Topic 2: Messages and lifecycle

Requests, responses, and notifications

MCP messages are JSON-RPC 2.0 messages. There are three shapes:

• A request has an id, a method, and usually params. It expects exactly one answer. In 2026-07-28 the id must be a string or an integer, never null, and must not repeat an id the sender still has in flight.
• A response carries the same id back, with either a result (success) or an error (failure), never both. Every 2026-07-28 result also carries resultType, which is "complete" for ordinary results.