--- title: "MCP glossary" description: "Definitions for the terms in MCP authentication and the protocol itself: PRM, CIMD, DCR, PKCE, resource indicators, Streamable HTTP, statelessness, and the MCP error codes. Every entry cites its RFC or specification section." canonicalUrl: "https://zuplo.com/learn/mcp/glossary" sourceUrl: "https://zuplo.com/learn/mcp/glossary" pageType: "other" generatedAt: "2026-08-18" --- # MCP glossary > 39 terms from MCP authentication and the protocol itself, each defined against > the RFC or specification section it comes from, and marked where the > 2026-07-28 revision changed it. _Current as of MCP 2026-07-28._ Each entry below is a pointer, not the full definition — the definition itself lives on the term's group page, at the linked anchor. This index gives the term, its group, a one-sentence gist, and the other strings it's also known as. ## Index (A–Z) ### A - **Audience** — The party a token is intended for. Also written: aud claim · aud · token audience · audience validation failed. _Authorization and tokens._ [Definition](/learn/mcp/glossary/authorization#aud) - **Authorization server metadata** (ASM) — A JSON document, published by an authorization server, that lists its endpoints and capabilities: issuer, authorization_endpoint, token_endpoint, scopes_supported, and which registration mechanisms it supports. Also written: RFC 8414 · oauth-authorization-server · AS metadata. _Discovery._ [Definition](/learn/mcp/glossary/discovery#authorization-server-metadata) ### C - **Client ID Metadata Documents** (CIMD) — A registration mechanism in which the client's client_id is an HTTPS URL that resolves to a JSON document of its own metadata, so the authorization server fetches the client's details on demand instead of storing a registration. Also written: client ID metadata document · client_id_metadata_document_supported · URL client_id · DCR replacement. _Credentials and registration._ [Definition](/learn/mcp/glossary/credentials#cimd) ### D - **Deprecated, Removed, and Active** — The feature lifecycle policy puts every specification feature in exactly one of three states. Also written: deprecated · removed · feature lifecycle · deprecation policy · deprecated features registry · SEP-2596. _Versions and governance._ [Definition](/learn/mcp/glossary/governance#deprecated-vs-removed) - **Dynamic Client Registration** (DCR) — A protocol, defined by RFC 7591, that lets a client register itself with an authorization server over HTTP and receive a client_id back, with no human in the loop. Also written: RFC 7591 · registration_endpoint · dynamic client registration not supported. _Credentials and registration._ [Definition](/learn/mcp/glossary/credentials#dcr) ### E - **Elicitation** — The mechanism by which a server asks for additional information from the user, through the client, while a request is in flight. Also written: elicitation/create · ElicitResult · form mode · url mode. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#elicitation) ### H - **HTTP+SSE transport** — The original two-endpoint HTTP transport from the 2024-11-05 revision: the client opened an SSE stream with GET and posted messages to a separate endpoint the stream advertised. Also written: HTTP+SSE · HTTP with SSE · SSE transport · old MCP transport · 2024-11-05 transport. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#http-sse) ### I - **Identity Assertion JWT Authorization Grant** (ID-JAG) — An IETF draft profile of the JWT authorization grant that gives a client delegated access to a resource in another trust domain on behalf of a user, without a separate user-approval step at that domain's authorization server. Also written: Cross-App Access · XAA · urn:ietf:params:oauth:token-type:id-jag · identity chaining. _Credentials and registration._ [Definition](/learn/mcp/glossary/credentials#id-jag) - **initialize handshake** — The opening exchange of every MCP revision up to 2025-11-25: the client sent an initialize request carrying its protocol version and capabilities, the server replied with its own, and the client confirmed with a notifications/initialized. Also written: initialize · notifications/initialized · MCP handshake · initialize request. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#initialize) - **Issuer identification** — RFC 9207 adds an iss parameter to the OAuth authorization response so a client can tell which authorization server issued the code it just received, and an authorization_response_iss_parameter_supported metadata flag to advertise it. Also written: iss · iss parameter · RFC 9207 · authorization_response_iss_parameter_supported · mix-up attack. _Authorization and tokens._ [Definition](/learn/mcp/glossary/authorization#iss) ### M - **MCP error codes** — MCP partitions the JSON-RPC implementation-defined error range: -32000 to -32019 is legacy, allocated by implementations before the policy existed, and -32020 to -32099 is reserved for the specification. Also written: -32020 · -32021 · -32022 · -32002 · -32042 · HeaderMismatch · MissingRequiredClientCapability · UnsupportedProtocolVersion. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#error-codes) - **Mcp-Method** — An HTTP header mirroring the JSON-RPC method field of the request body, required on all Streamable HTTP POST requests. Also written: mcp method header · mirrored headers. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#mcp-method) - **Mcp-Name** — An HTTP header mirroring params.name or params.uri from the request body, required on tools/call, resources/read, and prompts/get requests. Also written: mcp name header · tool name header. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#mcp-name) - **MCP-Protocol-Version** — The HTTP header that carries the protocol version on every Streamable HTTP POST, for example MCP-Protocol-Version: 2026-07-28. Also written: protocol version header · unsupported protocol version. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#mcp-protocol-version) - **Mcp-Session-Id** — The HTTP header that carried a protocol-level session identifier in the 2025-03-26 through 2025-11-25 revisions. Also written: mcp session id header · MCP session · no valid session ID provided. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#mcp-session-id) - **\_meta** — A reserved field on MCP requests, results, and notifications that carries metadata rather than payload. Also written: io.modelcontextprotocol/protocolVersion · clientCapabilities · clientInfo · serverInfo · meta field. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#meta) - **Multi Round-Trip Requests** (MRTR) — The pattern that replaced server-initiated requests in 2026-07-28. Also written: InputRequiredResult · resultType · input_required · inputRequests · inputResponses. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#mrtr) ### O - **Official extension** — An optional addition to the specification that defines capability beyond the core protocol, identified as {vendor-prefix}/{extension-name}. Also written: official extensions · MCP extensions · io.modelcontextprotocol/ui · io.modelcontextprotocol/tasks · io.modelcontextprotocol/oauth-client-credentials · io.modelcontextprotocol/enterprise-managed-authorization · MCP Apps · extensions capability. _Versions and governance._ [Definition](/learn/mcp/glossary/governance#official-extensions) - **OPTIONAL authorization** — The MCP specification states that authorization is OPTIONAL for implementations, and that HTTP-based implementations SHOULD — not MUST — conform to its OAuth profile when they do support it. Also written: authorization is optional · MCP API key auth · custom header auth MCP · is API key authentication allowed in MCP · out of scope. _Authorization and tokens._ [Definition](/learn/mcp/glossary/authorization#optional-authorization) ### P - **Prompt** — A server-defined, optionally parameterized template of structured messages and instructions for interacting with a language model, listed with prompts/list and resolved with prompts/get. Also written: prompts/get · prompts/list · MCP prompt · user-controlled. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#prompt) - **Proof Key for Code Exchange** (PKCE) — An extension to the OAuth authorization code flow that stops a stolen authorization code from being redeemed by anyone else. Also written: RFC 7636 · code_challenge · code_verifier · S256. _Credentials and registration._ [Definition](/learn/mcp/glossary/credentials#pkce) - **Protected resource metadata** (PRM) — A JSON document, published by a protected resource, that names the authorization servers able to issue tokens for it. Also written: RFC 9728 · oauth-protected-resource · resource metadata. _Discovery._ [Definition](/learn/mcp/glossary/discovery#protected-resource-metadata) - **Protocol revision** — MCP versions are dates in YYYY-MM-DD form, naming the last date on which backward-incompatible changes were made. Also written: MCP version · 2026-07-28 · protocol version · Draft Current Final. _Versions and governance._ [Definition](/learn/mcp/glossary/governance#protocol-revision) ### R - **Resource** — Data a server exposes to give a language model context — files, database schemas, or application-specific information — each identified uniquely by a URI and read with resources/read. Also written: resources/read · resources/list · MCP resource · application-driven. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#mcp-resource) - **Resource indicators** — RFC 8707 defines a resource request parameter that names the target service a token is being requested for, so the authorization server can audience-restrict the token it issues. Also written: RFC 8707 · resource parameter · resource indicator missing or unknown · canonical server URI. _Authorization and tokens._ [Definition](/learn/mcp/glossary/authorization#resource) - **Roots** — Filesystem locations a client tells a server it considers relevant, each identified by a file:// URI. Also written: roots/list · MCP roots · filesystem roots · workspace roots. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#roots) ### S - **Sampling** — The mechanism by which a server asks the client to run a language model completion on its behalf, so the client keeps control of model access, selection, and cost, and the server needs no model API key of its own. Also written: sampling/createMessage · MCP sampling · server requests completion. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#sampling) - **Scope** — The OAuth mechanism for expressing what an access token is allowed to do. Also written: scopes_supported · insufficient_scope · 403 insufficient scope · step-up authorization. _Authorization and tokens._ [Definition](/learn/mcp/glossary/authorization#scope) - **server/discover** — A method that returns a server's supported protocol versions, capabilities, and identity in a single request. Also written: DiscoverResult · mcp discover method · supportedVersions. _Discovery._ [Definition](/learn/mcp/glossary/discovery#server-discover) - **Specification Enhancement Proposal** (SEP) — A SEP is a design document that describes a new feature for MCP or for its processes, and it is the mechanism required for any protocol change, breaking change, or governance change. Also written: SEP-2575 · SEP-2322. _Versions and governance._ [Definition](/learn/mcp/glossary/governance#sep) - **Statelessness** — MCP is a stateless protocol as of the 2026-07-28 revision. Also written: stateless · stateless MCP · sessions removed · MCP is stateless. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#statelessness) - **stdio** — The transport in which the client launches the MCP server as a subprocess and the two exchange newline-delimited JSON-RPC messages over the subprocess's standard input and output. Also written: standard input output transport · local MCP server. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#stdio) - **Streamable HTTP** — The HTTP transport for MCP, introduced in the 2025-03-26 revision. Also written: streamable http transport · MCP endpoint · remote MCP transport. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#streamable-http) - **subscriptions/listen** — A long-lived request that opens a server-to-client notification stream filtered to the event types the client names. Also written: resources/subscribe · MCP notifications stream · subscriptionId. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#subscriptions-listen) ### T - **Token exchange** — An OAuth grant, defined by RFC 8693, in which a client presents one token and receives a different one with a different audience, subject, or scope. Also written: RFC 8693 · urn:ietf:params:oauth:grant-type:token-exchange · subject_token · delegation · impersonation. _Authorization and tokens._ [Definition](/learn/mcp/glossary/authorization#token-exchange) - **Token passthrough** — The anti-pattern in which an MCP server accepts a token from a client without validating that it was issued to the MCP server, then passes it through to a downstream API. Also written: forwarding access tokens · confused deputy. _Authorization and tokens._ [Definition](/learn/mcp/glossary/authorization#token-passthrough) - **Tool** — A named, schema-described capability a server exposes so that a language model can interact with an external system. Also written: tools/call · tools/list · MCP tool · model-controlled. _Protocol surface._ [Definition](/learn/mcp/glossary/protocol#tool) ### W - **WWW-Authenticate** — The HTTP response header field a server uses to challenge a client for credentials, defined by RFC 9110 and given its Bearer-scheme parameters by RFC 6750. Also written: www authenticate header · 401 challenge · resource_metadata · Bearer challenge. _Discovery._ [Definition](/learn/mcp/glossary/discovery#www-authenticate) ### X - **x-mcp-header** — An extension property a server puts on a parameter inside a tool's inputSchema to have the client mirror that parameter's value into an HTTP header named Mcp-Param-{Name}. Also written: Mcp-Param · custom headers from tool parameters · tool parameter header. _Transport and headers._ [Definition](/learn/mcp/glossary/transport#x-mcp-header) ## Pairs that get mixed up Each of these is two things, and knowing which one you have is usually the whole diagnosis. - **[Protected resource metadata](/learn/mcp/glossary/discovery#protected-resource-metadata) versus [Authorization server metadata](/learn/mcp/glossary/discovery#authorization-server-metadata)** — Two different documents, published by two different parties. The resource says which authorization servers can issue tokens for it; the authorization server says where its own endpoints are. - **[DCR](/learn/mcp/glossary/credentials#dcr) versus [CIMD](/learn/mcp/glossary/credentials#cimd)** — Both get a client an identity with no human involved. DCR stores a registration at the authorization server; CIMD uses an HTTPS URL as the client ID and is fetched on demand, so nothing is stored and the ID is portable. - **[Resource](/learn/mcp/glossary/authorization#resource) versus [Audience](/learn/mcp/glossary/authorization#aud)** — The `resource` parameter is what the client asks for; the audience is what the issued token carries and what the server checks. An identity provider that ignores `resource` still issues a token — with the wrong audience. - **[Mcp-Session-Id](/learn/mcp/glossary/transport#mcp-session-id) versus [Statelessness](/learn/mcp/glossary/transport#statelessness)** — Not a rename. Protocol-level sessions were deleted, so state that has to outlive a request is now an explicit handle passed in tool arguments rather than a header the transport tracks. ## Browse a whole area These read in the order a request does: find the authorization server, pick a transport, get a credential, have it checked, then the surface that credential buys, then the process that changes all of it. - **[Discovery](/learn/mcp/glossary/discovery)** (4 terms) — How a client finds out where to authenticate, starting from a 401 it did not expect. - **[Transport and headers](/learn/mcp/glossary/transport)** (10 terms) — How MCP messages travel, and the headers the 2026-07-28 revision added, changed, or dropped. - **[Credentials and registration](/learn/mcp/glossary/credentials)** (4 terms) — How a client obtains a client identity and an access token before it calls anything. - **[Authorization and tokens](/learn/mcp/glossary/authorization)** (7 terms) — What a server checks on the token it receives, and what it must refuse to do with it. - **[Protocol surface](/learn/mcp/glossary/protocol)** (10 terms) — The primitives a server exposes, the shape of a result, and the error codes MCP defines. - **[Versions and governance](/learn/mcp/glossary/governance)** (4 terms) — How revisions are numbered, how features are retired, and how any of it changes. ## Next steps Discovery documents, audience binding, token validation, and the 2026-07-28 headers are the same work on every MCP server. A gateway in front of them does it once. - Start for free: https://portal.zuplo.com/signup - Read the docs: https://zuplo.com/docs/mcp-gateway/code-config/overview