A quick reference to common MCP terms.
Core Roles
- Host: the AI application the user works in.
- Client: the host's connection to a single server.
- Server: a program exposing capabilities over MCP.
Server Capabilities
- Tool: a function the model can call.
- Resource: data identified by a URI that the application can read.
- Resource template: a parameterised URI pattern.
- Prompt: a reusable template users can invoke.
Client Capabilities
- Sampling: a server requesting a model completion via the host.
- Elicitation: a server requesting information from the user via the host.
- Roots: locations the client indicates are in scope.
Protocol Terms
- JSON-RPC: the message format used for requests, responses and notifications.
- Initialisation: the handshake exchanging versions and capabilities.
- Notification: a one-way message needing no response.
- Capability negotiation: agreeing which features both sides support.
Transports
- stdio: messages over a subprocess's standard input and output.
- Streamable HTTP: messages over HTTP, with streaming for responses.
Security
- OAuth: the framework used for authorising remote servers.
- Tool poisoning: malicious instructions hidden in tool metadata.