structcli

Human-friendly, AI-native CLIs from Go structs

Declare your CLI contract once in Go structs. structcli turns it into flags, env vars, config-file loading, validation, organized help, and machine-readable contracts for agents.

• Less Cobra/Viper boilerplate
• Better CLIs for humans
• Better contracts for automation and LLMs

Stop writing plumbing. Start shipping commands.

⚡ Quick Start

Start with a plain Go struct:

package main

import (
	"fmt"
	"log"

	"github.com/leodido/structcli"
	"github.com/spf13/cobra"
	"go.uber.org/zap/zapcore"
)

type Options struct {
	LogLevel zapcore.Level
	Port     int
}

func main() {
	opts := &Options{}
	cli := &cobra.Command{Use: "myapp"}

	if err := structcli.Define(cli, opts); err != nil {
		log.Fatalln(err)
	}

	cli.PreRunE = func(c *cobra.Command, args []string) error {
		return structcli.Unmarshal(c, opts)
	}

	cli.RunE = func(c *cobra.Command, args []string) error {
		fmt.Println(opts)

		return nil
	}

	if err := cli.Execute(); err != nil {
		log.Fatalln(err)
	}
}

That single Define call creates the CLI surface from your struct, and Unmarshal hydrates it back from flags, env vars, config, and defaults.

❯ go run examples/minimal/main.go --help
# Usage:
#   myapp [flags]
#
# Flags:
#       --loglevel zapcore.Level    {debug,info,warn,error,dpanic,panic,fatal} (default info)
#       --port int

Add tags when you want aliases, env vars, shorthand, defaults, and descriptions:

type Options struct {
	LogLevel zapcore.Level `flag:"level" flagdescr:"Set logging level" flagenv:"true"`
	Port     int           `flagshort:"p" flagdescr:"Server port" flagenv:"true" default:"3000"`
}

❯ go run examples/simple/main.go -h
# Usage:
#   myapp [flags]
#
# Flags:
#       --level zapcore.Level   Set logging level {debug,info,warn,error,dpanic,panic,fatal} (default info)
#   -p, --port int              Server port (default 3000)

❯ MYAPP_LOGLEVEL=debug go run examples/simple/main.go
# &{debug 3000}

❯ MYAPP_LOGLEVEL=error MYAPP_PORT=9000 go run examples/simple/main.go --level dpanic
# &{dpanic 9000}

Built-in types like zapcore.Level are validated automatically too.

Out of the box, your CLI supports:

• 📝 Command-line flags (--level info, -p 8080)
• 🌍 Environment variables (MYAPP_PORT=8080)
• 💦 Options precedence (flags > env vars > config file > defaults)
• ✅ Automatic validation and type conversion
• 📚 Beautiful help output with proper grouping

Add the AI-native wiring below and it also gains machine-readable JSON Schema, structured JSON errors, semantic exit codes, and optional MCP tool-server mode for agents.

Build AI-Native CLIs

structcli does not just generate flags for humans. It can make your CLI legible to agents too.

Instead of scraping --help and guessing, an agent can discover the contract, call the command correctly, and recover from structured failures.

structcli.SetupJSONSchema(rootCmd, jsonschema.Options{})
structcli.SetupFlagErrors(rootCmd) // Optional, but recommended for typed flag-parse errors
structcli.SetupMCP(rootCmd, mcp.Options{}) // Optional, exposes the CLI as an MCP server over stdio
structcli.ExecuteOrExit(rootCmd)

With that wiring:

• --jsonschema exposes flags, defaults, required inputs, enums, and env bindings across the command tree
• HandleError / ExecuteOrExit emit structured JSON errors instead of forcing callers to parse human-oriented output
• --mcp exposes the same command tree as MCP tools over stdio, with typed inputs and structured tool-call failures
• semantic exit codes tell the caller whether it should fix input, fix config, retry, or escalate to a human

The same contract spans flags, env vars, config, validation, and enum constraints.

$ mycli srv --jsonschema
{
  "properties": {
    "port": {
      "type": "integer",
      "default": 3000,
      "x-structcli-env-vars": ["MYCLI_SRV_PORT"]
    }
  }
}

No --help parsing. No guessing what failed. Just a CLI that can explain itself and fail in machine-actionable ways.

Use exitcode.Category(code) and exitcode.IsRetryable(code) to decide what to do next. See jsonschema.WithFullTree() and jsonschema.WithEnumInDescription() for schema customization, and pass the same schema options through SetupJSONSchema with jsonschema.Options{SchemaOpts: ...}.

For build-time discovery, generate.WriteAll produces SKILL.md, llms.txt, and AGENTS.md from the same struct definitions — wire it into //go:generate and the files stay in sync automatically.

Read the full AI-native guide or walk through the runnable structured error example.

⬇️ Install

go get github.com/leodido/structcli

📦 Key Features

🧩 Declarative Flags Definition

Define flags once using Go struct tags.

No more boilerplate for Flags().StringVarP, Flags().IntVar, viper.BindPFlag, etc.

Yes, you can nest structs too.

type ServerOptions struct {
	// Basic flags
	Host string `flag:"host" flagdescr:"Server host" default:"localhost"`
	Port int    `flagshort:"p" flagdescr:"Server port" flagrequired:"true" flagenv:"true"`

	// Environment variable binding
	APIKey string `flagenv:"true" flagdescr:"API authentication key"`

	// Network contracts using net families
	BindIP        net.IP     `flag:"bind-ip" flaggroup:"Network" flagdescr:"Bind interface IP" flagenv:"true"`
	BindMask      net.IPMask `flag:"bind-mask" flaggroup:"Network" flagdescr:"Bind interface mask" flagenv:"true"`
	AdvertiseCIDR net.IPNet  `flag:"advertise-cidr" flaggroup:"Network" flagdescr:"Advertised service subnet (CIDR)" flagenv:"true"`
	TrustedPeers  []net.IP   `flag:"trusted-peers" flaggroup:"Network" flagdescr:"Trusted peer IPs (comma separated)" flagenv:"true"`

	// Flag grouping for organized help
	LogLevel zapcore.Level `flag:"log-level" flaggroup:"Logging" flagdescr:"Set log level"`
	LogFile  string        `flag:"log-file" flaggroup:"Logging" flagdescr:"Log file path" flagenv:"true"`

	// Nested structs for organization
	Database DatabaseConfig `flaggroup:"Database"`

	// Custom type
	TargetEnv Environment `flagcustom:"true" flag:"target-env" flagdescr:"Set the target environment"`
}

type DatabaseConfig struct {
	URL      string `flag:"db-url" flagdescr:"Database connection URL"`
	MaxConns int    `flagdescr:"Max database connections" default:"10" flagenv:"true"`
}

See full example for more details.

🛠️ Automatic Environment Variable Binding

Automatically generate environment variables binding them to configuration files (YAML, JSON, TOML, etc.) and flags.

From the previous options struct, you get the following env vars automatically:

• FULL_SRV_PORT
• FULL_SRV_APIKEY
• FULL_SRV_BIND_IP
• FULL_SRV_BIND_MASK
• FULL_SRV_ADVERTISE_CIDR
• FULL_SRV_TRUSTED_PEERS
• FULL_SRV_DATABASE_MAXCONNS
• FULL_SRV_LOGFILE, FULL_SRV_LOG_FILE

Every struct field with the flagenv:"true" tag gets an environment variable (two if the struct field also has the flag:"..." tag, see struct field LogFile).

The prefix of the environment variable name is the CLI name plus the command name to which those options are attached to.

Environment variables are command-scoped for command-local options. For example, if Port is attached to the srv command, FULL_SRV_PORT is used (not FULL_PORT).

⚙️ Configuration File Support

Easily set up configuration file discovery (flag, environment variable, and fallback paths) with a single line of code.

structcli.SetupConfig(rootCmd, config.Options{AppName: "full"})

Enable strict config-key validation with:

structcli.SetupConfig(rootCmd, config.Options{
  AppName:      "full",
  ValidateKeys: true, // opt-in
})

When enabled, Unmarshal fails if command-relevant config contains unknown keys.

Call SetupConfig before attaching/defining options when you rely on app-prefixed environment variables, so the env prefix is initialized before env annotations are generated.

The line above:

• creates --config global flag
• creates FULL_CONFIG env var
• sets /etc/full/, $HOME/.full/, $PWD/.full/ as fallback paths for config.yaml

Magic, isn't it?

What's left? Tell your CLI to load the configuration file (if any).

rootC.PersistentPreRunE = func(c *cobra.Command, args []string) error {
	_, configMessage, configErr := structcli.UseConfigSimple(c)
	if configErr != nil {
		return configErr
	}
	if configMessage != "" {
		c.Println(configMessage)
	}

	return nil
}

UseConfigSimple(c) loads config into the root config scope and merges only the relevant section into c's effective scope.

🧠 Viper Model Scopes

structcli uses two different viper scopes on purpose:

• structcli.GetConfigViper(rootOrLeafCmd) -> root-scoped config source (config file data tree)
• structcli.GetViper(cmd) -> command-scoped effective values (flags/env/defaults + command-relevant config)

This separation keeps config-file loading isolated from runtime command state.

If you need imperative values in tests or application code, write to the right scope:

// 1) Effective override for one command context
structcli.GetViper(cmd).Set("timeout", 60)

// 2) Config-tree style injection (top-level + command section)
structcli.GetConfigViper(rootCmd).Set("srv", map[string]any{
  "port": 8443,
})

Global viper.Set(...) is not used by structcli.Unmarshal(...) resolution. Use GetViper/GetConfigViper instead.

📜 Configuration Is First-Class Citizen

Configuration can mirror your command hierarchy.

Settings can be global (at the top level) or specific to a command or subcommand. The most specific section always takes precedence.

# Global settings apply to all commands unless overridden by a specific section.
# `dryrun` matches the `DryRun` struct field name.
dryrun: true
verbose: 1 # A default verbosity level for all commands.

# Config for the `srv` command (`full srv`)
srv:
  # `port` matches the `Port` field name.
  port: 8433
  # Network options
  bind-ip: "10.20.0.10"
  bind-mask: "ffffff00"
  advertise-cidr: "10.20.0.0/24"
  trusted-peers: "10.20.0.11,10.20.0.12"
  # `log-level` matches the `flag:"log-level"` tag.
  log-level: "warn"
  # `logfile` matches the `LogFile` field name.
  logfile: /var/log/mysrv.log

  # Flattened keys can set options in nested structs.
  # `db-url` (from `flag:"db-url"` tag) maps to ServerOptions.Database.URL.
  db-url: "postgres://user:pass@db/prod"

  # Nested keys are also supported.
  database:
    # Struct field key style
    url: "postgres://user:pass@db/prod"
    # Alias key style (from `flag:"db-url"`)
    db-url: "postgres://user:pass@db/prod"

# Config for the `usr` command group.
usr:
  # This nested section matches the `usr add` command (`full usr add`).
  # Its settings are ONLY applied to 'usr add'.
  add:
    name: "Config User"
    email: "config.user@example.com"
    age: 42
    # Command specific override
    dry: false
# NOTE: Per the library's design, there is no other fallback other than from the top-level.
# A command like 'usr delete' would ONLY use the global keys above (if those keys/flags are attached to it),
# as an exact 'usr.delete' section is not defined.

This configuration system supports:

• Hierarchical Structure: Nest keys to match your command path (e.g., usr: { add: { ... } }).
• Strict Precedence: Only settings from the global scope and the exact command path section are merged. There is no automatic fallback to parent command sections.
• Flexible Keys: You can use struct field names and aliases (flag:"...") in both flattened and nested forms.
• Supported Forms for Nested Fields: db-url, database.url, database: { url: ... }, and database: { db-url: ... }.

✅ Built-in Validation & Transformation

Supports validation, transformation, and custom flag type definitions through simple interfaces.

Your struct must implement Options (via Attach) and can optionally implement ValidatableOptions and TransformableOptions.

type UserConfig struct {
	Email string `flag:"email" flagdescr:"User email" validate:"email"`
	Age   int    `flag:"age" flagdescr:"User age" validate:"min=18,max=120"`
	Name  string `flag:"name" flagdescr:"User name" mod:"trim,title"`
}

func (o *ServerOptions) Validate(ctx context.Context) []error {
    // Automatic validation
}

func (o *ServerOptions) Transform(ctx context.Context) error {
    // Automatic transformation
}

See a full working example here.

🚧 Automatic Debugging Support

Create a --debug-options flag (plus a FULL_DEBUG_OPTIONS env var) for troubleshooting config/env/flags resolution.

structcli.SetupDebug(rootCmd, debug.Options{})

❯ go run examples/full/main.go srv --debug-options --config examples/full/config.yaml -p 3333
#
# Aliases:
# map[string]string{"database.url":"db-url", "logfile":"log-file", "loglevel":"log-level", "targetenv":"target-env"}
# Override:
# map[string]interface {}{}
# PFlags:
# map[string]viper.FlagValue{"apikey":viper.pflagValue{flag:(*pflag.Flag)(0x14000109ea0)}, "database.maxconns":viper.pflagValue{flag:(*pflag.Flag)(0x140002181e0)}, "db-url":viper.pflagValue{flag:(*pflag.Flag)(0x14000218140)}, "host":viper.pflagValue{flag:(*pflag.Flag)(0x14000109d60)}, "log-file":viper.pflagValue{flag:(*pflag.Flag)(0x140002180a0)}, "log-level":viper.pflagValue{flag:(*pflag.Flag)(0x14000218000)}, "port":viper.pflagValue{flag:(*pflag.Flag)(0x14000109e00)}, "target-env":viper.pflagValue{flag:(*pflag.Flag)(0x14000218320)}}
# Env:
# map[string][]string{"apikey":[]string{"SRV_APIKEY"}, "database.maxconns":[]string{"SRV_DATABASE_MAXCONNS"}, "log-file":[]string{"SRV_LOGFILE", "SRV_LOG_FILE"}}
# Key/Value Store:
# map[string]interface {}{}
# Config:
# map[string]interface {}{"apikey":"secret-api-key", "database":map[string]interface {}{"maxconns":3}, "db-url":"postgres://user:pass@localhost/mydb", "host":"production-server", "log-file":"/var/log/mysrv.log", "log-level":"debug", "port":8443}
# Defaults:
# map[string]interface {}{"database":map[string]interface {}{"maxconns":"10"}, "host":"localhost"}
# Values:
# map[string]interface {}{"apikey":"secret-api-key", "database":map[string]interface {}{"maxconns":3, "url":"postgres://user:pass@localhost/mydb"}, "db-url":"postgres://user:pass@localhost/mydb", "host":"production-server", "log-file":"/var/log/mysrv.log", "log-level":"debug", "logfile":"/var/log/mysrv.log", "loglevel":"debug", "port":3333, "target-env":"dev", "targetenv":"dev"}

↪️ Sharing Options Between Commands

In complex CLIs, multiple commands often need access to the same global configuration and shared resources (like a logger or a database connection). structcli provides a powerful pattern using the ContextOptions interface to achieve this without resorting to global variables, by propagating a single "source of truth" through the command context.

The pattern allows you to:

• Populate a shared options struct once from flags, environment variables, or a config file.
• Initialize "computed state" (like a logger) based on those options.
• Share this single, fully-prepared "source of truth" with any subcommand that needs it.

🍩 In a Nutshell

Create a shared struct that implements the ContextOptions interface. This struct will hold both the configuration flags and the computed state (e.g., the logger).

// This struct holds our shared state.
type CommonOptions struct {
    LogLevel zapcore.Level `flag:"loglevel" flagdescr:"Logging level" default:"info"`
    Logger   *zap.Logger   `flagignore:"true"` // This field is computed, not a flag.
}

// The Context/FromContext methods enable the propagation pattern.
func (o *CommonOptions) Context(ctx context.Context) context.Context { /* ... */ }
func (o *CommonOptions) FromContext(ctx context.Context) error { /* ... */ }

// Initialize is a custom method to create the computed state.
func (o *CommonOptions) Initialize() error { /* ... */ }

Initialize the state in the root command. Use a PersistentPreRunE hook on your root command to populate your struct and initialize any resources. Invoking structcli.Unmarshal will automatically inject the prepared object into the context for all subcommands to use.

rootC.PersistentPreRunE = func(c *cobra.Command, args []string) error {
	// Populate the master `commonOpts` from flags, env, and config file.
	if err := structcli.Unmarshal(c, commonOpts); err != nil {
		return err
	}
	// Use the populated values to initialize the computed state (the logger).
	if err := commonOpts.Initialize(); err != nil {
		return err
	}

	return nil
}

Finally, retrieve the state in subcommands. In your subcommand's RunE, simply call .FromContext() to retrieve the shared, initialized object.

func(c *cobra.Command, args []string) error {
    // Create a receiver and retrieve the master state from the context.
    config := &CommonOptions{}
    if err := config.FromContext(c.Context()); err != nil {
        return err
    }
    config.Logger.Info("Executing subcommand...")

    return nil
},

This pattern ensures that subcommands remain decoupled while having access to a consistent, centrally-managed state.

For a complete, runnable implementation of this pattern, see the loginsvc example located in the /examples/loginsvc directory.

🪃 Custom Type Handlers

Declare options (flags, env vars, config file keys) with custom types by implementing methods on your options struct.

Implement these methods on your options structs:

• Define<FieldName>: return a pflag.Value that knows how to handle your custom type, along with an enhanced description.
• Decode<FieldName>: decode the input into your custom type.
• Complete<FieldName> (optional): provide shell completion candidates for the generated flag value. structcli.Define() auto-registers it.

type Environment string

const (
	EnvDevelopment Environment = "dev"
	EnvStaging     Environment = "staging"
	EnvProduction  Environment = "prod"
)

type ServerOptions struct {
	...
	// Custom type
	TargetEnv Environment `flagcustom:"true" flag:"target-env" flagdescr:"Set the target environment"`
}

// DefineTargetEnv returns a pflag.Value for the custom Environment type.
func (o *ServerOptions) DefineTargetEnv(name, short, descr string, structField reflect.StructField, fieldValue reflect.Value) (pflag.Value, string) {
    enhancedDesc := descr + " {dev,staging,prod}"
    fieldPtr := fieldValue.Addr().Interface().(*Environment)
    *fieldPtr = "dev" // Set default

    return structclivalues.NewString((*string)(fieldPtr)), enhancedDesc
}

// DecodeTargetEnv converts the string input to the Environment type.
func (o *ServerOptions) DecodeTargetEnv(input any) (any, error) {
	// ... (validation and conversion logic)
    return EnvDevelopment, nil
}

// CompleteTargetEnv provides shell completion for --target-env.
func (o *ServerOptions) CompleteTargetEnv(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
    return []string{"dev", "staging", "prod"}, cobra.ShellCompDirectiveNoFileComp
}

func (o *ServerOptions) Attach(c *cobra.Command) error {
	return structcli.Define(c, o)
}

Complete<FieldName> works for any field that becomes a flag (not only flagcustom:"true" fields).

Completion precedence:

• If a completion function is already registered on a flag before structcli.Define(), structcli preserves it.
• If structcli.Define() auto-registers Complete<FieldName>, a later manual RegisterFlagCompletionFunc on the same flag returns Cobra's already registered error.

In values we provide pflag.Value implementations for standard types.

See full example for more details.

🧱 Built-in Custom Types

Note on JSON output: net.IPMask is a byte slice under the hood, so Go's encoding/json renders it as base64 (for example 255.255.255.0 appears as ////AA==). This is expected.

All built-in types support:

• Command-line flags with validation and help text
• Environment variables with automatic binding
• Configuration files (YAML, JSON, TOML)
• Type validation with helpful error messages

Slices and maps use the same contract across flags, env vars, and config.

See examples/collections/main.go for a runnable version of this example.

type AdvancedOptions struct {
	Retries   []uint          `flag:"retries" flagenv:"true"`
	Backoffs  []time.Duration `flag:"backoffs" flagenv:"true"`
	FeatureOn []bool          `flag:"feature-on" flagenv:"true"`
	Labels    map[string]string `flag:"labels" flagenv:"true"`
	Limits    map[string]int    `flag:"limits" flagenv:"true"`
	Counts    map[string]int64  `flag:"counts" flagenv:"true"`
}

❯ myapp --retries 1,2,3 --backoffs 1s,5s --feature-on true,false --labels env=prod,team=platform --limits cpu=8,memory=16 --counts ok=10,fail=3
❯ MYAPP_RETRIES=1,2,3 MYAPP_BACKOFFS=1s,5s MYAPP_FEATURE_ON=true,false MYAPP_LABELS=env=prod,team=platform MYAPP_LIMITS=cpu=8,memory=16 MYAPP_COUNTS=ok=10,fail=3 myapp
❯ go run examples/collections/main.go --config examples/collections/config.yaml

retries: "1,2,3"
backoffs:
  - 1s
  - 5s
feature-on: "true,false"
labels:
  env: prod
  team: platform
limits:
  cpu: 8
  memory: 16
counts: "ok=10,fail=3"

🎨 Beautiful, Organized Help Output

Organize your --help output into logical groups for better readability.

❯ go run examples/full/main.go --help
# A demonstration of the structcli library with beautiful CLI features
#
# Usage:
#   full [flags]
#   full [command]
#
# Available Commands:
#   completion  Generate the autocompletion script for the specified shell
#   help        Help about any command
#   srv         Start the server
#   usr         User management
#
# Global Flags:
#       --config string   config file (fallbacks to: {/etc/full,{executable_dir}/.full,$HOME/.full}/config.{yaml,json,toml})
#       --debug-options   enable debug output for options
#
# Utility Flags:
#       --dry-run
#   -v, --verbose count

❯ go run examples/full/main.go srv --help
# Start the server with the specified configuration
#
# Usage:
#   full srv [flags]
#   full srv [command]
#
# Available Commands:
#   version     Print version information
#
# Flags:
#       --apikey string       API authentication key
#       --host string         Server host (default "localhost")
#   -p, --port int            Server port
#       --target-env string   Set the target environment {dev,staging,prod} (default "dev")
#
# Database Flags:
#       --database.maxconns int   Max database connections (default 10)
#       --db-url string           Database connection URL
#
# Logging Flags:
#       --log-file string           Log file path
#       --log-level zapcore.Level   Set log level {debug,info,warn,error,dpanic,panic,fatal} (default info)
#
# Network Flags:
#       --advertise-cidr ipNet      Advertised service subnet (CIDR) (default 127.0.0.0/24)
#       --bind-ip ip                Bind interface IP (default 127.0.0.1)
#       --bind-mask ipMask          Bind interface mask (default ffffff00)
#       --trusted-peers ipSlice     Trusted peer IPs (comma separated) (default [127.0.0.2,127.0.0.3])
#
# Global Flags:
#       --config string   config file (fallbacks to: {/etc/full,{executable_dir}/.full,$HOME/.full}/config.{yaml,json,toml})
#       --debug-options   enable debug output for options
#
# Use "full srv [command] --help" for more information about a command.

🏷️ Available Struct Tags

Use these tags in your struct fields to control the behavior:

flagpreset is syntactic sugar: it creates alias flags that set the canonical flag value. Format: <alias>=<value>; multiple entries can be separated by ; or ,. Example: flagpreset:"logeverything=5;logquiet=0" makes --logeverything behave like --loglevel=5. If both alias and canonical flags are passed, the last assignment in argv wins. It does not bypass transform/validate flow.

📖 Documentation

For comprehensive documentation and advanced usage patterns, visit the documentation.

Start here for repo-local guides:

• AI-Native CLIs guide
• Structured error example walkthrough
• Examples directory

🤝 Contributing

Contributions are welcome!

Please feel free to submit a Pull Request.