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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 all server-to-client notifications. Clients opt in to specific types (toolsListChanged, promptsListChanged, resourcesListChanged, resourceSubscriptions); the server acknowledges and tags notifications with io.modelcontextprotocol/subscriptionId (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).

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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).

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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.