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name mcp
description The Model Context Protocol (MCP) is an open, JSON-RPC 2.0 standard for connecting AI applications to external tools, data, and prompts; use it when building or integrating MCP clients/servers and especially when you must target a specific date-versioned protocol revision.
metadata
version learned-date target-model cutoff
1.0.0
2026-06-22
claude-opus-4-8
2026-01

The Model Context Protocol (MCP) is an open standard for connecting AI applications to external tools, data sources, and prompts over a uniform JSON-RPC 2.0 interface.

Overview

MCP defines a client–server protocol: a host (an AI application) runs one client per connection to an MCP server, and servers expose capabilities the model can use. Every message is JSON-RPC 2.0. Servers expose three core primitives — tools (callable functions), resources (readable context), and prompts (reusable templates) — while clients may expose sampling, roots, and (in newer revisions) elicitation.

When to use

Use this skill when writing or integrating an MCP client or server, choosing a transport, or — most importantly — when you must pin or negotiate a specific protocol revision. MCP is date-versioned, and revisions are not backward-compatible by default, so the wrong assumption about the version silently breaks integrations.

Protocol versioning

MCP protocol revisions are identified by a date string YYYY-MM-DD, not semver. Known revisions:

Revision Notable changes
2024-11-05 First public spec. stdio and HTTP+SSE transports.
2025-03-26 Added Streamable HTTP transport (replacing the older HTTP+SSE), OAuth 2.1-based authorization, tool annotations, and audio content.
2025-06-18 Structured tool output, elicitation, resource links in tool results, removal of JSON-RPC batching, and a required MCP-Protocol-Version header for HTTP.

Negotiation happens in initialize. The client sends the latest revision it supports as protocolVersion; the server replies with the version it will actually use (its latest supported revision ≤ the client's, or its own latest if it can't match). The client must disconnect if it cannot accept the server's chosen version.

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2025-06-18",
    "capabilities": {},
    "clientInfo": { "name": "example-client", "version": "1.0.0" }
  }
}
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "protocolVersion": "2025-06-18",
    "capabilities": { "tools": {}, "resources": {} },
    "serverInfo": { "name": "example-server", "version": "1.0.0" }
  }
}

Modern vs deprecated

  • Transport: Prefer Streamable HTTP (since 2025-03-26) for remote servers. The original HTTP+SSE transport is superseded — do not build new servers on it. stdio remains the standard for local subprocess servers.
  • Header (HTTP): From 2025-06-18, clients must send the negotiated revision on every subsequent HTTP request as MCP-Protocol-Version: 2025-06-18. Servers that receive no header should assume 2025-03-26 for backward compatibility.
  • Batching: JSON-RPC batching was removed in 2025-06-18; do not rely on it when targeting that revision.

Instructions

  1. Pick the SDK, not raw JSON-RPC, unless you have a reason: TypeScript @modelcontextprotocol/sdk, Python mcp.
  2. Negotiate explicitly: send the newest revision you support in initialize.protocolVersion; branch behaviour on the version the server returns rather than assuming.
  3. For HTTP, echo the negotiated version back via the MCP-Protocol-Version header on every request (required from 2025-06-18).
  4. Feature-gate capabilities by revision (e.g. only use elicitation/structured output when the negotiated version is 2025-06-18 or later).
npm install @modelcontextprotocol/sdk
# or: pip install mcp

Sources

Created with skill3.