AI library

Tooling

Model Context Protocol (MCP)

Last reviewed 15 October 2026

Overview

MCP is an open protocol for connecting AI applications to tools and data. A service builds one MCP server; any MCP-capable client (Claude, ChatGPT, IDEs, coding agents, custom agents) can use it. Anthropic introduced it in November 2024; within a year it was the de facto standard, with official SDKs in many languages.

Key concepts

  • Host / client / server: the host app (for example Claude Code) runs a client that connects to each server.
  • Primitives: tools (functions the model can call), resources (data the app can read), prompts (reusable templates).
  • Stateless core: since the 2026-07-28 spec there is no initialize handshake and no protocol session; each request stands alone, so remote servers can run behind ordinary load balancers (routing uses Mcp-Method and Mcp-Name headers).
  • Transports: stdio for local servers; HTTP for remote ones.
  • Authorization: OAuth-based for remote servers, aligned with OpenID Connect.
  • Extensions: a formal extensions framework and deprecation policy. Tasks (for long-running operations) is an opt-in extension.
  • MCP Apps: servers can render interactive HTML UIs in a sandboxed iframe.
  • MCP connector: some APIs (including Claude's) can connect to remote MCP servers directly.

How to use it

  • Use existing servers for common services before writing your own.
  • Install only servers you trust; a server's tool descriptions and outputs go straight into the model's context (see security).
  • Keep tool lists small; prefer clients that load tools on demand.
  • Build a server when you want one integration usable from many AI clients.

Current state

  • 2026-10-14: Claude Code 2.1.299 extends the 2026-07-28 protocol default to cloud session and Remote Control connectors, so remote connections now negotiate it too. MCP permission rules were tightened: mcp__a no longer matches a server named a__b, and policy-blocked servers are reported as blocked without being contacted.
  • 2026-10-06: Claude Code 2.1.292 negotiates the 2026-07-28 protocol by default for local stdio servers (opt out with MCP_PROTOCOL_NEGOTIATION=legacy); servers that do not answer the newer check fall back to the old handshake. Its default cap on MCP tool descriptions rose to 4,096 characters on 2026-10-09.
  • 2026-10-02: Google ADK for Python 2.11 adds opt-in support for MCP SDK 2.x, an example of frameworks starting to move to the newer protocol.
  • 2026-07-28: The current specification (the largest revision since launch, now described in Key concepts) deprecated Roots, Sampling, and Logging with a 12-month removal window, so they may disappear from around July 2027. Servers and clients built on earlier versions need changes.
  • 2026-07-28: The maintainers reported SDK downloads approaching 500 million a month.

Open questions

  • How quickly clients and servers migrate to the stateless spec. Partly answered: Google ADK supports MCP SDK 2.x only as an opt-in (2026-10-02), while Claude Code has used the new protocol by default for local servers since 2026-10-06 and for its cloud and Remote Control connectors since 2026-10-14. Still unknown: other major clients, and how many servers support it.

Sources