MCP glossary

A one-line definition for every MCP term in this guide, so you can look one up without rereading a whole page.

Core roles #

• Host: the AI application the user interacts with; contains clients.
• Client: the connector inside a host that holds one connection to one server.
• Server: a program exposing capabilities over MCP.

Primitives #

• Tool: a model-callable function with side effects or computed results.
• Resource: read-only data the application supplies as context, addressed by URI.
• Prompt: a user-invoked, parameterized message template.
• Sampling: a server requesting a model completion from the client.
• Roots: the files or directories a client permits a server to work within.
• Elicitation: a server asking the user for input mid-operation.

Protocol and transport #

• JSON-RPC 2.0: the message format MCP uses.
• Initialize: the handshake that negotiates capabilities.
• stdio: local transport over standard input and output.
• Streamable HTTP: the current remote transport.
• Capability negotiation: each side declaring what it supports at connect time.