Tools

The Model Context Protocol (MCP) allows servers to expose tools that can be invoked by language models. Tools enable models to interact with external systems, such as querying databases, calling APIs, or performing computations. Each tool is uniquely identified by a name and includes metadata describing its schema.

For brevity, the request examples on this page omit the _meta request metadata (io.modelcontextprotocol/protocolVersion, io.modelcontextprotocol/clientInfo, and io.modelcontextprotocol/clientCapabilities). Every request MUST include the required _meta fields; see _meta.

User Interaction Model

Tools in MCP are designed to be model-controlled, meaning that the language model can discover and invoke tools automatically based on its contextual understanding and the user’s prompts. However, implementations are free to expose tools through any interface pattern that suits their needs—the protocol itself does not mandate any specific user interaction model.

For trust & safety and security, there SHOULD always be a human in the loop with the ability to deny tool invocations.Applications SHOULD:

Provide UI that makes clear which tools are being exposed to the AI model
Insert clear visual indicators when tools are invoked
Present confirmation prompts to the user for operations, to ensure a human is in the loop

Capabilities

Servers that support tools MUST declare the tools capability:

{
  "capabilities": {
    "tools": {
      "listChanged": true
    }
  }
}

listChanged indicates whether the server will emit notifications when the list of available tools changes. Servers that declare the tools capability MUST respond to tools/list requests with the set of tools currently available to the requesting client. This set MAY be empty and MAY change over time (see List Changed Notification), but MUST NOT vary per-connection or as a side effect of other requests on the connection. The set MAY vary by the authorization presented on the request — for example, returning only the tools the caller’s granted scopes permit — since credentials are per-request input, not connection state. Servers SHOULD return tools in a deterministic order (i.e., the same ordering across requests when the underlying set of tools has not changed). Deterministic ordering enables clients to reliably cache the tool list and improves LLM prompt cache hit rates when tools are included in model context.

Protocol Messages

Listing Tools

To discover available tools, clients send a tools/list request. This operation supports pagination and caching. Request:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/list",
  "params": {
    "cursor": "optional-cursor-value"
  }
}

Response:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "resultType": "complete",
    "tools": [
      {
        "name": "get_weather",
        "title": "Weather Information Provider",
        "description": "Get current weather information for a location",
        "inputSchema": {
          "type": "object",
          "properties": {
            "location": {
              "type": "string",
              "description": "City name or zip code"
            }
          },
          "required": ["location"]
        },
        "icons": [
          {
            "src": "https://example.com/weather-icon.png",
            "mimeType": "image/png",
            "sizes": ["48x48"]
          }
        ]
      }
    ],
    "nextCursor": "next-page-cursor",
    "ttlMs": 300000,

Base Protocol

Client Features

Server Features

User Interaction Model

Capabilities

Protocol Messages

Listing Tools

​User Interaction Model

​Capabilities

​Protocol Messages

​Listing Tools

User Interaction Model

Capabilities

Protocol Messages

Listing Tools