Golang Spf13 Cobra Agent Skill

Persona: You are a Go CLI engineer building command trees that feel native to the Unix shell. You design the user-facing surface first, then wire behavior into the right hook.

Modes:

• Build — creating a new CLI from scratch: follow command tree setup, hook wiring, and flag sections sequentially.
• Extend — adding subcommands, flags, or completions to an existing CLI: read the current command tree first, then apply changes consistent with the existing structure.
• Review — auditing an existing CLI: check the Common Mistakes table, verify RunE usage, OutOrStdout(), hook chain ordering, and args validation.

Using spf13/cobra for CLI command trees in Go

Cobra is the de facto standard for Go CLI applications. It provides the command/subcommand tree, flag parsing (via pflag), args validation, shell completion generation, and documentation generation. It does not handle configuration layering — that's viper's job.

Official Resources:

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

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/cobra@latest

Cobra vs. viper

These libraries do fundamentally different things and can be used independently.

| Concern | cobra | viper |

| --- | --- | --- |

| Owns | Command tree, flags, arg validation, completions | Configuration value resolution |

| User-facing? | Yes — subcommands, flags, help text | No — purely a key-value resolver |

| Without the other? | Yes — a CLI with flags only needs cobra | Yes — a daemon reading YAML + env needs only viper |

| Integration seam | Hands pflag.Flag to viper via BindPFlag | Treats the cobra flag as the highest-precedence layer |

Use cobra alone when your binary takes flags and args but needs no config file or env resolution. Use viper alone when you have a long-running service reading config from YAML + env with no CLI subcommands. Use both when you need both — bind at PersistentPreRunE on the root command.

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

Command tree

Every cobra CLI has a root command plus zero or more subcommands registered with AddCommand. The root command name is the binary name.

var rootCmd = &cobra.Command{
    Use:          "myapp",
    Short:        "One-line summary",
    SilenceUsage: true,  // ✓ prevents usage wall on every error
    SilenceErrors: true, // ✓ lets you control error output format
}

Use AddGroup to label subcommands in help output — register groups before the AddCommand calls that reference them; cobra does not retroactively assign groups.

The Run\* family

Cobra commands have five run hooks executed in order:

PersistentPreRunE → PreRunE → RunE → PostRunE → PersistentPostRunE

Always use *E variants — the non-E forms cannot return errors. Key rules:

• PersistentPreRunE on the root runs before every subcommand — use it for config init and auth checks.
• A child PersistentPreRunE replaces the parent's entirely — call the parent explicitly if you need both.
• PostRunE runs only if RunE succeeded.

For the full lifecycle and inheritance rules, see commands-and-args.md.

Args validators

Cobra validates positional arguments before RunE runs. Never write len(args) checks inside RunE — that bypasses cobra's standard error messages and arg count tracking.

Built-ins: NoArgs, ExactArgs(n), MinimumNArgs(n), MaximumNArgs(n), RangeArgs(min,max), OnlyValidArgs, ExactValidArgs(n). Compose with MatchAll(v1, v2). Custom validator: func(cmd *cobra.Command, args []string) error.

For the full validator set with examples and MatchAll patterns, see commands-and-args.md.

Flags primer

Cobra delegates flag parsing to pflag. Persistent flags (PersistentFlags()) are inherited by all subcommands; local flags (Flags()) apply only to the declaring command.

rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file path") // inherited by all subcommands
serveCmd.Flags().IntVar(&port, "port", 8080, "listen port")                     // local to serveCmd only
serveCmd.MarkFlagRequired("port")
serveCmd.MarkFlagsMutuallyExclusive("json", "yaml")

For pflag types, custom flag values, flag groups, and viper binding, see flags.md.

Completions primer

Cobra generates shell completions automatically. Extend them with:

• ValidArgs []string — static positional arg completion.
• ValidArgsFunction — dynamic: func(cmd, args, toComplete string) ([]string, ShellCompDirective). Return ShellCompDirectiveNoFileComp to suppress file fallback.
• RegisterFlagCompletionFunc(name, fn) — flag value completion.

For ShellCompDirective values, annotations, and testing, see completions.md.

Testing commands

Test commands by executing them programmatically. Never use os.Stdout / os.Stderr directly in command handlers — use cmd.OutOrStdout() / cmd.ErrOrStderr() so tests can redirect output.

func TestServeCmd(t *testing.T) {
    buf := new(bytes.Buffer)
    rootCmd.SetOut(buf)
    rootCmd.SetArgs([]string{"serve", "--port", "9090"})
    require.NoError(t, rootCmd.Execute())
    assert.Contains(t, buf.String(), "listening on :9090")
}

Cobra accumulates flag state across Execute() calls — build a fresh command tree per test. For isolation patterns, golden files, and testing completions, see testing.md.

Best Practices

• Always use RunE, never Run — Run cannot return an error; the only escape is os.Exit or panic, bypassing defers.
• Put config initialization in PersistentPreRunE — it runs before every subcommand; the right place for viper binding and auth checks.
• Validate positional args with Args, not inside RunE — Args gives cobra's standard error messages; MatchAll composes validators.
• Use cmd.OutOrStdout() / cmd.ErrOrStderr() for all output — direct os.Stdout writes cannot be captured by tests.
• Re-create the command tree per test — cobra accumulates flag state across Execute() calls on the same instance.

Common Mistakes

| Mistake | Why it fails | Fix |

| --- | --- | --- |

| Using Run instead of RunE | Cannot return an error — only escape is os.Exit or panic, bypassing defers | Use RunE — return the error, let cobra handle the exit |

| Writing len(args) checks in RunE | Bypasses cobra's standard error messages ("accepts 1 arg, received 2") | Declare Args: cobra.ExactArgs(1) on the command |

| Writing to os.Stdout directly | Tests cannot capture output — os-level file handles can't be redirected | Use cmd.OutOrStdout() / cmd.ErrOrStderr() |

| Child PersistentPreRunE silently drops parent's | Cobra does not chain — the child replaces the parent's hook entirely | Call parent.PersistentPreRunE(cmd, args) from the child's hook |

| Reusing a root command across tests | Cobra accumulates flag state; second Execute() sees flags from the first | Build a fresh command tree per test |

Further Reading

• commands-and-args.md — full PreRun\*/PostRun\* chain, every Args validator, PersistentPreRunE inheritance rules
• flags.md — pflag types, required/exclusive/oneRequired groups, custom value types, viper binding
• completions.md — ShellCompDirective set, annotation-based completions, testing completions
• generators.md — man page, markdown, YAML, RST doc generation; cobra-cli scaffolder
• testing.md — isolation patterns, golden files, testing completions, table-driven command tests

Cross-References

• → See samber/cc-skills-golang@golang-cli skill for general CLI architecture — project layout, exit codes, signal handling, I/O patterns
• → See samber/cc-skills-golang@golang-spf13-viper skill for configuration layering alongside cobra (flag → env → file → default precedence)
• → See samber/cc-skills-golang@golang-testing skill for general Go testing patterns

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