structcli

2024-02-12T00:00:00+00:00
Declare your CLI contract once in Go structs. structcli</code> turns it into flags, env vars, config-file loading, validation, organized help, and machine-readable contracts for agents.</p>

Less Cobra/Viper boilerplate</li>
Better CLIs for humans</li>
Better contracts for automation and LLMs</li>
</ul>
Stop writing plumbing. Start shipping commands.</p>
⚡ Quick Start§</a>
</h2>
</a></p>
Start with a plain Go struct:</p>
package</span> main</span></span>
</span>
import</span> (</span></span>
	"fmt"</span></span>
	"log"</span></span>
</span>
	"github.com/leodido/structcli"</span></span>
	"github.com/spf13/cobra"</span></span>
	"go.uber.org/zap/zapcore"</span></span>
)</span></span>
</span>
type</span> Options</span> struct</span> {</span></span>
	LogLevel</span> zapcore</span>.</span>Level</span></span>
	Port</span>     int</span></span>
}</span></span>
</span>
func</span> main</span>() {</span></span>
	opts</span> := &</span>Options</span>{}</span></span>
	cli</span> := &</span>cobra</span>.</span>Command</span>{</span>Use</span>:</span> "myapp"</span>}</span></span>
</span>
	if</span> err</span> :=</span> structcli</span>.</span>Define</span>(</span>cli</span>,</span> opts</span>);</span> err</span> !=</span> nil</span> {</span></span>
		log</span>.</span>Fatalln</span>(</span>err</span>)</span></span>
	}</span></span>
</span>
	cli</span>.</span>PreRunE</span> =</span> func</span>(</span>c</span> *</span>cobra</span>.</span>Command</span>,</span> args</span> []</span>string</span>)</span> error</span> {</span></span>
		return</span> structcli</span>.</span>Unmarshal</span>(</span>c</span>,</span> opts</span>)</span></span>
	}</span></span>
</span>
	cli</span>.</span>RunE</span> =</span> func</span>(</span>c</span> *</span>cobra</span>.</span>Command</span>,</span> args</span> []</span>string</span>)</span> error</span> {</span></span>
		fmt</span>.</span>Println</span>(</span>opts</span>)</span></span>
</span>
		return</span> nil</span></span>
	}</span></span>
</span>
	if</span> err</span> :=</span> cli</span>.</span>Execute</span>();</span> err</span> !=</span> nil</span> {</span></span>
		log</span>.</span>Fatalln</span>(</span>err</span>)</span></span>
	}</span></span>
}</span></span></code></pre>
That single Define</code> call creates the CLI surface from your struct, and Unmarshal</code> hydrates it back from flags, env vars, config, and defaults.</p>
❯</span> go run examples/minimal/main.go</span> --help</span></span>
# Usage:</span></span>
#   myapp [flags]</span></span>
#</span></span>
# Flags:</span></span>
#       --loglevel zapcore.Level    {debug,info,warn,error,dpanic,panic,fatal} (default info)</span></span>
#       --port int</span></span></code></pre>
Add tags when you want aliases, env vars, shorthand, defaults, and descriptions:</p>
type</span> Options</span> struct</span> {</span></span>
	LogLevel</span> zapcore</span>.</span>Level</span> `flag:"level" flagdescr:"Set logging level" flagenv:"true"`</span></span>
	Port</span>     int</span>           `flagshort:"p" flagdescr:"Server port" flagenv:"true" default:"3000"`</span></span>
}</span></span></code></pre>❯</span> go run examples/simple/main.go</span> -h</span></span>
# Usage:</span></span>
#   myapp [flags]</span></span>
#</span></span>
# Flags:</span></span>
#       --level zapcore.Level   Set logging level {debug,info,warn,error,dpanic,panic,fatal} (default info)</span></span>
#   -p, --port int              Server port (default 3000)</span></span></code></pre>❯</span> MYAPP_LOGLEVEL=debug go run examples/simple/main.go</span></span>
# &{debug 3000}</span></span></code></pre>❯</span> MYAPP_LOGLEVEL=error MYAPP_PORT=</span>9000</span> go run examples/simple/main.go</span> --level</span> dpanic</span></span>
# &{dpanic 9000}</span></span></code></pre>
Built-in types like zapcore.Level</code> are validated automatically too.</p>
Out of the box, your CLI supports:</p>

📝 Command-line flags (--level info</code>, -p 8080</code>)</li>
🌍 Environment variables (MYAPP_PORT=8080</code>)</li>
💦 Options precedence (flags > env vars > config file > defaults)</li>
✅ Automatic validation and type conversion</li>
📚 Beautiful help output with proper grouping</li>
</ul>
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.</p>
Build AI-Native CLIs§</a>
</h2>
structcli</code> does not just generate flags for humans. It can make your CLI legible to agents too.</p>
Instead of scraping --help</code> and guessing, an agent can discover the contract, call the command correctly, and recover from structured failures.</p>
structcli</span>.</span>SetupJSONSchema</span>(</span>rootCmd</span>,</span> jsonschema</span>.</span>Options</span>{})</span></span>
structcli</span>.</span>SetupFlagErrors</span>(</span>rootCmd</span>)</span> // Optional, but recommended for typed flag-parse errors</span></span>
structcli</span>.</span>SetupMCP</span>(</span>rootCmd</span>,</span> mcp</span>.</span>Options</span>{})</span> // Optional, exposes the CLI as an MCP server over stdio</span></span>
structcli</span>.</span>ExecuteOrExit</span>(</span>rootCmd</span>)</span></span></code></pre>
With that wiring:</p>

--jsonschema</code> exposes flags, defaults, required inputs, enums, and env bindings across the command tree</li>
HandleError</code> / ExecuteOrExit</code> emit structured JSON errors instead of forcing callers to parse human-oriented output</li>
--mcp</code> exposes the same command tree as MCP tools over stdio, with typed inputs and structured tool-call failures</li>
semantic exit codes tell the caller whether it should fix input, fix config, retry, or escalate to a human</li>
</ul>
The same contract spans flags, env vars, config, validation, and enum constraints.</p>
$</span> mycli srv</span> --jsonschema</span></span>
{</span></span>
  "properties":</span> {</span></span>
    "port":</span> {</span></span>
      "type":</span> "integer",</span></span>
      "default":</span> 3000,</span></span>
      "x-structcli-env-vars":</span> [</span>"MYCLI_SRV_PORT"</span>]</span></span>
    }</span></span>
  }</span></span>
}</span></span></code></pre>
No --help</code> parsing. No guessing what failed. Just a CLI that can explain itself and fail in machine-actionable ways.</p>
Use exitcode.Category(code)</code> and exitcode.IsRetryable(code)</code> to decide what to do next. See jsonschema.WithFullTree()</code> and jsonschema.WithEnumInDescription()</code> for schema customization, and pass the same schema options through SetupJSONSchema</code> with jsonschema.Options{SchemaOpts: ...}</code>.</p>
For CLIs that capture output streams during command construction, configure mcp.Options.CommandFactory</code> so each MCP tool call builds a fresh command with the tool-call stdout and stderr writers. This keeps MCP protocol output separate from command output while preserving the existing command tree schema. If the command constructor requires stdin, the factory can wire a non-interactive reader such as strings.NewReader("")</code>.</p>
For build-time discovery, generate.WriteAll</code> produces SKILL.md, llms.txt, and AGENTS.md from the same struct definitions — wire it into //go:generate</code> and the files stay in sync automatically.</p>
Read the full AI-native guide</a> or walk through the runnable structured error example</a>.</p>
⬇️ Install§</a>
</h2>
go</span> get github.com/leodido/structcli</span></span></code></pre>📦 Key Features§</a>
</h2>
🧩 Declarative Flags Definition§</a>
</h3>
Define flags once using Go struct tags.</p>
No more boilerplate for Flags().StringVarP</code>, Flags().IntVar</code>, viper.BindPFlag</code>, etc.</p>
Yes, you can nest</em> structs too.</p>
type</span> ServerOptions</span> struct</span> {</span></span>
	// Basic flags</span></span>
	Host</span> string</span> `flag:"host" flagdescr:"Server host" default:"localhost"`</span></span>
	Port</span> int</span>    `flagshort:"p" flagdescr:"Server port" flagrequired:"true" flagenv:"true"`</span></span>
</span>
	// Environment variable binding</span></span>
	APIKey</span> string</span> `flagenv:"true" flagdescr:"API authentication key"`</span></span>
</span>
	// Network contracts using net families</span></span>
	BindIP</span>        net</span>.</span>IP</span>     `flag:"bind-ip" flaggroup:"Network" flagdescr:"Bind interface IP" flagenv:"true"`</span></span>
	BindMask</span>      net</span>.</span>IPMask</span> `flag:"bind-mask" flaggroup:"Network" flagdescr:"Bind interface mask" flagenv:"true"`</span></span>
	AdvertiseCIDR</span> net</span>.</span>IPNet</span>  `flag:"advertise-cidr" flaggroup:"Network" flagdescr:"Advertised service subnet (CIDR)" flagenv:"true"`</span></span>
	TrustedPeers</span>  []</span>net</span>.</span>IP</span>   `flag:"trusted-peers" flaggroup:"Network" flagdescr:"Trusted peer IPs (comma separated)" flagenv:"true"`</span></span>
</span>
	// Flag grouping for organized help</span></span>
	LogLevel</span> zapcore</span>.</span>Level</span> `flag:"log-level" flaggroup:"Logging" flagdescr:"Set log level"`</span></span>
	LogFile</span>  string</span>        `flag:"log-file" flaggroup:"Logging" flagdescr:"Log file path" flagenv:"true"`</span></span>
</span>
	// Nested structs for organization</span></span>
	Database</span> DatabaseConfig</span> `flaggroup:"Database"`</span></span>
</span>
	// Custom type</span></span>
	TargetEnv</span> Environment</span> `flagcustom:"true" flag:"target-env" flagdescr:"Set the target environment"`</span></span>
}</span></span>
</span>
type</span> DatabaseConfig</span> struct</span> {</span></span>
	URL</span>      string</span> `flag:"db-url" flagdescr:"Database connection URL"`</span></span>
	MaxConns</span> int</span>    `flagdescr:"Max database connections" default:"10" flagenv:"true"`</span></span>
}</span></span></code></pre>
See full example</a> for more details.</p>
🛠️ Automatic Environment Variable Binding§</a>
</h3>
Automatically generate environment variables binding them to configuration files (YAML, JSON, TOML, etc.) and flags.</p>
From the previous options struct, you get the following env vars automatically:</p>

FULL_SRV_PORT</code></li>
FULL_SRV_APIKEY</code></li>
FULL_SRV_BIND_IP</code></li>
FULL_SRV_BIND_MASK</code></li>
FULL_SRV_ADVERTISE_CIDR</code></li>
FULL_SRV_TRUSTED_PEERS</code></li>
FULL_SRV_DATABASE_MAXCONNS</code></li>
FULL_SRV_LOGFILE</code>, FULL_SRV_LOG_FILE</code></li>
</ul>
Every struct field with the flagenv:"true"</code> tag gets an environment variable (two if the struct field also has the flag:"..."</code> tag, see struct field LogFile</code>).</p>
The prefix of the environment variable name is the CLI name plus the command name to which those options are attached to.</p>
Environment variables are command-scoped for command-local options.
For example, if Port</code> is attached to the srv</code> command, FULL_SRV_PORT</code> is used (not FULL_PORT</code>).</p>
⚙️ Configuration File Support§</a>
</h3>
Easily set up configuration file discovery (flag, environment variable, and fallback paths) with a single line of code.</p>
structcli</span>.</span>SetupConfig</span>(</span>rootCmd</span>,</span> config</span>.</span>Options</span>{</span>AppName</span>:</span> "full"</span>})</span></span></code></pre>
Enable strict config-key validation with:</p>
structcli</span>.</span>SetupConfig</span>(</span>rootCmd</span>,</span> config</span>.</span>Options</span>{</span></span>
  AppName</span>:</span>      "full"</span>,</span></span>
  ValidateKeys</span>:</span> true</span>,</span> // opt-in</span></span>
})</span></span></code></pre>
When enabled, Unmarshal</code> fails if command-relevant config contains unknown keys.</p>
Call SetupConfig</code> before attaching/defining options when you rely on app-prefixed environment variables, so the env prefix is initialized before env annotations are generated.</p>
The line above:</p>

creates --config</code> global flag</li>
creates FULL_CONFIG</code> env var</li>
sets /etc/full/</code>, $HOME/.full/</code>, $PWD/.full/</code> as fallback paths for config.yaml</code></li>
</ul>
Magic, isn't it?</p>
What's left? Tell your CLI to load the configuration file (if any).</p>
rootC</span>.</span>PersistentPreRunE</span> =</span> func</span>(</span>c</span> *</span>cobra</span>.</span>Command</span>,</span> args</span> []</span>string</span>)</span> error</span> {</span></span>
	_</span>,</span> configMessage</span>,</span> configErr</span> :=</span> structcli</span>.</span>UseConfigSimple</span>(</span>c</span>)</span></span>
	if</span> configErr</span> !=</span> nil</span> {</span></span>
		return</span> configErr</span></span>
	}</span></span>
	if</span> configMessage</span> !=</span> ""</span> {</span></span>
		c</span>.</span>Println</span>(</span>configMessage</span>)</span></span>
	}</span></span>
</span>
	return</span> nil</span></span>
}</span></span></code></pre>
UseConfigSimple(c)</code> loads config into the root config scope and merges only the relevant section into c</code>'s effective scope.</p>
🧠 Viper Model Scopes§</a>
</h4>
structcli</code> uses two different viper scopes on purpose:</p>

structcli.GetConfigViper(rootOrLeafCmd)</code> -> root-scoped config source</strong> (config file data tree)</li>
structcli.GetViper(cmd)</code> -> command-scoped effective values</strong> (flags/env/defaults + command-relevant config)</li>
</ul>
This separation keeps config-file loading isolated from runtime command state.</p>
If you need imperative values in tests or application code, write to the right scope:</p>
// 1) Effective override for one command context</span></span>
structcli</span>.</span>GetViper</span>(</span>cmd</span>).</span>Set</span>(</span>"timeout"</span>,</span> 60</span>)</span></span>
</span>
// 2) Config-tree style injection (top-level + command section)</span></span>
structcli</span>.</span>GetConfigViper</span>(</span>rootCmd</span>).</span>Set</span>(</span>"srv"</span>,</span> map</span>[</span>string</span>]</span>any</span>{</span></span>
  "port"</span>:</span> 8443</span>,</span></span>
})</span></span></code></pre>
Global viper.Set(...)</code> is not used by structcli.Unmarshal(...)</code> resolution.
Use GetViper</code>/GetConfigViper</code> instead.</p>
📜 Configuration Is First-Class Citizen§</a>
</h4>
Configuration can mirror your command hierarchy.</p>
Settings can be global (at the top level) or specific to a command or subcommand. The most specific section always takes precedence.</p>
# Global settings apply to all commands unless overridden by a specific section.</span></span>
# `dryrun` matches the `DryRun` struct field name.</span></span>
dryrun</span>:</span> true</span></span>
verbose</span>:</span> 1</span> # A default verbosity level for all commands.</span></span>
</span>
# Config for the `srv` command (`full srv`)</span></span>
srv</span>:</span></span>
  # `port` matches the `Port` field name.</span></span>
  port</span>:</span> 8433</span></span>
  # Network options</span></span>
  bind-ip</span>:</span> "10.20.0.10"</span></span>
  bind-mask</span>:</span> "ffffff00"</span></span>
  advertise-cidr</span>:</span> "10.20.0.0/24"</span></span>
  trusted-peers</span>:</span> "10.20.0.11,10.20.0.12"</span></span>
  # `log-level` matches the `flag:"log-level"` tag.</span></span>
  log-level</span>:</span> "warn"</span></span>
  # `logfile` matches the `LogFile` field name.</span></span>
  logfile</span>:</span> /var/log/mysrv.log</span></span>
</span>
  # Flattened keys can set options in nested structs.</span></span>
  # `db-url` (from `flag:"db-url"` tag) maps to ServerOptions.Database.URL.</span></span>
  db-url</span>:</span> "postgres://user:pass@db/prod"</span></span>
</span>
  # Nested keys are also supported.</span></span>
  database</span>:</span></span>
    # Struct field key style</span></span>
    url</span>:</span> "postgres://user:pass@db/prod"</span></span>
    # Alias key style (from `flag:"db-url"`)</span></span>
    db-url</span>:</span> "postgres://user:pass@db/prod"</span></span>
</span>
# Config for the `usr` command group.</span></span>
usr</span>:</span></span>
  # This nested section matches the `usr add` command (`full usr add`).</span></span>
  # Its settings are ONLY applied to 'usr add'.</span></span>
  add</span>:</span></span>
    name</span>:</span> "Config User"</span></span>
    email</span>:</span> "config.user@example.com"</span></span>
    age</span>:</span> 42</span></span>
    # Command specific override</span></span>
    dry</span>:</span> false</span></span>
# NOTE: Per the library's design, there is no other fallback other than from the top-level.</span></span>
# A command like 'usr delete' would ONLY use the global keys above (if those keys/flags are attached to it),</span></span>
# as an exact 'usr.delete' section is not defined.</span></span></code></pre>
This configuration system supports:</p>

Hierarchical Structure</strong>: Nest keys to match your command path (e.g., usr: { add: { ... } }</code>).</li>
Strict Precedence</strong>: Only settings from the global scope and the exact command path section are merged. There is no automatic fallback to parent command sections.</li>
Flexible Keys</strong>: You can use struct field names and aliases (flag:"..."</code>) in both flattened and nested forms.</li>
Supported Forms for Nested Fields</strong>: db-url</code>, database.url</code>, database: { url: ... }</code>, and database: { db-url: ... }</code>.</li>
</ul>
✅ Built-in Validation & Transformation§</a>
</h3>
Supports validation, transformation, and custom flag type definitions through simple interfaces.</p>
Your struct must implement Options</code> (via Attach</code>) and can optionally implement ValidatableOptions</code> and TransformableOptions</code>.</p>
type</span> UserConfig</span> struct</span> {</span></span>
	Email</span> string</span> `flag:"email" flagdescr:"User email" validate:"email"`</span></span>
	Age</span>   int</span>    `flag:"age" flagdescr:"User age" validate:"min=18,max=120"`</span></span>
	Name</span>  string</span> `flag:"name" flagdescr:"User name" mod:"trim,title"`</span></span>
}</span></span>
</span>
func</span> (</span>o </span>*</span>ServerOptions</span>)</span> Validate</span>(</span>ctx</span> context</span>.</span>Context</span>) []</span>error</span> {</span></span>
    // Automatic validation</span></span>
}</span></span>
</span>
func</span> (</span>o </span>*</span>ServerOptions</span>)</span> Transform</span>(</span>ctx</span> context</span>.</span>Context</span>)</span> error</span> {</span></span>
    // Automatic transformation</span></span>
}</span></span></code></pre>
See a full working example here</a>.</p>
🚧 Automatic Debugging Support§</a>
</h3>
Create a --debug-options</code> flag (plus a FULL_DEBUG_OPTIONS</code> env var) for troubleshooting config/env/flags resolution.</p>
structcli</span>.</span>SetupDebug</span>(</span>rootCmd</span>,</span> debug</span>.</span>Options</span>{})</span></span></code></pre>❯</span> go run examples/full/main.go srv</span> --debug-options --config</span> examples/full/config.yaml</span> -p</span> 3333</span></span>
#</span></span>
# Aliases:</span></span>
# map[string]string{"database.url":"db-url", "logfile":"log-file", "loglevel":"log-level", "targetenv":"target-env"}</span></span>
# Override:</span></span>
# map[string]interface {}{}</span></span>
# PFlags:</span></span>
# 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)}}</span></span>
# Env:</span></span>
# map[string][]string{"apikey":[]string{"SRV_APIKEY"}, "database.maxconns":[]string{"SRV_DATABASE_MAXCONNS"}, "log-file":[]string{"SRV_LOGFILE", "SRV_LOG_FILE"}}</span></span>
# Key/Value Store:</span></span>
# map[string]interface {}{}</span></span>
# Config:</span></span>
# 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}</span></span>
# Defaults:</span></span>
# map[string]interface {}{"database":map[string]interface {}{"maxconns":"10"}, "host":"localhost"}</span></span>
# Values:</span></span>
# 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"}</span></span></code></pre>↪️ Sharing Options Between Commands§</a>
</h3>
In complex CLIs, multiple commands often need access to the same global configuration and shared resources (like a logger or a database connection). structcli</code> provides a powerful pattern using the ContextOptions</a> interface to achieve this without resorting to global variables, by propagating a single "source of truth" through the command context.</p>
The pattern allows you to:</p>

Populate a shared options struct once from flags, environment variables, or a config file.</li>
Initialize "computed state" (like a logger) based on those options.</li>
Share this single, fully-prepared "source of truth" with any subcommand that needs it.</li>
</ul>
🍩 In a Nutshell§</a>
</h4>
Create a shared struct that implements the ContextOptions</code> interface. This struct will hold both the configuration flags and the computed state (e.g., the logger).</p>
// This struct holds our shared state.</span></span>
type</span> CommonOptions</span> struct</span> {</span></span>
    LogLevel</span> zapcore</span>.</span>Level</span> `flag:"loglevel" flagdescr:"Logging level" default:"info"`</span></span>
    Logger</span>   *</span>zap</span>.</span>Logger</span>   `flagignore:"true"`</span> // This field is computed, not a flag.</span></span>
}</span></span>
</span>
// The Context/FromContext methods enable the propagation pattern.</span></span>
func</span> (</span>o </span>*</span>CommonOptions</span>)</span> Context</span>(</span>ctx</span> context</span>.</span>Context</span>)</span> context</span>.</span>Context</span> {</span> /* ... */</span> }</span></span>
func</span> (</span>o </span>*</span>CommonOptions</span>)</span> FromContext</span>(</span>ctx</span> context</span>.</span>Context</span>)</span> error</span> {</span> /* ... */</span> }</span></span>
</span>
// Initialize is a custom method to create the computed state.</span></span>
func</span> (</span>o </span>*</span>CommonOptions</span>)</span> Initialize</span>()</span> error</span> {</span> /* ... */</span> }</span></span></code></pre>
Initialize the state in the root command. Use a PersistentPreRunE</code> hook on your root command to populate your struct and initialize any resources.
Invoking structcli.Unmarshal</code> will automatically inject the prepared object into the context for all subcommands to use.</p>
rootC</span>.</span>PersistentPreRunE</span> =</span> func</span>(</span>c</span> *</span>cobra</span>.</span>Command</span>,</span> args</span> []</span>string</span>)</span> error</span> {</span></span>
	// Populate the master `commonOpts` from flags, env, and config file.</span></span>
	if</span> err</span> :=</span> structcli</span>.</span>Unmarshal</span>(</span>c</span>,</span> commonOpts</span>);</span> err</span> !=</span> nil</span> {</span></span>
		return</span> err</span></span>
	}</span></span>
	// Use the populated values to initialize the computed state (the logger).</span></span>
	if</span> err</span> :=</span> commonOpts</span>.</span>Initialize</span>();</span> err</span> !=</span> nil</span> {</span></span>
		return</span> err</span></span>
	}</span></span>
</span>
	return</span> nil</span></span>
}</span></span></code></pre>
Finally, retrieve the state in subcommands. In your subcommand's RunE</code>, simply call .FromContext()</code> to retrieve the shared, initialized object.</p>
func</span>(</span>c </span>*</span>cobra</span>.</span>Command</span>,</span> args</span> []</span>string</span>)</span> error</span> {</span></span>
    // Create a receiver and retrieve the master state from the context.</span></span>
    config</span> := &</span>CommonOptions</span>{}</span></span>
    if</span> err</span> :=</span> config</span>.</span>FromContext</span>(</span>c</span>.</span>Context</span>());</span> err</span> !=</span> nil</span> {</span></span>
        return</span> err</span></span>
    }</span></span>
    config</span>.</span>Logger</span>.</span>Info</span>(</span>"Executing subcommand..."</span>)</span></span>
</span>
    return</span> nil</span></span>
},</span></span></code></pre>
This pattern ensures that subcommands remain decoupled while having access to a consistent, centrally-managed state.</p>
For a complete, runnable implementation of this pattern, see the loginsvc example located in the /examples/loginsvc</a> directory.</p>
🪃 Custom Type Handlers§</a>
</h3>
Declare options (flags, env vars, config file keys) with custom types by implementing methods on your options struct.</p>
Implement these methods on your options structs:</p>

Define<FieldName></code>: return a pflag.Value</code> that knows how to handle your custom type, along with an enhanced description.</li>
Decode<FieldName></code>: decode the input into your custom type.</li>
Complete<FieldName></code> (optional): provide shell completion candidates for the generated flag value. structcli.Define()</code> auto-registers it.</li>
</ul>
type</span> Environment string</span></span>
</span>
const</span> (</span></span>
	EnvDevelopment</span> Environment</span> =</span> "dev"</span></span>
	EnvStaging</span>     Environment</span> =</span> "staging"</span></span>
	EnvProduction</span>  Environment</span> =</span> "prod"</span></span>
)</span></span>
</span>
type</span> ServerOptions</span> struct</span> {</span></span>
	...</span></span>
	// Custom type</span></span>
	TargetEnv</span> Environment</span> `flagcustom:"true" flag:"target-env" flagdescr:"Set the target environment"`</span></span>
}</span></span>
</span>
// DefineTargetEnv returns a pflag.Value for the custom Environment type.</span></span>
func</span> (</span>o </span>*</span>ServerOptions</span>)</span> DefineTargetEnv</span>(</span>name</span>,</span> short</span>,</span> descr</span> string</span>,</span> structField</span> reflect</span>.</span>StructField</span>,</span> fieldValue</span> reflect</span>.</span>Value</span>) (</span>pflag</span>.</span>Value</span>,</span> string</span>) {</span></span>
    enhancedDesc</span> :=</span> descr</span> +</span> " {dev,staging,prod}"</span></span>
    fieldPtr</span> :=</span> fieldValue</span>.</span>Addr</span>().</span>Interface</span>().(*</span>Environment</span>)</span></span>
    *</span>fieldPtr</span> =</span> "dev"</span> // Set default</span></span>
</span>
    return</span> structclivalues</span>.</span>NewString</span>((*</span>string</span>)(</span>fieldPtr</span>)),</span> enhancedDesc</span></span>
}</span></span>
</span>
// DecodeTargetEnv converts the string input to the Environment type.</span></span>
func</span> (</span>o </span>*</span>ServerOptions</span>)</span> DecodeTargetEnv</span>(</span>input</span> any</span>) (</span>any</span>,</span> error</span>) {</span></span>
	// ... (validation and conversion logic)</span></span>
    return</span> EnvDevelopment</span>,</span> nil</span></span>
}</span></span>
</span>
// CompleteTargetEnv provides shell completion for --target-env.</span></span>
func</span> (</span>o </span>*</span>ServerOptions</span>)</span> CompleteTargetEnv</span>(</span>cmd</span> *</span>cobra</span>.</span>Command</span>,</span> args</span> []</span>string</span>,</span> toComplete</span> string</span>) ([]</span>string</span>,</span> cobra</span>.</span>ShellCompDirective</span>) {</span></span>
    return</span> []</span>string</span>{</span>"dev"</span>,</span> "staging"</span>,</span> "prod"</span>},</span> cobra</span>.</span>ShellCompDirectiveNoFileComp</span></span>
}</span></span>
</span>
func</span> (</span>o </span>*</span>ServerOptions</span>)</span> Attach</span>(</span>c</span> *</span>cobra</span>.</span>Command</span>)</span> error</span> {</span></span>
	return</span> structcli</span>.</span>Define</span>(</span>c</span>,</span> o</span>)</span></span>
}</span></span></code></pre>
Complete<FieldName></code> works for any field that becomes a flag (not only flagcustom:"true"</code> fields).</p>
Completion precedence:</p>

If a completion function is already registered on a flag before structcli.Define()</code>, structcli preserves it.</li>
If structcli.Define()</code> auto-registers Complete<FieldName></code>, a later manual RegisterFlagCompletionFunc</code> on the same flag returns Cobra's already registered</code> error.</li>
</ul>
In values</a> we provide pflag.Value</code> implementations for standard types.</p>
See full example</a> for more details.</p>
🧱 Built-in Custom Types§</a>
</h3>
Type</th> Description</th> Example Values</th> Special Features</th></tr></thead>

zapcore.Level</code></td> Zap logging levels</td> debug</code>, info</code>, warn</code>, error</code>, dpanic</code>, panic</code>, fatal</code></td> Enum validation</td></tr>
slog.Level</code></td> Standard library logging levels</td> debug</code>, info</code>, warn</code>, error</code>, error+2</code>, ...</td> Level offsets: ERROR+2</code>, INFO-4</code></td></tr>
time.Duration</code></td> Time durations</td> 30s</code>, 5m</code>, 2h</code>, 1h30m</code></td> Go duration parsing</td></tr>
[]time.Duration</code></td> Duration slices</td> 30s,5m</code>, 1s,2m30s</code></td> Comma-separated / repeated flags</td></tr>
[]bool</code></td> Boolean slices</td> true,false,true</code></td> Comma-separated / repeated flags</td></tr>
[]uint</code></td> Unsigned integer slices</td> 1,2,3,42</code></td> Comma-separated / repeated flags</td></tr>
[]byte</code></td> Raw textual bytes</td> hello</code>, abc123</code></td> Raw textual input</td></tr>
structcli.Hex</code></td> Hex-decoded textual input</td> 68656c6c6f</code>, 48656c6c6f</code></td> Hex decoding</td></tr>
structcli.Base64</code></td> Base64-decoded textual input</td> aGVsbG8=</code>, YWJjMTIz</code></td> Base64 decoding</td></tr>
net.IP</code></td> IP address</td> 127.0.0.1</code>, 10.42.0.10</code>, 2001:db8::1</code></td> IP parsing</td></tr>
net.IPMask</code></td> IPv4 mask</td> 255.255.255.0</code>, ffffff00</code></td> Dotted or hex mask parsing</td></tr>
net.IPNet</code></td> CIDR subnet</td> 10.42.0.0/24</code>, 2001:db8::/64</code></td> CIDR parsing</td></tr>
[]net.IP</code></td> IP slices</td> 10.0.0.1,10.0.0.2</code></td> Comma-separated / repeated flags</td></tr>
[]string</code></td> String slices</td> item1,item2,item3</code></td> Comma-separated</td></tr>
[]int</code></td> Integer slices</td> 1,2,3,42</code></td> Comma-separated</td></tr>
map[string]string</code></td> String maps</td> env=prod,team=platform</code></td> key=value</code> pairs</td></tr>
map[string]int</code></td> Integer maps</td> cpu=2,memory=4</code></td> key=value</code> pairs with int parsing</td></tr>
map[string]int64</code></td> 64-bit integer maps</td> ok=1,fail=2</code></td> key=value</code> pairs with int64 parsing</td></tr>
</tbody></table>
Note on JSON output: net.IPMask</code> is a byte slice under the hood, so Go's encoding/json</code>
renders it as base64 (for example 255.255.255.0</code> appears as ////AA==</code>). This is expected.</p>
All built-in types support:</p>

Command-line flags with validation and help text</li>
Environment variables with automatic binding</li>
Configuration files (YAML, JSON, TOML)</li>
Type validation with helpful error messages</li>
</ul>
Slices and maps use the same contract across flags, env vars, and config.</p>
See examples/collections/main.go</a> for a runnable version of this example.</p>
type</span> AdvancedOptions</span> struct</span> {</span></span>
	Retries</span>   []</span>uint</span>          `flag:"retries" flagenv:"true"`</span></span>
	Backoffs</span>  []</span>time</span>.</span>Duration</span> `flag:"backoffs" flagenv:"true"`</span></span>
	FeatureOn</span> []</span>bool</span>          `flag:"feature-on" flagenv:"true"`</span></span>
	Labels</span>    map</span>[</span>string</span>]</span>string</span> `flag:"labels" flagenv:"true"`</span></span>
	Limits</span>    map</span>[</span>string</span>]</span>int</span>    `flag:"limits" flagenv:"true"`</span></span>
	Counts</span>    map</span>[</span>string</span>]</span>int64</span>  `flag:"counts" flagenv:"true"`</span></span>
}</span></span></code></pre>❯</span> myapp</span> --retries</span> 1,2,3</span> --backoffs</span> 1s,5s</span> --feature-on true</span>,</span>false --labels</span> env=prod,team=platform</span> --limits</span> cpu=8,memory=</span>16</span> --counts</span> ok=10,fail=</span>3</span></span>
❯</span> MYAPP_RETRIES=1,2,3 MYAPP_BACKOFFS=1s,5s MYAPP_FEATURE_ON=</span>true</span>,</span>false</span> MYAPP_LABELS=env=prod,team=platform MYAPP_LIMITS=cpu=8,memory=</span>16</span> MYAPP_COUNTS=ok=10,fail=</span>3</span> myapp</span></span>
❯</span> go run examples/collections/main.go</span> --config</span> examples/collections/config.yaml</span></span></code></pre>retries</span>:</span> "1,2,3"</span></span>
backoffs</span>:</span></span>
  -</span> 1s</span></span>
  -</span> 5s</span></span>
feature-on</span>:</span> "true,false"</span></span>
labels</span>:</span></span>
  env</span>:</span> prod</span></span>
  team</span>:</span> platform</span></span>
limits</span>:</span></span>
  cpu</span>:</span> 8</span></span>
  memory</span>:</span> 16</span></span>
counts</span>:</span> "ok=10,fail=3"</span></span></code></pre>🎨 Beautiful, Organized Help Output§</a>
</h3>
Organize your --help</code> output into logical groups for better readability.</p>
❯</span> go run examples/full/main.go</span> --help</span></span>
# A demonstration of the structcli library with beautiful CLI features</span></span>
#</span></span>
# Usage:</span></span>
#   full [flags]</span></span>
#   full [command]</span></span>
#</span></span>
# Available Commands:</span></span>
#   completion  Generate the autocompletion script for the specified shell</span></span>
#   help        Help about any command</span></span>
#   srv         Start the server</span></span>
#   usr         User management</span></span>
#</span></span>
# Global Flags:</span></span>
#       --config string   config file (fallbacks to: {/etc/full,{executable_dir}/.full,$HOME/.full}/config.{yaml,json,toml})</span></span>
#       --debug-options   enable debug output for options</span></span>
#</span></span>
# Utility Flags:</span></span>
#       --dry-run</span></span>
#   -v, --verbose count</span></span></code></pre>❯</span> go run examples/full/main.go srv</span> --help</span></span>
# Start the server with the specified configuration</span></span>
#</span></span>
# Usage:</span></span>
#   full srv [flags]</span></span>
#   full srv [command]</span></span>
#</span></span>
# Available Commands:</span></span>
#   version     Print version information</span></span>
#</span></span>
# Flags:</span></span>
#       --apikey string       API authentication key</span></span>
#       --host string         Server host (default "localhost")</span></span>
#   -p, --port int            Server port</span></span>
#       --target-env string   Set the target environment {dev,staging,prod} (default "dev")</span></span>
#</span></span>
# Database Flags:</span></span>
#       --database.maxconns int   Max database connections (default 10)</span></span>
#       --db-url string           Database connection URL</span></span>
#</span></span>
# Logging Flags:</span></span>
#       --log-file string           Log file path</span></span>
#       --log-level zapcore.Level   Set log level {debug,info,warn,error,dpanic,panic,fatal} (default info)</span></span>
#</span></span>
# Network Flags:</span></span>
#       --advertise-cidr ipNet      Advertised service subnet (CIDR) (default 127.0.0.0/24)</span></span>
#       --bind-ip ip                Bind interface IP (default 127.0.0.1)</span></span>
#       --bind-mask ipMask          Bind interface mask (default ffffff00)</span></span>
#       --trusted-peers ipSlice     Trusted peer IPs (comma separated) (default [127.0.0.2,127.0.0.3])</span></span>
#</span></span>
# Global Flags:</span></span>
#       --config string   config file (fallbacks to: {/etc/full,{executable_dir}/.full,$HOME/.full}/config.{yaml,json,toml})</span></span>
#       --debug-options   enable debug output for options</span></span>
#</span></span>
# Use "full srv [command] --help" for more information about a command.</span></span></code></pre>🏷️ Available Struct Tags§</a>
</h2>
Use these tags in your struct fields to control the behavior:</p>
Tag</th> Description</th> Example</th></tr></thead>

flag</code></td> Sets a custom name for the flag (otherwise, generated from the field name)</td> flag:"log-level"</code></td></tr>
flagpreset</code></td> Defines CLI-only preset aliases for this field's flag. Each preset is <alias-flag-name>=<value-for-this-field-flag></code>. No env/config keys are created.</td> flagpreset:"logeverything=5;logquiet=0"</code></td></tr>
flagshort</code></td> Sets a single-character shorthand for the flag</td> flagshort:"l"</code></td></tr>
flagdescr</code></td> Provides the help text for the flag</td> flagdescr:"Logging level"</code></td></tr>
default</code></td> Sets the default value for the flag</td> default:"info"</code></td></tr>
flagenv</code></td> Enables binding to an environment variable ("true"</code>/"false"</code>)</td> flagenv:"true"</code></td></tr>
flagrequired</code></td> Marks the flag as required ("true"</code>/"false"</code>)</td> flagrequired:"true"</code></td></tr>
flaghidden</code></td> Hides the flag from help/usage output and machine-readable schemas while keeping it fully functional ("true"</code>/"false"</code>)</td> flaghidden:"true"</code></td></tr>
flaggroup</code></td> Assigns the flag to a group in the help message</td> flaggroup:"Database"</code></td></tr>
flagignore</code></td> Skips creating a flag for this field ("true"</code>/"false"</code>)</td> flagignore:"true"</code></td></tr>
flagcustom</code></td> Uses a custom Define<FieldName></code> method for advanced flag creation and a custom Decode<FieldName></code> method for advanced value decoding</td> flagcustom:"true"</code></td></tr>
flagtype</code></td> Specifies a special flag type. Currently supports count</code></td> flagtype:"count"</code></td></tr>
</tbody></table>
flagpreset</code> is syntactic sugar: it creates alias flags that set the canonical flag value.
Format: <alias>=<value></code>; multiple entries can be separated by ;</code> or ,</code>.
Example: flagpreset:"logeverything=5;logquiet=0"</code> makes --logeverything</code> behave like --loglevel=5</code>.
If both alias and canonical flags are passed, the last assignment in argv wins.
It does not bypass transform/validate flow.</p>
📖 Documentation§</a>
</h2>
For comprehensive documentation and advanced usage patterns, visit the documentation</a>.</p>
Start here for repo-local guides:</p>

AI-Native CLIs guide</a></li>
Structured error example walkthrough</a></li>
Examples directory</a></li>
</ul>
🤝 Contributing§</a>
</h2>
Contributions are welcome!</p>
Please feel free to submit a Pull Request.</p>
🤝 Contributing §</a> </h2>
Contributions are welcome!</p>
Please feel free to submit a Pull Request.</p>

leodido.dev - Viper

structcli
