Tool use with Claude

Claude is capable of interacting with tools and functions, allowing you to extend Claude's capabilities to perform a wider variety of tasks.

Here's an example of how to provide tools to Claude using the Messages API:

curl https://api.anthropic.com/v1/messages \
  -H "content-type: application/json" \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-sonnet-4-5",
    "max_tokens": 1024,
    "tools": [
      {
        "name": "get_weather",
        "description": "Get the current weather in a given location",
        "input_schema": {
          "type": "object",
          "properties": {
            "location": {
              "type": "string",
              "description": "The city and state, e.g. San Francisco, CA"
            }
          },
          "required": ["location"]
        }
      }
    ],
    "messages": [
      {
        "role": "user",
        "content": "What is the weather like in San Francisco?"
      }
    ]
  }'

How tool use works

Claude supports two types of tools:

1. Client tools: Tools that execute on your systems, which include:

   • User-defined custom tools that you create and implement
   • Anthropic-defined tools like computer use and text editor that require client implementation

2. Server tools: Tools that execute on Anthropic's servers, like the web search and web fetch tools. These tools must be specified in the API request but don't require implementation on your part.

Client tools

Integrate client tools with Claude in these steps:

Note: Steps 3 and 4 are optional. For some workflows, Claude's tool use request (step 2) might be all you need, without sending results back to Claude.

Server tools

Server tools follow a different workflow:

Using MCP tools with Claude

If you're building an application that uses the Model Context Protocol (MCP), you can use tools from MCP servers directly with Claude's Messages API. MCP tool definitions use a schema format that's similar to Claude's tool format. You just need to rename inputSchema to input_schema.

Converting MCP tools to Claude format

When you build an MCP client and call list_tools() on an MCP server, you'll receive tool definitions with an inputSchema field. To use these tools with Claude, convert them to Claude's format:

from mcp import ClientSession

async def get_claude_tools(mcp_session: ClientSession):
    """Convert MCP tools to Claude's tool format."""
    mcp_tools = await mcp_session.list_tools()

    claude_tools = []
    for tool in mcp_tools.tools:
        claude_tools.append({
            "name": tool.name,
            "description": tool.description or "",
            "input_schema": tool.inputSchema  # Rename inputSchema to input_schema
        })

    return claude_tools

Then pass these converted tools to Claude:

import anthropic

client = anthropic.Anthropic()
claude_tools = await get_claude_tools(mcp_session)

response = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1024,
    tools=claude_tools,
    messages=[{"role": "user", "content": "What tools do you have available?"}]
)

When Claude responds with a tool_use block, execute the tool on your MCP server using call_tool() and return the result to Claude in a tool_result block.

For a complete guide to building MCP clients, see Build an MCP client.

Tool use examples

Here are a few code examples demonstrating various tool use patterns and techniques. For brevity's sake, the tools are simple tools, and the tool descriptions are shorter than would be ideal to ensure best performance.

Pricing

Tool use requests are priced based on:

1. The total number of input tokens sent to the model (including in the tools parameter)
2. The number of output tokens generated
3. For server-side tools, additional usage-based pricing (e.g., web search charges per search performed)

Client-side tools are priced the same as any other Claude API request, while server-side tools may incur additional charges based on their specific usage.

The additional tokens from tool use come from:

• The tools parameter in API requests (tool names, descriptions, and schemas)
• tool_use content blocks in API requests and responses
• tool_result content blocks in API requests

When you use tools, we also automatically include a special system prompt for the model which enables tool use. The number of tool use tokens required for each model are listed below (excluding the additional tokens listed above). Note that the table assumes at least 1 tool is provided. If no tools are provided, then a tool choice of none uses 0 additional system prompt tokens.

These token counts are added to your normal input and output tokens to calculate the total cost of a request.

Refer to our models overview table for current per-model prices.

When you send a tool use prompt, just like any other API request, the response will output both input and output token counts as part of the reported usage metrics.

Next Steps

Explore our repository of ready-to-implement tool use code examples in our cookbooks: