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Documentation Index

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This document lists changes made to the Model Context Protocol (MCP) specification since the previous revision, 2025-11-25.

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Major changes

  1. Remove protocol-level sessions and the Mcp-Session-Id header from the Streamable HTTP transport. List endpoints (tools/list, resources/list, prompts/list) no longer vary per-connection. Servers that need cross-call state use explicit, server-minted handles passed as ordinary tool arguments (SEP-2567).
  2. Make MCP stateless: remove the initialize/notifications/initialized handshake. Every request now carries its protocol version, client identity, and client capabilities in _meta (io.modelcontextprotocol/protocolVersion, io.modelcontextprotocol/clientInfo, io.modelcontextprotocol/clientCapabilities). Version mismatches return UnsupportedProtocolVersionError (SEP-2575).
  3. Add server/discover: servers MUST implement this RPC to advertise their supported protocol versions, capabilities, and identity. Clients MAY call it before any other request for up-front version selection, or use it as a backward-compatibility probe on STDIO (SEP-2575).
  4. Replace the HTTP GET endpoint and resources/subscribe/resources/unsubscribe with subscriptions/listen: a single long-lived POST-response stream for opted-in server-to-client change notifications. Clients opt in to specific types (toolsListChanged, promptsListChanged, resourcesListChanged, resourceSubscriptions); the server acknowledges and tags notifications with io.modelcontextprotocol/subscriptionId. Request-scoped notifications such as notifications/progress and notifications/message continue to flow on the response stream of the request they relate to, not the subscriptions/listen stream (SEP-2575).
  5. Remove ping, logging/setLevel, and notifications/roots/list_changed. Log level is now set per-request via io.modelcontextprotocol/logLevel in _meta; servers MUST NOT emit notifications/message for requests that did not include this field (SEP-2575).
  6. Move experimental tasks out of the core protocol and into an official extension (io.modelcontextprotocol/tasks). The redesigned extension replaces the blocking tasks/result method with polling via tasks/get and a new tasks/update for client-to-server input, removes tasks/list, and allows servers to return task handles unsolicited without per-request opt-in (SEP-2663).
  7. Multi Round-Trip Requests (MRTR) pattern introduced which replaces the previous approach of sending server-initiated requests, such as roots/list, sampling/createMessage, or elicitation/create. Servers return inputRequests, a new resultType containing the additional information needed to process the request. Clients respond with inputResponses on the next request providing the requested information. (SEP-2322).
  8. Deprecate the Roots, Sampling, and Logging features (SEP-2577). These features remain fully functional during the deprecation window but new implementations should not add support for them.

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Minor changes

  1. Add extensions field to ClientCapabilities and ServerCapabilities to support optional extensions beyond the core protocol.
  2. Document OpenTelemetry trace context propagation conventions for _meta keys (traceparent, tracestate, baggage) (SEP-414).
  3. Servers SHOULD return tools from tools/list in a deterministic order to enable client-side caching and improve LLM prompt cache hit rates.
  4. Require standard MCP request headers (Mcp-Method, Mcp-Name) on Streamable HTTP POST requests, and add support for custom headers from tool parameters via x-mcp-header (SEP-2243).
  5. Require ttlMs and cacheScope fields on results returned by tools/list, prompts/list, resources/list, resources/read, and resources/templates/list via a new CacheableResult interface. ttlMs is a freshness hint (in milliseconds) allowing clients to cache responses and reduce polling; cacheScope ("public" or "private") controls whether shared intermediaries may cache the response. Both fields complement existing listChanged notifications (SEP-2549).
  6. Change resource not found error code from -32002 to -32602 (Invalid Params) to align with JSON-RPC specification.

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Other schema changes

N/A

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Governance and process updates

N/A

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Process changes

  1. Formalize PR-based SEP workflow with markdown files in seps/ directory, PR-derived numbering, sponsor responsibilities, and status management via PR labels (SEP-1850).

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Full changelog

For a complete list of all changes that have been made since the last protocol revision, see GitHub.