MCP Server Primitives: Tools

Tools are one of MCP's three server primitives, representing executable actions that an agent can discover and invoke through the protocol. Unlike resources (which expose data for reading) and prompts (which provide templates for interaction), tools are the active capability — they let agents take actions like querying a database, creating a file, sending a message, or triggering a deployment. Each tool is defined with a name, description, and JSON Schema for its input parameters, following the same patterns as native function calling but standardized across the MCP protocol so any conforming client can use any conforming server's tools. Understanding MCP tools specifically is important because they represent the primary mechanism by which agents affect the external world through the protocol layer. This concept connects to function calling for the underlying mechanism tools build upon, tool definition patterns for best practices in designing MCP tool schemas, and MCP server primitives resources and prompts for the other two primitive types.