Defining tools with MCP

Overview

• Building an MCP (Model Context Protocol) server to add functionality to an existing CLI chatbot
• Server will provide tools that Claude can use to interact with documents
• Implementation goes in mcpserver. py file in the project root

MCP Python SDK Benefits

• Official MCP Python SDK simplifies server and tool creation
• Creates MCP server with just one line of code
• Automatically generates JSON schemas for tools behind the scenes
• Much easier than manually writing JSON schema definitions

Tool Definition Pattern

• Use @MCP. tool decorator to define a tool
• Specify tool name and description
• Define function with typed parameters using Pydantic Field class
• SDK automatically generates the required JSON schema

First Tool: Read Document

• Name: read_doc_contents
• Purpose: Read and return contents of a document
• Parameter: doc_id (string) - ID of document to read
• Implementation: Look up document in docs dictionary and return its contents
• Error handling: Raise ValueError if document ID doesn't exist

Second Tool: Edit Document

• Name: edit_document
• Purpose: Find and replace text within a document
• Parameters:
• doc_id (string) - ID of document to edit
• old_string (string) - Text to find (must match exactly including whitespace)
• new_string (string) - Replacement text
• Implementation: Use Python string . replace() method
• Error handling: Raise ValueError if document ID doesn't exist

Key Imports

• from pydantic import Field - For defining tool parameters with descriptions

Takeaways

• MCP Python SDK simplifies server creation and automatically generates JSON schemas for tools, eliminating manual schema writing
• Use @MCP. tool decorator with typed Pydantic Field parameters to define tools that Claude can call
• read_doc_contents tool retrieves document text by doc_id from a docs dictionary, raising ValueError if ID doesn't exist
• edit_document tool finds and replaces exact text matches within documents using Python's . replace() method
• Both tools require error handling for invalid document IDs