decopin-cli

What is decopin-cli?

Build CLIs the way Next.js builds web apps: file conventions, JSX output, and types that come from your declarations. TypeScript + Bun.

Most CLIs keep their real interface implicit. The arguments are declared, but whether the command reads stdin, which environment variables it needs, the shape of what it prints and whether it touches the disk all live inside the code, unstated. decopin gives each of them a file with a fixed name, which makes the interface a declaration — and anything that can read a declaration then comes for free: --help, shell completion, the TypeScript types, and an MCP server the CLI already is. There is no second definition to keep in sync, because there is no second definition.

Output is JSX. There is no React — decopin ships its own small renderer.

// app/hello/cmd.tsx
import { Line, Text, type CmdProps } from 'decopin-cli';

export default function Command({ args, options }: CmdProps<'hello'>) {
  return (
    <Line>
      <Text bold color="green">
        hello, {args.name}
      </Text>
    </Line>
  );
}

$ bun run build
$ ./dist/index.js hello world
hello, world

Setup

bunx decopin-cli init my-cli   # or `init` alone for the current folder
cd my-cli
bun run build
./dist/index.js hello          # hello, world

Add files to an existing project with gen:

bunx decopin-cli gen --conv cmd --path app/hello
bunx decopin-cli gen --conv argv --path app/hello
bunx decopin-cli gen --inherited layout --path app/user
bunx decopin-cli gen --root-only env --path app
bunx decopin-cli gen --help

Choose one of --conv, --inherited, or --root-only; the help lists all supported names. --path is a directory relative to the current working directory and defaults to the app root. Use --app src/app for a custom root (and --path src/app/hello for a command inside it). Root-only files must go at that root. Inherited files use the same templates as their convention counterparts and apply to the subtree where they are placed. Each invocation creates one .tsx file and any missing directories. Existing .tsx, .ts, and legacy command files are kept. Run bun run build afterward.

To set things up by hand instead

bun add decopin-cli
bun add -d @types/bun

Your tsconfig.json needs the JSX settings. Without them TypeScript and Bun both look for React and the build fails with Could not resolve: react/jsx-runtime (decopin build / decopin dev warn you about these two settings, unless your tsconfig uses extends).

{
  "compilerOptions": {
    "jsx": "react-jsx",
    "jsxImportSource": "decopin-cli/jsx",
    "moduleResolution": "bundler",
    "noEmit": true,
    "allowImportingTsExtensions": true
  },
  "include": ["app/**/*", ".decopin/types.d.ts"]
}

bunx decopin build   # scan app/ and produce dist/index.js
bunx decopin dev     # watch app/ and rebuild types + dist/index.js on every save

Files, not configuration

A CLI has more inputs than argv. It reads stdin, or does not. It depends on environment variables it never lists. Its error output changes shape two subcommands down. In most CLIs these live inside the code, unstated, and the type checker cannot see any of them.

decopin gives each of them a file with a fixed name. There are three kinds, and they differ in where they may go:

| Kind | Files | Where it goes | | --------------- | ---------------------------------------------------------------------------------------------- | ------------------------------------------------- | | Conventions | cmd.tsx argv.tsx data.tsx output.tsx stdin.tsx help.tsx shell.tsx complete.tsx | next to the command; applies to that command only | | Inherited | layout.tsx middleware.tsx error.tsx not-found.tsx | any directory; applies to everything below it | | Root-only | env.tsx version.tsx global-error.tsx | app/ only; applies to the whole CLI |

app/
├── env.tsx               root-only: the environment, validated once at startup
├── version.tsx           root-only: what --version prints
├── global-error.tsx      root-only: the last resort
├── hello/
│   ├── argv.tsx          convention: arguments and options
│   └── cmd.tsx           convention: the view
└── user/                 a directory without cmd.tsx is a group
    ├── error.tsx         inherited: list/ and import/ fail through here
    ├── layout.tsx        inherited: wraps their output
    ├── list/
    │   ├── argv.tsx
    │   └── cmd.tsx
    └── import/
        ├── stdin.tsx     convention: this command reads stdin, and only this one
        └── cmd.tsx

Every file you add does two things. It changes what the command (or the subtree, or the whole CLI) does, and it changes the type of CmdProps<'…'> the next time decopin build or decopin dev runs. What you leave out is not guessed: a command without stdin.tsx gets stdin: never, and never reads it.

Declared once, and the machines read it

Because the inputs are declared instead of implied, the CLI is already an MCP server. Run it with the reserved __mcp command and an agent host can list and call your commands:

{
  "mcpServers": {
    "mycli": { "command": "mycli", "args": ["__mcp"] }
  }
}

Nothing new to declare, and nothing to annotate. argv.tsx is the inputSchema, output.tsx is the outputSchema, data.tsx is the structuredContent, env.tsx says what a call needs, and the tool annotations (readOnlyHint, openWorldHint) come from decopin build counting which side effects each command can reach — a proven absence, not an assertion. --strict-effects turns that into a build-time guarantee. A command whose point is to change the parent shell (shell.tsx) is left out, because under an MCP host there is no parent shell to change. The full mechanics are in the MCP guide.

The directory tree is also the subcommand tree, and a group lists what is under it, with each command's description and defaults:

$ ./dist/index.js user
Usage: decopin-cli user <command> [options]

Commands:
  import  Import users from JSON on stdin.
  list    List users. (default: --limit=10, --verbose=false)
  show    Show one user, or suggest a close name.

Run "decopin-cli user <command> --help" for details.

argv.tsx, briefly

Arguments are the one input every CLI library declares, so this is the familiar part. What you write in argv.tsx drives validation, --help, and the types, and you never touch the validation library:

// app/hello/argv.tsx
import { Arg, Argv, Option, Type } from 'decopin-cli';

export default function DefineArgv() {
  return (
    <Argv description="Greet someone.">
      <Arg
        name="name"
        type="string"
        default="world"
        description="who to greet"
      />
      <Option
        name="loud"
        alias="l"
        type="boolean"
        default={false}
        description="shout it"
      />
      <Option name="times" alias="t" default={1} description="repeat count">
        <Type.Number min={1} max={5} integer />
      </Option>
      <Option name="style" default="plain" description="how to decorate">
        <Type.Enum values={['plain', 'bold', 'rainbow']} />
      </Option>
    </Argv>
  );
}

$ ./dist/index.js hello --help
Usage: decopin-cli hello [name] [options]

Greet someone.

Arguments:
  name                              who to greet (default: "world")

Options:
  -l, --loud                        shout it (default: false)
  -t, --times <number>              repeat count (default: 1)
      --style <plain|bold|rainbow>  how to decorate (default: "plain")
  -h, --help                        show this help

import type { CmdProps } from 'decopin-cli';

export default function Command({ args, options }: CmdProps<'hello'>) {
  args.name; // string
  options.times; // number
  options.style; // "plain" | "bold" | "rainbow"
  return null;
}

The two files below are the ones that do the same for inputs a CLI usually leaves implicit. The rest are covered in the documentation.

stdin.tsx: does this command read stdin?

A command without stdin.tsx never touches stdin. The most common CLI accident, running a command in a terminal and having it hang on input you did not know it wanted, cannot happen by construction. The file that opts in also says what the input is:

// app/count/stdin.tsx
import { Stdin } from 'decopin-cli';

export default function DefineStdin() {
  return <Stdin mode="lines" required />;
}

// app/count/cmd.tsx
import { Line, type CmdProps } from 'decopin-cli';

export default function Command({ stdin }: CmdProps<'count'>) {
  // stdin is string[], because mode="lines" and required
  return <Line>{stdin.length}</Line>;
}

$ printf 'a\nb\n\nc\n' | ./dist/index.js count
4

| mode | What the command receives | | ------- | ------------------------------------------------------------- | | text | string (the whole input). trim drops the trailing newline | | lines | string[] (split on newlines) | | json | the type you declared in children, otherwise unknown |

Without required, a command run in a terminal receives undefined, and the type says | undefined, so the code that forgets to handle it does not compile. With required, a run without a pipe is a usage error (exit 2) instead of a hang.

mode="json" takes the same Type.* children as argv.tsx, so structured input gets the same treatment as arguments: validated before the command runs, and typed from the declaration.

// app/user/import/stdin.tsx
import { Stdin, Type } from 'decopin-cli';

export default function DefineStdin() {
  return (
    <Stdin mode="json" required>
      <Type.Array minItems={1}>
        <Type.Object>
          <Type.Field name="name" required>
            <Type.String minLength={1} />
          </Type.Field>
          <Type.Field name="admin" defaultValue={false}>
            <Type.Boolean />
          </Type.Field>
        </Type.Object>
      </Type.Array>
    </Stdin>
  );
}

// app/user/import/cmd.tsx
import { Line, type CmdProps } from 'decopin-cli';

export default function Command({ stdin }: CmdProps<'user/import'>) {
  // stdin is { name: string; admin: boolean }[]
  return <Line>imported {stdin.length}</Line>;
}

--help knows about it too, so the requirement is visible before anyone runs the command:

$ ./dist/index.js count --help
Usage: decopin-cli count [options]

Count lines coming from stdin.

Stdin:
  lines            required (pipe something in)
...

env.tsx: which environment does this CLI need?

Environment variables are the input nobody documents. They are read wherever process.env happens to be mentioned, parsed as strings, and a missing or misspelled one surfaces as a failure far from its cause. env.tsx is root-only: one file lists every variable the CLI reads, and it is checked once, before any command runs.

// app/env.tsx
import { Env, Type, Var } from 'decopin-cli';

export default function DefineEnv() {
  return (
    <Env>
      <Var name="DECOPIN_LOG" default="info" description="log level">
        <Type.Enum values={['debug', 'info', 'warn', 'error']} />
      </Var>
      <Var name="DECOPIN_RETRIES" default={3} description="how many retries">
        <Type.Number min={0} max={10} integer />
      </Var>
      <Var name="DECOPIN_TOKEN" type="string" description="API token" />
    </Env>
  );
}

Every command receives the validated result as env, already converted and typed. A variable with a default is never undefined; one without is optional, and the type says so:

// app/config/cmd.tsx
import { KeyValue, type CmdProps } from 'decopin-cli';

export default function Command({ env }: CmdProps<'config'>) {
  return (
    <KeyValue
      data={{
        log: env.DECOPIN_LOG, // "debug" | "info" | "warn" | "error"
        retries: env.DECOPIN_RETRIES, // number, not string
        token: env.DECOPIN_TOKEN ?? '(not set)', // string | undefined
      }}
    />
  );
}

A bad value fails fast, before the command, and says which variable and why. With --json the same failure is structured (code: "env"), so a script calling the CLI can tell a setup problem from a runtime one:

$ DECOPIN_LOG=verbose ./dist/index.js config
Invalid usage: DECOPIN_LOG: Invalid type: Expected ("debug" | "info" | "warn" | "error") but received "verbose"
exit code 2

Reading process.env directly still works, of course. The point of the file is that the list of what the CLI depends on exists, in one place, and that the type checker enforces it: env.DECOPIN_LGO is a compile error.

Where the types come from

decopin build and decopin dev write .decopin/types.d.ts from the files above, and your tsconfig.json includes it. CmdProps<'user/import'> is the command's path, and each prop on it traces back to one file:

| Prop | Comes from | When the file is absent | | --------- | ----------------------------- | -------------------------------- | | args | <Arg> in argv.tsx | {} | | options | <Option> in argv.tsx | {} | | stdin | stdin.tsx | never, and stdin is never read | | env | env.tsx at the root | {} | | data | data.tsx (or output.tsx) | never | | dryRun | the reserved --dry-run flag | always boolean | | argv | what was left after routing | always readonly string[] | | cwd | the working directory | always string |

data.tsx is inferred from its return type; output.tsx replaces the inference with a declaration and checks the value at run time before it is displayed or printed as --json. The same declarations feed --help, shell completion, and the MCP tool schema, so there is one description of each command and it is the one the compiler reads.

Working examples

demo/app/ is the example, and the build and the tests keep it honest.

| Command | What it shows | | ---------------------------------------------- | --------------------------------------------------------------------- | | demo/app/hello | positional args, options, enums | | demo/app/count | stdin.tsx (lines), a help.tsx override, bundled boolean aliases | | demo/app/upper | optional stdin (undefined in a terminal) | | demo/app/config | reading validated env.tsx values | | demo/app/user | subcommands, layout.tsx, middleware.tsx, an inherited error.tsx | | demo/app/user/import | mode="json" with Type.Object | | demo/app/user/show | notFound() with an automatic suggestion, complete.tsx | | demo/app/deploy | help() when the input cannot be acted on | | demo/app/publish | authRequired() / missingTool() with fix hints | | demo/app/stats | data.tsx split from the view, output.tsx, and --json | | demo/app/crash | error.tsx and <Exit> | | demo/app/go | shell.tsx: cd and export in the parent shell |

Documentation

Everything else is at https://yuyakinjo.github.io/decopin-cli/, one page per file convention plus the guides: data.tsx and --json, inherited error.tsx / layout.tsx / middleware.tsx, shell.tsx for changing the parent shell, shell completion, exposing commands as MCP tools, Partial Repainting, output components, startup cost, versioning and releasing.

The reasoning behind the design is in docs/decisions.md, written in Japanese. The behaviour is pinned by table-driven tests in test/contract/, and the examples in this README and in the documentation are type-checked and executed by test/docs/, so they cannot drift from the code.

Development

bun run ci            # build, then typecheck / test / lint / format in parallel
bun run site          # build the documentation site into site/dist
bun run site:dev      # build and preview at http://localhost:4173 (restart after edits)
bun run bench         # startup time
bun run format        # rewrite files (ci only checks)

Versioning

Versions are timestamps, not SemVer: YYYY.MMdd.HHmm in UTC. This changes what ^ and ~ mean for you: ^2026.828.1430 allows anything below 2027.0.0. Pin the exact version if that matters to you. Nothing is removed without a deprecation warning from decopin build and a year of grace; the current list and the release process are in the documentation.

License

MIT