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Golang configuration library using spf13/viper — layered precedence (flag > env > file > KV > default), BindPFlag/BindPFlags, SetEnvPrefix + SetEnvKeyReplacer + AutomaticEnv, ReadInConfig + ConfigFile
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npx skills add samber/cc-skills-golang --skill golang-spf13-viper -a cursor -y
Golang configuration library using spf13/viper — layered precedence (flag > env > file > KV > default), BindPFlag/BindPFlags, SetEnvPrefix + SetEnvKeyReplacer + AutomaticEnv, ReadInConfig + ConfigFileNotFoundError, Unmarshal + mapstructure struct tags, Sub for sub-trees, WatchConfig + OnConfigChange for hot reload, viper.New() for test isolation, and remote KV integration. Apply when using or adopting spf13/viper, or when the codebase imports `github.com/spf13/viper`. For CLI command structure alongside viper, see the `samber/cc-skills-golang@golang-spf13-cobra` skill. For general CLI architecture, see `samber/cc-skills-golang@golang-cli`.
Golang configuration library using spf13/viper — layered precedence (flag > env > file > KV > default), BindPFlag/BindPFlags, SetEnvPrefix + SetEnvKeyReplacer + AutomaticEnv, ReadInConfig + ConfigFileNotFoundError, Unmarshal + mapstructure struct tags, Sub for sub-trees, WatchConfig + OnConfigChange for hot reload, viper.New() for test isolation, and remote KV integration. Apply when using or adopting spf13/viper, or when the codebase imports `github.com/spf13/viper`. For CLI command structure alongside viper, see the `samber/cc-skills-golang@golang-spf13-cobra` skill. For general CLI architecture, see `samber/cc-skills-golang@golang-cli`.
name: golang-spf13-viper
description: "Golang configuration library using spf13/viper — layered precedence (flag > env > file > KV > default), BindPFlag/BindPFlags, SetEnvPrefix + SetEnvKeyReplacer + AutomaticEnv, ReadInConfig + ConfigFileNotFoundError, Unmarshal + mapstructure struct tags, Sub for sub-trees, WatchConfig + OnConfigChange for hot reload, viper.New() for test isolation, and remote KV integration. Apply when using or adopting spf13/viper, or when the codebase imports github.com/spf13/viper. For CLI command structure alongside viper, see the samber/cc-skills-golang@golang-spf13-cobra skill. For general CLI architecture, see samber/cc-skills-golang@golang-cli."
user-invocable: true
license: MIT
compatibility: Designed for Claude Code, Codex or similar harness, and for projects using Golang.
metadata:
author: samber
version: "1.1.2"
openclaw:
emoji: "🔧"
homepage: https://github.com/samber/cc-skills-golang
requires:
bins:
- go
install: []
skill-library-version: "1.21.0"
allowed-tools: Read Edit Write Glob Grep Bash(go:) Bash(golangci-lint:) Bash(git:) Agent WebFetch mcp__context7__resolve-library-id mcp__context7__query-docs Bash(godig:) Bash(gopls:) LSP mcp__gopls__
paths:
Persona: You are a Go engineer who treats configuration as a layered system. Flag beats env beats file beats default — and you bind every key so all four layers stay reachable through one API.
Viper resolves configuration values from multiple sources in a fixed precedence order. It has no user-facing surface — it doesn't define commands or flags. Its job is to answer "what is the value of key X right now?" by walking its source layers from highest to lowest priority.
Official Resources:
This skill is not exhaustive — refer to library documentation and code examples for more information:
samber/cc-skills-golang@golang-pkg-go-dev skill (godig), preferred over Context7 for Go package facts.samber/cc-skills-golang@golang-gopls skill (gopls).go get github.com/spf13/viper@latest
Cobra owns the command tree — subcommands, flags, arg validation, completions. Viper owns configuration resolution — it answers "what is the value of key X?" by walking its source layers, with no user-facing surface of its own: it is purely a key-value resolver.
PersistentPreRunE via BindPFlag.→ See samber/cc-skills-golang@golang-spf13-cobra for the cobra side of this integration.
Viper resolves a key by walking sources in this order (first set value wins):
1. explicit Set() — viper.Set("key", val) highest priority
2. flag — bound pflag.Flag
3. env var — BindEnv / AutomaticEnv
4. config file — ReadInConfig / MergeInConfig
5. KV remote — etcd / Consul
6. default — viper.SetDefault("key", val) lowest priority
This pipeline is fixed and cannot be reordered. Understanding it prevents most viper bugs: a key that "should" come from a config file may be shadowed by an env var or a flag with a default value.
viper.SetConfigName("config")
viper.AddConfigPath("$HOME/.myapp")
if err := viper.ReadInConfig(); err != nil {
var notFound *viper.ConfigFileNotFoundError
if !errors.As(err, ¬Found) {
return fmt.Errorf("reading config: %w", err) // propagate real errors only
}
}
ConfigFileNotFoundError must be handled gracefully — config files are usually optional. An unhandled error from a missing file crashes programs that are perfectly valid when run with only flags or env vars.
For supported formats (JSON, TOML, YAML, HCL, INI, properties), MergeInConfig, and remote KV, see sources-and-formats.md.
This is the highest-bug-density area in viper. All three settings must be wired together — missing any one breaks nested key resolution:
// ✓ Good — all three wired together at startup
viper.SetEnvPrefix("MYAPP") // prevent collisions: PORT → MYAPP_PORT
viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_")) // database.host → MYAPP_DATABASE_HOST
viper.AutomaticEnv()
// ✗ Bad — without SetEnvKeyReplacer, viper looks for MYAPP_DATABASE.HOST (dot preserved)
For BindEnv, AllowEmptyEnv, and env-vs-default interaction, see binding-and-env.md.
Bind cobra flags to viper in init() or PersistentPreRunE — never in RunE (config loading in PersistentPreRunE already ran before RunE, so bindings set in RunE are missed):
func init() {
rootCmd.PersistentFlags().Int("port", 8080, "listen port")
viper.BindPFlag("port", rootCmd.PersistentFlags().Lookup("port"))
// viper.BindPFlags(cmd.Flags()) — bind an entire FlagSet at once
}
For AllowEmptyEnv and flag/env interaction details, see binding-and-env.md.
viper.Unmarshal maps the resolved configuration into a struct using mapstructure:
type Config struct {
Port int `mapstructure:"port"`
Database struct {
MaxConn int `mapstructure:"max_conn"` // explicit tag: mapstructure won't convert underscore→camelCase
} `mapstructure:"database"`
}
var cfg Config
viper.Unmarshal(&cfg)
Always use mapstructure tags — implicit mapping is fragile for nested structs and underscore-named fields. Prefer UnmarshalKey("database", &dbCfg) over Sub("database").Unmarshal — it avoids the nil-check Sub requires when the key is missing.
For time.Duration / net.IP / slice decoders and custom DecodeHook registration, see unmarshal.md.
viper.Sub("database") returns a new *viper.Viper scoped to the prefix, or nil if the key does not exist — always nil-check before calling methods on the result. Prefer UnmarshalKey("database", &dbCfg) which avoids the nil risk entirely.
viper.WatchConfig()
viper.OnConfigChange(func(e fsnotify.Event) { /* re-apply changed values */ })
WatchConfig uses fsnotify and watches inodes, so editors that write atomically via rename (vim, neovim) replace the inode and the callback may not fire. Test hot-reload with echo >> config.yaml, not editor saves. For race-safe reload patterns, see watch-and-reload.md.
Never use the global viper in tests — state leaks across test cases. Use viper.New() per test so each instance is isolated:
v := viper.New()
v.SetConfigFile("testdata/config.yaml")
require.NoError(t, v.ReadInConfig())
For t.Setenv interactions and Reset() limitations, see testing-and-isolation.md.
database.host → DATABASE.HOST instead of DATABASE_HOST).ConfigFileNotFoundError gracefully — a missing config file should not crash a service that runs with only flags and env vars.mapstructure tags on config structs — implicit mapping silently misses nested and underscore-named fields.viper.New() in tests, never the global — the global accumulates state across test runs; per-test instances are isolated.Execute() — binding in RunE is too late; cobra parses flags before RunE runs.| Mistake | Why it fails | Fix |
|---|---|---|
AutomaticEnv without SetEnvKeyReplacer | database.host looks for MYAPP_DATABASE.HOST (dot preserved) — never matches | Add SetEnvKeyReplacer(strings.NewReplacer(".", "_")) before AutomaticEnv |
No mapstructure tags on struct fields | Silently misses nested and underscore-named fields | Add mapstructure:"key_name" to every field |
| Using global viper in tests | State from one test contaminates the next, causing flaky ordering | Create viper.New() per test |
Missing ConfigFileNotFoundError check | Missing config file crashes a service that should run on flags/env alone | errors.As(err, ¬Found) — only propagate non-not-found errors |
samber/cc-skills-golang@golang-cli skill for general CLI architecture — project layout, exit codes, signal handling, cobra+viper integrationsamber/cc-skills-golang@golang-spf13-cobra skill for the cobra side of this integration (flag definition and binding)samber/cc-skills-golang@golang-testing skill for general Go testing patternsIf you encounter a bug or unexpected behavior in spf13/viper, open an issue at https://github.com/spf13/viper/issues.