Custom tools allow you to extend Claude Code’s capabilities with your own functionality through in-process MCP servers, enabling Claude to interact with external services, APIs, or perform specialized operations.

​
Creating Custom Tools

Use the createSdkMcpServer and tool helper functions to define type-safe custom tools:

import { query, tool, createSdkMcpServer } from "@anthropic-ai/claude-code";
import { z } from "zod";

// Create an SDK MCP server with custom tools
const customServer = createSdkMcpServer({
  name: "my-custom-tools",
  version: "1.0.0",
  tools: [
    tool(
      "get_weather",
      "Get current weather for a location",
      {
        location: z.string().describe("City name or coordinates"),
        units: z.enum(["celsius", "fahrenheit"]).default("celsius").describe("Temperature units")
      },
      async (args) => {
        // Call weather API
        const response = await fetch(
          `https://api.weather.com/v1/current?q=${args.location}&units=${args.units}`
        );
        const data = await response.json();
        
        return {
          content: [{
            type: "text",
            text: `Temperature: ${data.temp}°\nConditions: ${data.conditions}\nHumidity: ${data.humidity}%`
          }]
        };
      }
    )
  ]
});

​
Using Custom Tools

Pass the custom server to the query function via the mcpServers option as a dictionary/object.

Important: Custom MCP tools require streaming input mode. You must use an async generator/iterable for the prompt parameter - a simple string will not work with MCP servers.

​
Tool Name Format

When MCP tools are exposed to Claude, their names follow a specific format:

  • Pattern: mcp__{server_name}__{tool_name}
  • Example: A tool named get_weather in server my-custom-tools becomes mcp__my-custom-tools__get_weather

​
Configuring Allowed Tools

You can control which tools Claude can use via the allowedTools option:

import { query } from "@anthropic-ai/claude-code";

// Use the custom tools in your query with streaming input
async function* generateMessages() {
  yield {
    type: "user" as const,
    message: {
      role: "user" as const,
      content: "What's the weather in San Francisco?"
    }
  };
}

for await (const message of query({
  prompt: generateMessages(),  // Use async generator for streaming input
  options: {
    mcpServers: {
      "my-custom-tools": customServer  // Pass as object/dictionary, not array
    },
    // Optionally specify which tools Claude can use
    allowedTools: [
      "mcp__my-custom-tools__get_weather",  // Allow the weather tool
      // Add other tools as needed
    ],
    maxTurns: 3
  }
})) {
  if (message.type === "result" && message.subtype === "success") {
    console.log(message.result);
  }
}

​
Multiple Tools Example

When your MCP server has multiple tools, you can selectively allow them:

const multiToolServer = createSdkMcpServer({
  name: "utilities",
  version: "1.0.0",
  tools: [
    tool("calculate", "Perform calculations", { /* ... */ }, async (args) => { /* ... */ }),
    tool("translate", "Translate text", { /* ... */ }, async (args) => { /* ... */ }),
    tool("search_web", "Search the web", { /* ... */ }, async (args) => { /* ... */ })
  ]
});

// Allow only specific tools with streaming input
async function* generateMessages() {
  yield {
    type: "user" as const,
    message: {
      role: "user" as const,
      content: "Calculate 5 + 3 and translate 'hello' to Spanish"
    }
  };
}

for await (const message of query({
  prompt: generateMessages(),  // Use async generator for streaming input
  options: {
    mcpServers: {
      utilities: multiToolServer
    },
    allowedTools: [
      "mcp__utilities__calculate",   // Allow calculator
      "mcp__utilities__translate",   // Allow translator
      // "mcp__utilities__search_web" is NOT allowed
    ]
  }
})) {
  // Process messages
}

​
Type Safety with Python

The @tool decorator supports various schema definition approaches for type safety:

import { z } from "zod";

tool(
  "process_data",
  "Process structured data with type safety",
  {
    // Zod schema defines both runtime validation and TypeScript types
    data: z.object({
      name: z.string(),
      age: z.number().min(0).max(150),
      email: z.string().email(),
      preferences: z.array(z.string()).optional()
    }),
    format: z.enum(["json", "csv", "xml"]).default("json")
  },
  async (args) => {
    // args is fully typed based on the schema
    // TypeScript knows: args.data.name is string, args.data.age is number, etc.
    console.log(`Processing ${args.data.name}'s data as ${args.format}`);
    
    // Your processing logic here
    return {
      content: [{
        type: "text",
        text: `Processed data for ${args.data.name}`
      }]
    };
  }
)

​
Error Handling

Handle errors gracefully to provide meaningful feedback:

tool(
  "fetch_data",
  "Fetch data from an API",
  {
    endpoint: z.string().url().describe("API endpoint URL")
  },
  async (args) => {
    try {
      const response = await fetch(args.endpoint);
      
      if (!response.ok) {
        return {
          content: [{
            type: "text",
            text: `API error: ${response.status} ${response.statusText}`
          }]
        };
      }
      
      const data = await response.json();
      return {
        content: [{
          type: "text",
          text: JSON.stringify(data, null, 2)
        }]
      };
    } catch (error) {
      return {
        content: [{
          type: "text",
          text: `Failed to fetch data: ${error.message}`
        }]
      };
    }
  }
)

​
Example Tools

​
Database Query Tool

const databaseServer = createSdkMcpServer({
  name: "database-tools",
  version: "1.0.0",
  tools: [
    tool(
      "query_database",
      "Execute a database query",
      {
        query: z.string().describe("SQL query to execute"),
        params: z.array(z.any()).optional().describe("Query parameters")
      },
      async (args) => {
        const results = await db.query(args.query, args.params || []);
        return {
          content: [{
            type: "text",
            text: `Found ${results.length} rows:\n${JSON.stringify(results, null, 2)}`
          }]
        };
      }
    )
  ]
});

​
API Gateway Tool

const apiGatewayServer = createSdkMcpServer({
  name: "api-gateway",
  version: "1.0.0",
  tools: [
    tool(
      "api_request",
      "Make authenticated API requests to external services",
      {
        service: z.enum(["stripe", "github", "openai", "slack"]).describe("Service to call"),
        endpoint: z.string().describe("API endpoint path"),
        method: z.enum(["GET", "POST", "PUT", "DELETE"]).describe("HTTP method"),
        body: z.record(z.any()).optional().describe("Request body"),
        query: z.record(z.string()).optional().describe("Query parameters")
      },
      async (args) => {
        const config = {
          stripe: { baseUrl: "https://api.stripe.com/v1", key: process.env.STRIPE_KEY },
          github: { baseUrl: "https://api.github.com", key: process.env.GITHUB_TOKEN },
          openai: { baseUrl: "https://api.openai.com/v1", key: process.env.OPENAI_KEY },
          slack: { baseUrl: "https://slack.com/api", key: process.env.SLACK_TOKEN }
        };
        
        const { baseUrl, key } = config[args.service];
        const url = new URL(`${baseUrl}${args.endpoint}`);
        
        if (args.query) {
          Object.entries(args.query).forEach(([k, v]) => url.searchParams.set(k, v));
        }
        
        const response = await fetch(url, {
          method: args.method,
          headers: { Authorization: `Bearer ${key}`, "Content-Type": "application/json" },
          body: args.body ? JSON.stringify(args.body) : undefined
        });
        
        const data = await response.json();
        return {
          content: [{
            type: "text",
            text: JSON.stringify(data, null, 2)
          }]
        };
      }
    )
  ]
});

​
Calculator Tool

const calculatorServer = createSdkMcpServer({
  name: "calculator",
  version: "1.0.0",
  tools: [
    tool(
      "calculate",
      "Perform mathematical calculations",
      {
        expression: z.string().describe("Mathematical expression to evaluate"),
        precision: z.number().optional().default(2).describe("Decimal precision")
      },
      async (args) => {
        try {
          // Use a safe math evaluation library in production
          const result = eval(args.expression); // Example only!
          const formatted = Number(result).toFixed(args.precision);
          
          return {
            content: [{
              type: "text",
              text: `${args.expression} = ${formatted}`
            }]
          };
        } catch (error) {
          return {
            content: [{
              type: "text",
              text: `Error: Invalid expression - ${error.message}`
            }]
          };
        }
      }
    ),
    tool(
      "compound_interest",
      "Calculate compound interest for an investment",
      {
        principal: z.number().positive().describe("Initial investment amount"),
        rate: z.number().describe("Annual interest rate (as decimal, e.g., 0.05 for 5%)"),
        time: z.number().positive().describe("Investment period in years"),
        n: z.number().positive().default(12).describe("Compounding frequency per year")
      },
      async (args) => {
        const amount = args.principal * Math.pow(1 + args.rate / args.n, args.n * args.time);
        const interest = amount - args.principal;
        
        return {
          content: [{
            type: "text",
            text: `Investment Analysis:\n` +
                  `Principal: $${args.principal.toFixed(2)}\n` +
                  `Rate: ${(args.rate * 100).toFixed(2)}%\n` +
                  `Time: ${args.time} years\n` +
                  `Compounding: ${args.n} times per year\n\n` +
                  `Final Amount: $${amount.toFixed(2)}\n` +
                  `Interest Earned: $${interest.toFixed(2)}\n` +
                  `Return: ${((interest / args.principal) * 100).toFixed(2)}%`
          }]
        };
      }
    )
  ]
});