Protocol surface

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. The 2026-07-28 revision removed it. Version, identity, and capabilities now travel in the _meta field of every request.

Deleted from this revision as a breaking change (SEP-2575), not deprecated first. SDKs may still expose an initialize() call, so check your SDK version before changing working code.

Also written: initialize · notifications/initialized · initialize handshake · MCP handshake · initialize request

A reserved field on MCP requests, results, and notifications that carries metadata rather than payload. Since 2026-07-28 it is where each request declares what the removed handshake used to establish. Exactly two keys are required on every request — io.modelcontextprotocol/protocolVersion and io.modelcontextprotocol/clientCapabilities — while io.modelcontextprotocol/clientInfo on requests and io.modelcontextprotocol/serverInfo on results are both a SHOULD.

This revision made _meta load-bearing: it now carries per-request version and capability declarations, and extension identifiers follow its key naming rules.

Also written: _meta · io.modelcontextprotocol/protocolVersion · clientCapabilities · clientInfo · serverInfo · meta field

A named, schema-described capability a server exposes so that a language model can interact with an external system. Tools are listed with tools/list and invoked with tools/call. The specification describes them as model-controlled: the model can discover and invoke a tool on its own from context and the user's prompts.

Also written: tool · tools/call · tools/list · MCP tool · model-controlled

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. The specification describes resources as application-driven, with the host application deciding how to incorporate them.

Also written: resource · resources/read · resources/list · MCP resource · application-driven

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. The specification describes prompts as user-controlled, meaning the user explicitly selects one — that is about who decides when a prompt is used, not who writes it. The content is defined by the server.

Also written: prompt · prompts/get · prompts/list · MCP prompt · user-controlled

The pattern that replaced server-initiated requests in 2026-07-28. Instead of sending its own JSON-RPC request, a server that needs input answers the client's request with an InputRequiredResult — resultType: "input_required" plus an inputRequests map — and the client gathers the input and retries the original request with matching inputResponses. The retry must use a new JSON-RPC id.

Added in this revision (SEP-2322) as a breaking change: the previous pattern of server-initiated requests is no longer supported.

Also written: MRTR · multi round-trip requests · InputRequiredResult · resultType · input_required · inputRequests · inputResponses

The mechanism by which a server asks for additional information from the user, through the client, while a request is in flight. It has two modes: form mode, where the client renders a schema-validated form and sees the answer, and URL mode, where the user is sent out of band so the data never passes through the client.

The feature remains, but its delivery moved to MRTR. URL mode dates from 2025-11-25; the notifications/elicitation/complete message and elicitationId from that revision were removed.

Also written: elicitation · elicitation/create · ElicitResult · form mode · url mode

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. Deprecated as of 2026-07-28.

Deprecated in this revision (SEP-2577). It stays in the specification for at least twelve months, so the earliest removal is the first revision on or after 2027-07-28.

Also written: sampling · sampling/createMessage · MCP sampling · server requests completion

Filesystem locations a client tells a server it considers relevant, each identified by a file:// URI. They are informational guidance and not an access-control mechanism: the protocol does not enforce that servers stay within them. Deprecated as of 2026-07-28.

Deprecated in this revision (SEP-2577), with the same twelve-month floor as sampling: earliest removal is the first revision on or after 2027-07-28.

Also written: roots · roots/list · MCP roots · filesystem roots · workspace roots

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. Three codes are currently defined: -32020 HeaderMismatch, -32021 MissingRequiredClientCapability, and -32022 UnsupportedProtocolVersion.

This revision renumbered all three defined codes from the legacy range and retired -32002 and -32042.

Also written: MCP error codes · -32020 · -32021 · -32022 · -32002 · -32042 · HeaderMismatch · MissingRequiredClientCapability · UnsupportedProtocolVersion