# kotlin-development

Guides AI coding agents with strict Kotlin conventions for clean function design, data classes, immutability, and unit test structure.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 709 installs · ★ 286
- Author: Mindrally
- License: Apache-2.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/Mindrally/skills/tree/HEAD/kotlin-development/SKILL.md
- Updated: 2026-10-08
- HTML: https://affpapa.org/en/nevibecode/skills/kotlin-development

## What it does

This skill establishes coding standards for Kotlin projects. It instructs the assistant to declare types explicitly, avoid the `any` type, and keep function bodies free of blank lines. Naming conventions enforce PascalCase for types, camelCase for methods, and `underscores_case` for filenames and folders.

Function and class sizes face strict boundaries. Functions cannot exceed 20 instructions or four parameters, prompting developers to group arguments into parameter objects. Classes must stay under 200 instructions, with caps of 10 public methods and 10 properties.

Enforcing these boundaries means legacy singletons and monolithic classes will fail the guidelines without refactoring. For testing, the guide requires Arrange-Act-Assert structure for unit suites and Given-When-Then formatting for acceptance tests.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill kotlin-development -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/kotlin-development/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/kotlin-development/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill kotlin-development -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/kotlin-development/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill kotlin-development -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/kotlin-development/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install Mindrally/skills kotlin-development
```

2. **Manually: put the files in place**
- File: `.github/skills/kotlin-development/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill kotlin-development -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/Mindrally/skills --path kotlin-development
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/kotlin-development/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill kotlin-development -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/Mindrally/skills/tree/HEAD/kotlin-development/SKILL.md

## When to use it
- refactor long Kotlin routines to under 20 instructions
- convert mutable structures to immutable data classes using val
- structure unit test suites using the Arrange-Act-Assert pattern
- eliminate nested conditionals with early returns and higher-order functions

## Example prompts
- Review this Kotlin service against the 20-instruction limit and suggest early returns
- Convert this mutable state model into an immutable Kotlin data class
- Generate unit tests for this repository using the Arrange-Act-Assert convention

## Source text · SKILL.md · Apache-2.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 1ea7e0a2d0005f02.

BEGIN THIRD-PARTY FILE 1ea7e0a2d0005f02
```text
---
name: kotlin-development
description: Kotlin development guidelines with best practices for clean code, naming conventions, function design, and data handling
metadata:
  maintainer: Mindrally
  source: https://github.com/Mindrally/skills
---

# Kotlin Development Best Practices

## General Principles

- Use English for all code and documentation
- Always declare the type of each variable and function
- Avoid the `any` type; create necessary types instead
- No blank lines within function bodies

## Naming Conventions

### Case Standards
- **PascalCase**: Classes, interfaces, enums
- **camelCase**: Variables, functions, methods, parameters
- **UPPERCASE**: Environment variables, constants
- **underscores_case**: Files and directories

### Naming Guidelines
- Start each function with a verb (get, set, create, update, delete, etc.)
- Use complete words over abbreviations
  - Allowed abbreviations: API, URL, i/j for loops, err, ctx, req/res/next
- Avoid magic numbers; define constants instead
- Boolean variables use prefixes: `isLoading`, `hasError`, `canDelete`

## Function Design

### Size and Responsibility
- Keep functions under 20 instructions
- Each function should have a single responsibility
- Extract logic into utility functions when complexity increases

### Naming Functions
- Use verb-based naming that describes the action
- Prefix boolean-returning functions with `is`, `has`, or `can`
- Prefix void functions with action verbs like `execute`, `save`, `send`

### Reducing Nesting
- Use early returns to handle edge cases first
- Extract nested logic into separate functions
- Employ higher-order functions (map, filter, reduce) to minimize loops
- Prefer arrow functions for simple operations; use named functions otherwise

### Parameters
- Use default parameters instead of null checks
- When functions have multiple parameters, consolidate them into objects (RO-RO pattern)
- Keep parameter lists short (max 3-4 parameters)

## Data Handling

### Data Classes
- Use data classes for data structures
- Encapsulate primitive types in composite types when they represent domain concepts
- Validate data internally within data classes

### Immutability
- Prefer immutability for data
- Use `val` for variables that don't change
- Use immutable collections when possible
- Create new instances instead of mutating existing ones

## Classes and Objects

### Size Guidelines
- Keep classes under 200 instructions
- Limit to fewer than 10 public methods
- Limit to fewer than 10 properties

### Design Principles
- Follow SOLID principles
- Favor composition over inheritance
- Use interfaces for abstraction
- Keep classes focused on a single responsibility

## Exception Handling

### When to Use Exceptions
- Reserve exceptions for truly unexpected errors
- Don't use exceptions for control flow
- Catch exceptions only to:
  - Fix an anticipated issue
```
END THIRD-PARTY FILE 1ea7e0a2d0005f02

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