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Purpose

MCP Tool Template provides a standardized structure for creating skills that wrap Model Context Protocol (MCP) tools. This template demonstrates how to integrate external MCP servers into the MAGSAG framework while maintaining contract validation, error handling, and observability requirements.

When to Use

• You need to integrate an external MCP tool as a reusable skill
• The MCP server is configured in .mcp/servers/<server-id>.yaml
• Input and output data structures are defined via JSON Schema contracts
• The tool will be invoked by agents or other orchestration workflows

Prerequisites

• MCP server configured in .mcp/servers/<server-id>.yaml with appropriate scopes
• Input schema defined at catalog/contracts/<input-contract>.json
• Output schema defined at catalog/contracts/<output-contract>.json
• Environment variables required by the MCP server (if any) are set
• Rate limits configured in the MCP server configuration

Procedures

Procedure 1: Invoke MCP Tool

1. Validate Input - Ensure the input payload conforms to the input schema contract
2. Prepare Request - Extract relevant fields from the payload and format them for the MCP tool
3. Call MCP Tool - Invoke the MCP server tool with prepared arguments
4. Handle Response - Process the MCP tool response and transform it to match the output schema
5. Error Handling - Catch and handle common errors (rate limits, timeouts, validation failures)
6. Validate Output - Ensure the output conforms to the output schema contract

Procedure 2: Fallback Behavior

1. If the MCP tool is unavailable, return a structured error response
2. Log the failure with appropriate context for debugging
3. Consider returning cached results if applicable
4. Emit warnings when operating in degraded mode

Examples

Example 1: Basic Tool Invocation

• Input: resources/examples/in.json
• Process:
  1. Validate input against schema
  2. Call MCP tool with extracted parameters
  3. Transform response to output format
  4. Validate output against schema
• Output: resources/examples/out.json

Implementation Notes

File Structure

<skill-name>/
├── SKILL.md                    # This file
├── impl/
│   └── mcp_tool.py            # Tool implementation
├── resources/
│   └── examples/
│       ├── in.json            # Sample input
│       └── out.json           # Sample output
└── tests/
    └── test_mcp_tool.py       # Unit tests

Key Functions

• async def run(payload: dict[str, Any], *, mcp: Optional[MCPRuntime] = None) -> dict[str, Any] – Main entry point with optional MCP runtime
• _validate(payload: dict[str, Any], schema: dict[str, Any], name: str) -> None – Input/output validation helper
• _prepare_request(payload: dict[str, Any]) -> dict[str, Any] – Request transformation helper
• async def _call_mcp_tool(**kwargs) -> Any – Local or mocked execution path when MCP runtime is unavailable
• async def _call_tool_via_runtime(mcp: MCPRuntime, request: dict[str, Any]) -> Any – Preferred MCP runtime integration path
• _process_response(raw_response: Any, payload: dict[str, Any]) -> dict[str, Any] – Response normalization helper

Development Checklist

• Copy template to catalog/skills/<your-skill-name>/
• Update name, description, and mcp.server_ref in SKILL.md frontmatter
• Define input/output contracts in catalog/contracts/
• Implement _call_mcp_tool() in impl/mcp_tool.py
• Create example payloads in resources/examples/
• Write tests in tests/test_mcp_tool.py
• Run mypy --strict and ruff to ensure code quality
• Run pytest to verify all tests pass
• Register skill in catalog/registry/skills.yaml if needed

Additional Resources

• .mcp/README.md - MCP server configuration guide
• docs/guides/agent-development.md - Skill development guidelines
• MCP Official Documentation: https://modelcontextprotocol.io/

Troubleshooting

• MCP Server Not Found: Verify the server is configured in .mcp/servers/<server-id>.yaml
• Rate Limit Exceeded: Check the limits.rate_per_min setting in SKILL.md and MCP server config
• Schema Validation Errors: Ensure contracts match actual payload structure
• Timeout Errors: Increase latency threshold or optimize MCP tool query
• Missing Environment Variables: Check MCP server requirements in .env.example