What can you do with Model Context Protocol?

• Read the schema, not the summary — The protocol is defined in TypeScript at schema/<revision>/schema.ts and published as JSON Schema for toolchains that cannot consume it, so the authoritative answer to what fields a message carries is a file in this repository rather than any SDK's documentation.
• Know which dated revision you are on — Revisions are YYYY-MM-DD strings that advance only when a backwards-incompatible change lands, so a compatible improvement ships without moving the version. The current revision is 2026-07-28; 2025-11-25 and earlier are the handshake-based ones.
• Negotiate per request, not per session — From 2026-07-28 the version travels on every request as io.modelcontextprotocol/protocolVersion inside _meta, and in the MCP-Protocol-Version header on Streamable HTTP. A server rejects each request independently with UnsupportedProtocolVersionError listing what it supports, and server/discover is a mandatory RPC returning supported versions, capabilities and identity in one call.
• Opt into extensions instead of waiting for the core — Tasks give long-running operations durable handles with polling and mid-flight input, MCP Apps render charts, forms and video players inline in a conversation, and two authorization extensions cover machine-to-machine and enterprise-managed access. Extensions are disabled by default: the client advertises support in io.modelcontextprotocol/clientCapabilities on every request and the server in its server/discover result, so a feature exists only once both ends have opted in.
• Do not expect the protocol to enforce safety — The specification states that MCP cannot enforce its security principles at the protocol level: tool annotations are to be treated as untrusted unless the server is trusted, and consent flows are left to the host. Who may call which tool is your problem, or a gateway's.
• Find the running code in sibling repositories — This repository holds the specification and documentation only. The reference servers, the community registry and the official Python, TypeScript, Go, C#, Rust, Java, Kotlin, PHP, Swift and Ruby SDKs are separate repositories in the same organisation, each on its own release cadence — so support for a new revision arrives after the revision does, and which one you can actually speak depends on the SDK.