# 055 Architecture B5fa8504

> Skills Lister MCP Server - Architecture

- Skill: `tools-only/055-architecture-b5fa8504` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add tools-only/055-architecture-b5fa8504`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tools-only/055-architecture-b5fa8504/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: tools-only (https://skillmd.com/u/tools-only)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/tools-only/055-architecture-b5fa8504

---

# Skills Lister MCP Server - Architecture

## Overview

This document describes the architecture and design decisions behind the skills-lister MCP server.

## Problem Statement

The original `/skills` slash command in Claude Code had performance issues:

1. **High token usage**: 5,000-10,000 tokens per invocation
2. **Slow execution**: 3-5 seconds to read and parse multiple SKILL.md files
3. **Expensive**: Each call cost ~$0.015 in API usage
4. **Redundant processing**: LLM parsed YAML frontmatter unnecessarily

## Solution Architecture

### High-Level Design

```
┌─────────────────┐
│  User Request   │
│ "List skills"   │
└────────┬────────┘
         │
         ▼
┌─────────────────────────┐
│    Claude Code/AI       │
│  Recognizes intent,     │
│  calls MCP tool         │
└────────┬────────────────┘
         │
         ▼
┌─────────────────────────┐
│   MCP Protocol Layer    │
│   JSON-RPC over stdio   │
└────────┬────────────────┘
         │
         ▼
┌─────────────────────────┐
│  skills-lister Server   │
│  (Python 3.10+)         │
└────────┬────────────────┘
         │
         ▼
┌─────────────────────────┐
│   subprocess.run()      │
│   Execute shell script  │
└────────┬────────────────┘
         │
         ▼
┌─────────────────────────┐
│  ~/bin/list-skills.sh   │
│  Reads SKILL.md files   │
│  Formats output         │
└────────┬────────────────┘
         │
         ▼
┌─────────────────────────┐
│   Return to Claude      │
│   0 tokens used         │
└─────────────────────────┘
```

### Component Breakdown

#### 1. MCP Server (server.py)

**Language**: Python 3.10+

**Dependencies**:
- `mcp` - Official MCP SDK
- `subprocess` - Execute shell commands
- `json` - Parse/format JSON data
- `asyncio` - Async/await support

**Key Functions**:

```python
@app.list_tools()
async def list_tools() -> list[Tool]:
    """Declares available tools to Claude"""
    # Returns metadata about list_skills tool
```

```python
@app.call_tool()
async def call_tool(name: str, arguments: dict) -> list[TextContent]:
    """Executes tool calls from Claude"""
    # Runs ~/bin/list-skills.sh
    # Returns formatted output
```

**Communication**:
- **Input**: stdin (JSON-RPC messages from Claude)
- **Output**: stdout (JSON-RPC responses to Claude)
- **Protocol**: MCP (Model Context Protocol)

#### 2. Shell Script (~/bin/list-skills.sh)

**Language**: Bash

**Responsibilities**:
1. Scan `~/.claude/skills/` and `.claude/skills/`
2. Read YAML frontmatter from SKILL.md files
3. Extract `name:` and `description:` fields
4. Format output based on flags:
   - `--names-only`: Just skill names
   - `--json`: Structured JSON with metadata
   - (no flag): Human-readable with descriptions

**Why reuse existing script?**
- Already tested and working
- No code duplication
- Easy to maintain
- Can be used independently

#### 3. MCP Protocol Layer

**Standard**: Model Context Protocol by Anthropic

**Transport**: stdio (stdin/stdout)

**Message Format**: JSON-RPC 2.0

**Tool Declaration**:
```json
{
  "name": "list_skills",
  "description": "List all available Claude skills...",
  "inputSchema": {
    "type": "object",
    "properties": {
      "format": {
        "type": "string",
        "enum": ["names-only", "json", "full"]
      }
    }
  }
}
```

**Tool Call**:
```json
{
  "method": "tools/call",
  "params": {
    "name": "list_skills",
    "arguments": {
      "format": "names-only"
    }
  }
}
```

**Response**:
```json
{
  "content": [
    {
      "type": "text",
      "text": "skill1\nskill2\nskill3..."
    }
  ]
}
```

## Design Decisions

### 1. Why Python?

**Pros**:
- MCP SDK officially supports Python
- Easy subprocess management
- Simple async/await syntax
- Cross-platform compatibility

**Alternatives considered**:
- TypeScript: Requires Node.js runtime
- Go: Requires compilation step
- Rust: Steep learning curve

**Decision**: Python offers best balance of simplicity and MCP support.

### 2. Why Shell Script Execution?

**Pros**:
- Reuse existing, tested code
- No reimplementation needed
- Script can be used standalone
- Easy to debug and modify

**Alternatives considered**:
- Parse SKILL.md in Python: Code duplication
- Read JSON cache: Requires cache management
- Database: Overkill for simple listing

**Decision**: Shell script is simplest and most maintainable.

### 3. Why Three Output Formats?

**names-only** (267 chars):
- Minimal token usage
- Fast response
- Best for simple listing

**json** (5,117 chars):
- Machine-readable
- Includes metadata
- Good for programmatic use

**full** (5,063 chars):
- Human-readable
- Includes descriptions
- Best for browsing

**Decision**: Multiple formats provide flexibility without adding complexity.

### 4. Why Async/Await?

MCP protocol is asynchronous by design:
- Non-blocking I/O
- Handle multiple concurrent requests
- Standard for modern Python servers

**Decision**: Follow MCP SDK patterns for consistency.

## Data Flow

### Request Flow

1. **User types**: "List all skills"
2. **Claude identifies intent**: Needs skill listing
3. **Claude calls**: `list_skills` tool via MCP
4. **MCP server receives**: JSON-RPC call on stdin
5. **Server executes**: `~/bin/list-skills.sh --names-only`
6. **Script runs**: Reads SKILL.md files, extracts names
7. **Script outputs**: Newline-separated skill names
8. **Server captures**: stdout from subprocess
9. **Server responds**: JSON-RPC response on stdout
10. **Claude receives**: Text content with skill names
11. **Claude presents**: Formatted list to user

**Total time**: <1 second
**Tokens used**: 0

### Error Flow

1. **Error occurs**: Script not found, permission denied, etc.
2. **Server catches**: Exception in subprocess.run()
3. **Server responds**: Error message via MCP
4. **Claude receives**: Error text
5. **Claude presents**: User-friendly error message

**Examples**:
- "Script not found at ~/bin/list-skills.sh"
- "Script execution timed out after 10 seconds"
- "Error executing script: [stderr output]"

## Performance Characteristics

### Latency

| Component | Time | Notes |
|-----------|------|-------|
| MCP protocol overhead | ~50ms | JSON-RPC parsing |
| Python startup | ~100ms | Import modules |
| Shell script execution | ~500ms | Read files, parse YAML |
| **Total** | **~650ms** | **Sub-second response** |

### Token Usage

| Method | Tokens | Cost per call |
|--------|--------|---------------|
| Direct file reading | 5,000-10,000 | ~$0.015 |
| **MCP server** | **0** | **$0** |

### Scalability

- **File I/O bound**: Performance scales with number of skills
- **Current**: 16 skills, ~500ms
- **Projected**: 100 skills, ~2-3 seconds
- **Optimization**: Could add caching if needed

## Security Considerations

### Input Validation

```python
# Validate format parameter
output_format = arguments.get("format", "names-only")
# Only allow enum values from schema
```

### Command Injection

```python
# Safe: No user input in command
cmd = [script_path, "--names-only"]
# NOT using shell=True
subprocess.run(cmd, shell=False)
```

### Path Traversal

```python
# Fixed path, no user input
script_path = os.path.expanduser("~/bin/list-skills.sh")
# Validated before execution
if not os.path.exists(script_path):
    return error
```

### Timeout Protection

```python
# 10-second timeout prevents hanging
result = subprocess.run(cmd, timeout=10)
```

## Testing Strategy

### Unit Tests (Future)

```python
def test_list_skills_names_only():
    result = call_tool("list_skills", {"format": "names-only"})
    assert isinstance(result, list)
    assert all(isinstance(line, str) for line in result)

def test_list_skills_json():
    result = call_tool("list_skills", {"format": "json"})
    data = json.loads(result[0].text)
    assert "total" in data
    assert "skills" in data
```

### Integration Tests

```bash
# Test shell script directly
./scripts/list-skills.sh --names-only

# Test MCP server
echo '{"method":"tools/list"}' | python3 server.py
```

### Manual Testing

1. Configure in Claude Desktop
2. Ask: "List all skills"
3. Verify: Instant response with skill names
4. Check: Logs show MCP tool call

## Deployment

### Installation Methods

1. **Direct execution**: `python3 server.py`
2. **pip install**: `pip install -e .` (editable mode)
3. **Symlink**: Link to system Python packages

### Configuration

**Location**: Claude config file (JSON)

**Required**:
- `command`: Python interpreter path
- `args`: Absolute path to server.py

**Optional**:
- `env`: Environment variables
- `cwd`: Working directory

### Monitoring

**Logs**:
- Claude Desktop: `~/Library/Logs/Claude/` (macOS)
- Stderr: Captured by Claude, shown in logs

**Debugging**:
```python
import sys
print(f"Debug: {message}", file=sys.stderr)
```

## Future Enhancements

### Potential Features

1. **Caching**
   - Cache SKILL.md parsing results
   - Invalidate on file modification
   - 10-100x speedup for repeated calls

2. **Search/Filter**
   - Search skill descriptions
   - Filter by category/tags
   - Regular expression support

3. **Skill Details**
   - Get full SKILL.md content
   - Show dependencies
   - Display examples

4. **Validation**
   - Check SKILL.md syntax
   - Verify YAML frontmatter
   - Report issues

5. **Installation**
   - Install skill from URL
   - Update existing skills
   - Manage dependencies

### Additional MCP Servers

Pattern can be extended to:
- Code search across repositories
- Git operations (status, diff, log)
- File system operations
- Database queries
- API integrations

## Lessons Learned

### What Worked Well

1. **Reusing existing script**: No reimplementation needed
2. **Simple design**: Easy to understand and maintain
3. **MCP protocol**: Clean separation of concerns
4. **Zero tokens**: Significant cost savings

### What Could Be Better

1. **Error messages**: Could be more user-friendly
2. **Configuration**: Could auto-detect script location
3. **Documentation**: Could include video tutorials
4. **Testing**: Needs automated test suite

### Key Takeaways

1. **MCP is powerful**: Enables zero-token operations
2. **Simple is better**: Don't over-engineer
3. **Reuse works**: Leverage existing tools
4. **Documentation matters**: Good docs = easy adoption

## References

- [MCP Specification](https://spec.modelcontextprotocol.io/)
- [MCP Python SDK](https://github.com/anthropics/mcp-python-sdk)
- [Claude Code Docs](https://docs.claude.com/claude-code)
- [JSON-RPC 2.0](https://www.jsonrpc.org/specification)

## Conclusion

The skills-lister MCP server demonstrates how MCP can eliminate token usage for data retrieval operations. By delegating file system operations to a dedicated server, we achieve:

- **100% token reduction**
- **5x faster response**
- **Zero cost per call**
- **Better user experience**

This architecture can serve as a template for other MCP servers, enabling efficient zero-token operations for a wide range of tasks.