MCP glossary

Definitions follow the current MCP specification, revision 2026-07-28. Where a term belongs only to older revisions, the entry says so, because many servers in production still use them. For a narrative introduction, start with What is MCP?.

Terms A to F

API key

A secret string that identifies the caller to a service, usually sent in a request header. Some MCP servers accept an API key instead of OAuth; it is simpler but is not tied to a particular user's consent or scopes.

Bearer token

An access token sent in the Authorization: Bearer <token> header. Whoever holds it can use it, so it must be kept out of logs, URLs and shared links.

Cancellation

Stopping a request that is still running. Over Streamable HTTP, closing the request's response stream is the cancellation signal; over stdio, the client sends a notifications/cancelled message with the request ID.

Capabilities

The optional features each side declares it supports, such as tools, resources or prompts on a server. In the current revision a client sends its capabilities with every request, and a server advertises its own through server/discover.

Catalog

A curated list of servers that people can browse and connect to. The mcptest.io catalog lists public remote servers with their publisher, sign-in method and latest validation result.

Client

The component inside a host application that holds one connection to one MCP server. A host connected to three servers runs three clients.

Conformance

Whether an implementation behaves as the specification requires. The MCP project maintains a conformance test framework that checks clients and servers against the specification.

CORS

Cross-Origin Resource Sharing, the browser rule that decides whether a web page may read a response from another domain. A remote MCP server must send the right CORS headers before a browser-based client can call it directly instead of through a proxy.

Drift

A change in a server's behaviour or definitions between two points in time, such as a renamed tool, a new required argument or a different authentication requirement. Drift can break clients silently, which is why reports are worth comparing over time.

Dynamic client registration

An OAuth mechanism (RFC 7591) that lets a client register itself with an authorization server automatically. The 2026-07-28 revision deprecates it in favour of Client ID Metadata Documents, but keeps it for compatibility.

Elicitation

A way for a server to ask the user for information in the middle of an operation, through a form or by sending them to a URL. In the current revision it is carried inside an input_required result rather than as a separate server-to-client request.

Terms H to O

Host

The application a person uses, such as a chat app, a code editor or an agent framework. The host runs the clients, passes tool definitions to the model and decides what needs user approval.

Initialize

The handshake that opened every connection in revisions up to 2025-11-25: the client sent initialize, the server replied with its capabilities, and the client confirmed with notifications/initialized. The 2026-07-28 revision removes it.

JSON-RPC

The message format MCP uses: small JSON objects with a method, params and an id, answered by a result or an error. It is version 2.0 of a long-standing remote procedure call convention.

Logging

Log messages a server sends to the client with notifications/message. The current revision deprecates protocol-level logging and recommends standard error output or OpenTelemetry instead.

MCP-Protocol-Version

The HTTP header that states which protocol revision a request uses, for example MCP-Protocol-Version: 2026-07-28. It is required on every Streamable HTTP request since revision 2025-06-18.

Mcp-Session-Id

The HTTP header a server used to assign a session in Streamable HTTP revisions 2025-03-26 to 2025-11-25. The 2026-07-28 revision removes protocol-level sessions, so current servers do not issue it.

Notifications

One-way JSON-RPC messages that expect no reply, such as progress updates or a notice that the tool list changed. In the current revision, change notifications arrive on a stream the client opens with subscriptions/listen.

OAuth 2.1

The authorization framework MCP uses for remote servers. The user signs in at an authorization server, approves the requested access, and the client receives a short-lived access token for one specific MCP server.

Output schema

An optional JSON Schema on a tool that describes the shape of its structured result. If a tool declares one, the server must return structured content that matches it, and clients should validate it.

Terms P to R

PKCE

Proof Key for Code Exchange (RFC 7636), pronounced "pixie". The client creates a one-time secret before sign-in and proves it holds that secret when exchanging the authorization code, so an intercepted code is useless to anyone else.

Preflight

The OPTIONS request a browser sends before a cross-origin request that uses custom headers or methods. If the server does not answer it with the right CORS headers, the browser blocks the real request.

Progress

Notifications a server sends while a long request runs, carrying how much work is done and an optional message. Clients can use them to show a progress bar or to extend a timeout.

Prompt

A reusable message template a server offers, often with arguments, that a user can pick in the host. Prompts are listed with prompts/list and filled in with prompts/get.

Protected resource metadata

A JSON document (RFC 9728) that an MCP server publishes to say which authorization servers issue tokens for it and which scopes it supports. A client fetches it after a 401 response to start the sign-in flow.

Registry

A public index of MCP servers with machine-readable metadata. The official MCP Registry, launched in preview in September 2025, holds publisher-maintained entries that other directories and clients can build on.

Resource

Data a server makes readable by URI, such as a file, a database record or a document. Resources are listed with resources/list and fetched with resources/read; the host or user usually decides which to include.

Resource indicator

The resource parameter (RFC 8707) a client includes when requesting a token, naming the exact MCP server the token is for. It stops a token issued for one server from being accepted by another.

Roots

Directories or files a client tells a server it may work within, such as a project folder. The 2026-07-28 revision deprecates roots and suggests passing locations through tool arguments or configuration instead.

Terms S to T

Sampling

A feature that lets a server ask the client's model to generate text, so the server can use AI without its own model access. It is deprecated in the 2026-07-28 revision in favour of servers calling a model provider directly.

Scope

A named permission in an OAuth token, such as issues:read. Servers should ask for the narrowest scopes they need and request more only when an action requires them.

Server

A program that offers tools, resources or prompts to MCP clients. It can run locally on the user's machine or remotely at a web address.

Session

A connection-scoped context that older revisions kept between requests, identified over HTTP by Mcp-Session-Id. Current MCP has no protocol-level sessions; servers that need state across calls return an explicit handle as a tool result.

SSE

Server-Sent Events, a web standard for streaming messages from server to client over one HTTP response. Streamable HTTP can answer a request with an SSE stream; the older HTTP+SSE transport, now deprecated, used a separate long-lived SSE connection.

Stateless

Describes a protocol in which each request carries everything needed to process it. Since revision 2026-07-28, MCP requests include their protocol version and client capabilities, so any server instance can answer any request.

Stdio

The local transport: the host starts the server as a child process and exchanges messages over its standard input and output. It works only on the same machine.

Streamable HTTP

The remote transport: the client sends each message as an HTTP POST to one endpoint, and the server replies with JSON or an SSE stream. It replaced the original HTTP+SSE transport in revision 2025-03-26.

Structured content

A tool result's structuredContent field, which carries machine-readable JSON alongside the human-readable content. It lets clients and code use results without parsing text.

Tool

A named action a server offers, with a description and a JSON Schema for its inputs. The model decides when to call a tool with tools/call, and the host can require user approval first.

Transport

The channel that carries MCP messages between client and server. The current specification defines stdio for local servers and Streamable HTTP for remote ones; see remote vs local MCP servers.

Transport agnostic

A protocol is transport agnostic when its message format and meaning are defined independently of the channel carrying them. MCP's JSON-RPC operations can travel over stdio or Streamable HTTP; changing the transport does not change what a tool call means. Each transport still has its own framing, lifecycle and security rules, so transport agnostic does not mean every client can connect over every transport.

Look up a term in a live server

Most of these terms show up in a real connection's raw log. Open Context7 in the mcptest.io playground to see capabilities, the protocol version header, tool schemas and structured results in the JSON-RPC exchange as they happen.