Claude Skills: Modular Prompt Engineering for Backend Engineers | Software Engineering Blog

What are Claude Skills?

Claude Skills are an extension feature that injects specific work procedures or expert knowledge into Claude as a "package" format, allowing it to be loaded dynamically only when needed.

Unlike general prompt engineering where you write everything in a single prompt, Skills exist as independent modules. Claude loads the skill definition (SKILL.md) and scripts into context only when it determines that a user's request is related to a specific skill.

Think of it like...

Skills are like work manuals for Claude. Instead of explaining every procedure each time, you give Claude a reference manual it can consult when needed.

Core Concepts

Skill Structure Components

1.
Metadata: Name, description, trigger conditions
2.
Instructions: Acts as System Prompt - defines how Claude should behave
3.
Executable Code: Python, Bash, or other scripts that can be run
4.
Reference Files: Documentation, templates, examples

Token Efficiency

Skills are loaded only when needed, not all at once. This saves tokens and reduces hallucinations.

Reusability

Managed as SKILL.md files, making them easy to version control with Git and share with team members.

Dynamic Loading

Claude automatically determines when to load a skill based on conversation context.

Skills vs MCP: Key Differences

As a backend developer, you might confuse Skills with MCP (Model Context Protocol). These two features are complementary, not competing.

Aspect	Skills	MCP
Purpose	"How" - Methods & Procedures	"What" - Connections & Data
Role	Define specific task workflows	Real-time connection to external systems
Analogy	Work Manual (Task Guidelines)	USB Port (Tool & Equipment Connection)
Example	"Follow this checklist for PR review"	"Query my local DB", "Fetch GitHub issues"

When to Use Each

Use Skills when...

Defining team coding conventions
Creating deployment procedure guides
Standardizing code review checklists
Documenting troubleshooting workflows

Use MCP when...

Querying EKS cluster status
Searching internal Wiki
Accessing GitHub/GitLab APIs
Reading local files or databases

Practical Examples for Backend Engineers

1 Code Review Skill

Register a skill containing your team's Kotlin style guide and architecture principles. When you say "Review this code", Claude automatically loads the guide and reviews according to your standards.

code-review-skill/SKILL.md

# Code Review Skill

## When to Use
- User asks for code review
- User shares code snippets for feedback

## Instructions
1. Check for SOLID principles violations
2. Verify naming conventions (camelCase for variables)
3. Look for potential null pointer issues
4. Suggest Kotlin idiomatic improvements
5. Check for proper error handling

## Checklist
- [ ] No magic numbers
- [ ] Functions are single-responsibility
- [ ] Proper logging implemented
- [ ] Unit tests exist for business logic

2 Troubleshooting Skill

Define a checklist for analyzing error logs (DB connection check → Network check → etc.). During incident response, Claude follows that procedure to help analyze issues.

troubleshooting-skill/SKILL.md

# Troubleshooting Skill

## When to Use
- User reports an error or exception
- User shares error logs

## Diagnostic Procedure
1. **Identify Error Type**
   - Is it a timeout? Connection error? Business logic error?

2. **Check Infrastructure Layer**
   - Database connection pool status
   - Network connectivity (DNS, firewall)
   - Resource limits (CPU, memory)

3. **Check Application Layer**
   - Recent deployments
   - Configuration changes
   - Dependency updates

4. **Suggest Solutions**
   - Immediate mitigation
   - Root cause fix
   - Prevention measures

3 API Design Skill

Encode your team's REST API design guidelines. When designing new endpoints, Claude follows your conventions automatically.

api-design-skill/SKILL.md

# API Design Skill

## URL Conventions
- Use plural nouns: /users, /orders
- Use kebab-case: /order-items
- Version in path: /api/v1/...

## Response Format
{
  "success": true,
  "data": { ... },
  "error": null,
  "pagination": { "page": 1, "size": 20, "total": 100 }
}

## HTTP Status Codes
- 200: Success
- 201: Created
- 400: Bad Request (validation error)
- 401: Unauthorized
- 403: Forbidden
- 404: Not Found
- 500: Internal Server Error

Skill File Structure

A typical skill is organized as a folder containing related files:

Skill Directory Structure

my-custom-skill/
├── SKILL.md           # Main skill definition (required)
├── instructions.md    # Detailed instructions
├── examples/          # Example files
│   ├── good-example.kt
│   └── bad-example.kt
├── templates/         # Reusable templates
│   └── pr-template.md
└── scripts/           # Executable scripts
    └── validate.sh

SKILL.md Template

# Skill Name

## Description
Brief description of what this skill does.

## When to Activate
- Trigger condition 1
- Trigger condition 2

## Instructions
Step-by-step instructions for Claude to follow.

## Reference Files
- @examples/good-example.kt
- @templates/pr-template.md

## Output Format
Define expected output format here.

Best Practices for Writing Skills

✓

Be Specific with Triggers

Clearly define when the skill should activate. Vague triggers lead to unexpected behavior.

✓

Include Examples

Good and bad examples help Claude understand your expectations better than abstract rules.

✓

Keep It Focused

One skill = one responsibility. Don't create a "mega skill" that does everything.

✓

Version Control

Store skills in Git. Track changes, review updates, and share with your team.

✗

Avoid Sensitive Data

Never include API keys, passwords, or sensitive credentials in skill files.

How to Use Skills

In Claude Desktop App

Go to Settings → Capabilities → Skills
Enable the Skills feature
Choose from Built-in Skills or upload Custom Skills

Built-in Skills

• Excel/PDF generation
• Data analysis
• Code formatting
• And more...

Custom Skills

• Upload your skill folder
• Team-specific workflows
• Domain expertise encoding
• Company conventions

Pro Tip

Combine Skills with MCP for maximum productivity. Use Skills to define how to work, and MCP to provide what to work with (data, tools, connections).

Conclusion

Key Takeaways

✓ Skills = Work Manuals: Reusable prompt modules loaded on-demand
✓ Skills vs MCP: Skills define "how", MCP provides "what"
✓ Token Efficient: Only loaded when needed
✓ Team Friendly: Version control and share via Git

For backend engineers, Skills provide a powerful way to encode team knowledge, standardize workflows, and ensure consistent quality across code reviews, troubleshooting, and development practices. Start with one or two simple skills and expand as you discover more use cases.

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