Module 5: Local Server Configs & Python SDK
Mount servers in Claude Desktop JSON configs. Build calculator and expense tracker MCP servers using FastMCP SDK.
Day 9
Claude Desktop Configuration
Why this matters
Desktop Config: Desktop config is how hosts discover and launch your server process.
On macOS, Claude Desktop reads ~/Library/Application Support/Claude/claude_desktop_config.json. Each entry under mcpServers defines how to spawn a server:
{
"mcpServers": {
"weather": {
"command": "uv",
"args": ["run", "weather-mcp-server"],
"env": {
"API_KEY": "your-key-here"
}
}
}
}command+args— executable and flags (oftenuv run).env— secrets and config (never commit real keys).- Restart Claude after editing the config.
Common mistakes
- Confusing host with server (the host runs the client; the server is a separate process).
- Hard-coding API keys inside tool implementations instead of env vars.
- Using SSE locally when stdio is simpler and more secure.
Interview checkpoints
- Q: What is desktop config in MCP? A: One-sentence definition + when it runs in the lifecycle.
- Q: One production pitfall? A: Name transport, auth, or schema mismatch.
Practice
- Basic: Sketch Desktop Config on a whiteboard.
- Intermediate: Find it in a real Claude Desktop or Cursor config.
- Advanced: Break it on purpose and document the error message.
Recap
- You can explain desktop config clearly.
- You know host vs client vs server roles.
- You see how this connects to the next part.
Next: FastMCP SDK
Day 10
FastMCP Server SDK
Why this matters
FastMCP SDK: FastMCP removes boilerplate so you expose tools with decorators, not raw JSON-RPC handlers.
FastMCP is the high-level Python SDK: decorators register tools; the framework handles JSON-RPC plumbing.
from mcp.server.fastmcp import FastMCP
mcp = FastMCP("Calculator")
@mcp.tool()
def add_values(x: float, y: float) -> float:
"""Adds two numbers."""
return x + y
if __name__ == "__main__":
mcp.run()Type hints and docstrings become tool descriptions for the model. Run with mcp.run() using stdio transport by default.
Common mistakes
- Confusing host with server (the host runs the client; the server is a separate process).
- Hard-coding API keys inside tool implementations instead of env vars.
- Using SSE locally when stdio is simpler and more secure.
Interview checkpoints
- Q: What is fastmcp sdk in MCP? A: One-sentence definition + when it runs in the lifecycle.
- Q: One production pitfall? A: Name transport, auth, or schema mismatch.
Practice
- Basic: Sketch FastMCP SDK on a whiteboard.
- Intermediate: Find it in a real Claude Desktop or Cursor config.
- Advanced: Break it on purpose and document the error message.
Recap
- You can explain fastmcp sdk clearly.
- You know host vs client vs server roles.
- You see how this connects to the next part.
Next: SSE Proxies