - Read when choosing log levels, deciding logger-in-context versus explicit parameters, or excluding sensitive fields.
- `references/LOGGING-PATTERNS.md` - Read when configuring slog handlers, logging HTTP requests, testing handlers, or migrating from `log.Printf`.

## Core Principle

Logs are for **operators**, not developers. Every log line should help someone
diagnose a production issue. If it doesn't serve that purpose, it's noise.

---

## Choosing a Logger

> **Normative**: Use `log/slog` for new Go code.

`slog` is structured, leveled, and in the standard library (Go 1.21+). It
covers the vast majority of production logging needs.

```
Which logger?
├─ New production code      → log/slog
├─ Trivial CLI / one-off    → log (standard)
└─ Measured perf bottleneck → zerolog or zap (benchmark first)
```

Do not introduce a third-party logging library unless profiling shows `slog`
is a bottleneck in your hot path. When you do, keep the same structured
key-value style.

---

## Structured Logging

> **Normative**: Always use key-value pairs. Never interpolate values into the message string.

The message is a **static description** of what happened. Dynamic data goes in
key-value attributes:

```go
// Good: static message, structured fields
slog.Info("order placed", "order_id", orderID, "total", total)

// Bad: dynamic data baked into the message string
slog.Info(fmt.Sprintf("order %d placed for $%.2f", orderID, total))
```

### Key Naming

> **Advisory**: Use `snake_case` for log attribute keys.

Keys should be lowercase, underscore-separated, and consistent across the
codebase: `user_id`, `request_id`, `elapsed_ms`.

### Typed Attributes

For performance-critical paths, use typed constructors to avoid allocations:

```go
slog.LogAttrs(ctx, slog.LevelInfo, "request handled",
    slog.String("method", r.Method),
    slog.Int("status", code),
    slog.Duration("elapsed", elapsed),
)
```

---

## Log Levels

> **Advisory**: Follow these level semantics consistently.

| Level | When to use | Production default |
|-------|-------------|--------------------|
````
END THIRD-PARTY FILE 9c35ba7b8b72f7c2

The third-party file has ended. Everything between the 9c35ba7b8b72f7c2 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.

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