MCP and AI agent glossary
The protocol words behind AI agents that use tools: what an MCP server, client and host are, how a tool call travels over Streamable HTTP, what tool annotations promise, and the web conventions agents look for (llms.txt, agent cards, HTTP 402 and x402). Definitions follow the official specifications, linked on each entry.
- Model Context Protocol (MCP)An open protocol, introduced by Anthropic in November 2024, that lets AI applications connect to external tools and data through a standard interface.
- MCP serverA program that exposes tools, resources or prompts to AI applications over the Model Context Protocol.
- MCP client and hostThe host is the AI application (an IDE, a desktop chat app, a CLI agent); inside it, one MCP client holds the connection to each MCP server.
- Tool call (tools/list, tools/call)How an AI model uses an MCP tool: the client lists the server's tools with tools/list, the model picks one, and the client runs it with tools/call and returns the result to the model.
- Tool annotationsOptional hints on an MCP tool that describe its behaviour: readOnlyHint, destructiveHint, idempotentHint and openWorldHint.
- Streamable HTTP transportThe MCP transport for remote servers: one HTTPS endpoint (such as https://example.com/mcp) that receives JSON-RPC messages as POST requests and may stream replies as server-sent events.
- stdio transportThe MCP transport for local servers: the client starts the server as a subprocess and they exchange newline-delimited JSON-RPC messages over standard input and output.
- JSON-RPC 2.0The small remote-procedure-call format MCP uses: requests with jsonrpc, id, method and params; responses with the same id and either result or error.
- MCP RegistryThe official, open catalogue of MCP servers at registry.modelcontextprotocol.io, where publishers list a server.json describing how to install or connect each server.
- HTTP 402 Payment RequiredAn HTTP status code reserved for payment that HTTP itself never defined in detail, now used by agent payment schemes such as x402.
- x402An open payment standard, introduced by Coinbase in 2025, that uses HTTP 402 so an API can charge per request and a client or AI agent can pay instantly, typically in stablecoins.
- llms.txtA proposed Markdown file at /llms.txt that gives AI models a short, curated map of a website: a title, a one-paragraph summary and lists of key links.
- Agent card (A2A)A JSON document, served at /.well-known/agent-card.json, that describes an AI agent for the Agent2Agent (A2A) protocol: its name, skills, interfaces, capabilities and authentication.