Model Context Protocol

MCP is a JSON-RPC protocol (version 2.0) for connecting an LLM application to outside data and tools. Anthropic open-sourced it on 2024-11-25 with a specification, SDKs, local server support in the Claude desktop apps, and a repository of reference servers for Google Drive, Slack, GitHub, Git, Postgres and Puppeteer. It names three roles: hosts are the LLM applications, clients are the connectors inside them, servers expose context and capabilities.

Nearly all working knowledge of MCP dates from that launch. As of the current revision — 2026-07-28source, accessed 2026-08-28 — most of the launch protocol is gone, and the two revisions that removed it were pursuing one decision: make MCP stateless.

The timeline above shows those two and not the others, which is a choice and worth naming as one. The specification keeps its own complete history: the versioning page lists every revision, and each revision's changelog opens by naming the one it follows. Revisions sit between the two named here. They are absent because they are not part of this entry's argument, not because the history stops.

What was removed, and what replaced it:

• The handshake. The initialize / notifications/initialized exchange no longer exists. Every request now carries its own protocol version and client capabilities in _meta (io.modelcontextprotocol/protocolVersion, io.modelcontextprotocol/clientCapabilities), and a version the server cannot serve comes back as UnsupportedProtocolVersionError listing what it does support. A new RPC, server/discover, which servers MUST implement, is how a client asks up front instead of negotiating.
• Sessions. Protocol-level sessions and the Mcp-Session-Id header are gone from Streamable HTTP, and tools/list, resources/list and prompts/list no longer vary per connection. A server needing cross-call state now mints an explicit handle and receives it back as an ordinary tool argument — state became data the client can see rather than a connection property it cannot.
• Server-initiated requests. roots/list, sampling/createMessage and elicitation/create are replaced by the Multi Round-Trip Request pattern: the server returns a result whose resultType is input_required, carrying an inputRequests list, and the client retries the original request with inputResponses attached. Every result now carries a resultType, and results from older servers that omit it MUST be read as complete.
• Stream resumability. SSE event ids and Last-Event-ID redelivery are gone: a broken response stream loses the in-flight request and the client must re-issue it under a new request id. The HTTP GET endpoint and resources/subscribe are replaced by one opt-in subscriptions/listen stream.
• ping, logging/setLevel and notifications/roots/list_changed are removed outright; log level is now set per request in _meta, and a server MUST NOT emit log messages for a request that did not ask for them.

Three features are deprecated in the same revision rather than removed: Roots, Sampling and Logging, each with a migration named in the changelog — pass directories as tool parameters or resource URIs instead of Roots, call the model provider's API directly instead of Sampling, log to stderr or OpenTelemetry instead of Logging. Sampling was the one capability inversion in the launch protocol — the primitive that let a server ask the host to run an inference on its behalf — and it is now on a removal path.

Two things worth knowing about how the specification versions itself. The version string is a date that marks the last backwards-incompatible change, not a release: a revision that stays compatible does not bump it, so "the current version" moves rarely and abruptly. And the latest revision added a written feature lifecycle — Active, Deprecated, Removed — with a stated removal clock (at least twelve months before a deprecated feature may be removed, or ninety days under the expedited exceptionsource, accessed 2026-08-28) and a registry page listing every feature currently deprecated. HTTP+SSE, deprecated in prose since 2025-03-26, was reclassified under that policy, which is the first time MCP's removals have carried a published schedule rather than a note.