Golang Spf13 Viper Agent Skill

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.

Using spf13/viper for layered configuration in Go

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:

• pkg.go.dev/github.com/spf13/viper
• github.com/spf13/viper

This skill is not exhaustive. Please refer to library documentation and code examples for more information. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill (godig) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See samber/cc-skills-golang@golang-gopls skill (gopls). Context7 remains a fallback for docs not indexed on pkg.go.dev.

go get github.com/spf13/viper@latest

Viper vs. cobra

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. Viper has no user-facing surface; it is purely a key-value resolver. Use cobra alone for flag-only CLIs; viper alone for config-file daemons; both when you need both, binding flags at PersistentPreRunE via BindPFlag.

→ See samber/cc-skills-golang@golang-spf13-cobra for the cobra side of this integration.

The precedence pipeline

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.

Sources and config files

viper.SetConfigName("config")
viper.AddConfigPath("$HOME/.myapp")
if err := viper.ReadInConfig(); err != nil {
    var notFound *viper.ConfigFileNotFoundError
    if !errors.As(err, &notFound) {
        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.

Env binding and key replacers

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.

Flag binding (the cobra seam)

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.

Unmarshaling into structs

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.

Sub-trees

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.

Hot reload

viper.WatchConfig()
viper.OnConfigChange(func(e fsnotify.Event) { /* re-apply changed values */ })

WatchConfig uses fsnotify and watches inodes. Editors that write atomically via rename (vim, neovim) replace the inode — 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.

Test isolation

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.

Best Practices

• Set prefix + key replacer + AutomaticEnv together — missing any one causes nested env keys to silently not resolve (database.host → DATABASE.HOST instead of DATABASE_HOST).
• Handle ConfigFileNotFoundError gracefully — a missing config file should not crash a service that runs with only flags and env vars.
• Always use mapstructure tags on config structs — implicit mapping silently misses nested and underscore-named fields.
• Use viper.New() in tests, never the global — the global accumulates state across test runs; per-test instances are isolated.
• Bind flags before Execute() — binding in RunE is too late; cobra parses flags before RunE runs.

Common Mistakes

| 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, &notFound) — only propagate non-not-found errors |

Further Reading

• sources-and-formats.md — supported file formats, multi-path search, MergeInConfig, remote KV (etcd/Consul)
• binding-and-env.md — BindEnv, AutomaticEnv, SetEnvPrefix, SetEnvKeyReplacer, AllowEmptyEnv, timing rules
• unmarshal.md — Unmarshal, UnmarshalKey, mapstructure tags, custom DecodeHooks (Duration, IP, slice)
• watch-and-reload.md — WatchConfig, OnConfigChange, fsnotify caveats, atomic-rename trap, race-safe patterns
• testing-and-isolation.md — viper.New() per test, t.Setenv interactions, Reset() limitations, snapshot/restore

Cross-References

• → See samber/cc-skills-golang@golang-cli skill for general CLI architecture — project layout, exit codes, signal handling, cobra+viper integration
• → See samber/cc-skills-golang@golang-spf13-cobra skill for the cobra side of this integration (flag definition and binding)
• → See samber/cc-skills-golang@golang-testing skill for general Go testing patterns

If you encounter a bug or unexpected behavior in spf13/viper, open an issue at <https://github.com/spf13/viper/issues>.