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Current as of MCP 2026-07-28

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.

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A

Audience The party a token is intended for. Also written: aud claim · aud · token audience · audience validation failed Authorization and tokens Authorization server metadata 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. ASM Also written: RFC 8414 · oauth-authorization-server · AS metadata Discovery

C

Client ID Metadata Documents 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. CIMD Also written: client ID metadata document · client_id_metadata_document_supported · URL client_id · DCR replacement Credentials and registration

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 Dynamic Client Registration 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. DCR Also written: RFC 7591 · registration_endpoint · dynamic client registration not supported Credentials and registration

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

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

I

Identity Assertion JWT Authorization Grant 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. ID-JAG Also written: Cross-App Access · XAA · urn:ietf:params:oauth:token-type:id-jag · identity chaining Credentials and registration 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 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

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 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 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 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 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 _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 Multi Round-Trip Requests The pattern that replaced server-initiated requests in 2026-07-28. MRTR Also written: InputRequiredResult · resultType · input_required · inputRequests · inputResponses Protocol surface

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 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

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 Proof Key for Code Exchange An extension to the OAuth authorization code flow that stops a stolen authorization code from being redeemed by anyone else. PKCE Also written: RFC 7636 · code_challenge · code_verifier · S256 Credentials and registration Protected resource metadata A JSON document, published by a protected resource, that names the authorization servers able to issue tokens for it. PRM Also written: RFC 9728 · oauth-protected-resource · resource metadata Discovery 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

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 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 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

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 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 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 Specification Enhancement Proposal 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. SEP Also written: SEP-2575 · SEP-2322 Versions and governance 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 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 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 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

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 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 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

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

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

Disambiguation

Pairs that get mixed up

Each of these is two things, and knowing which one you have is usually the whole diagnosis.

PRM versus ASM: 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 versus 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 indicators versus Audience: 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 versus 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.

By topic

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.

4 terms Discovery How a client finds out where to authenticate, starting from a 401 it did not expect. See the terms

10 terms Transport and headers How MCP messages travel, and the headers the 2026-07-28 revision added, changed, or dropped. See the terms

4 terms Credentials and registration How a client obtains a client identity and an access token before it calls anything. See the terms

7 terms Authorization and tokens What a server checks on the token it receives, and what it must refuse to do with it. See the terms

10 terms Protocol surface The primitives a server exposes, the shape of a result, and the error codes MCP defines. See the terms

4 terms Versions and governance How revisions are numbered, how features are retired, and how any of it changes. See the terms

One place to enforce all of this

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.

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