Base Protocol

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Protocol Revision: draft

All messages between MCP clients and servers MUST follow the JSON-RPC 2.0 specification. The protocol defines three fundamental types of messages:

Type	Description	Requirements
`Requests`	Messages sent to initiate an operation	Must include unique ID and method name
`Responses`	Messages sent in reply to requests	Must include same ID as request
`Notifications`	One-way messages with no reply	Must not include an ID

Responses are further sub-categorized as either successful results or errors. Results can follow any JSON object structure, while errors must include an error code and message at minimum.

Protocol Layers

The Model Context Protocol consists of several key components that work together:

Base Protocol: Core JSON-RPC message types
Lifecycle Management: Connection initialization, capability negotiation, and session control
Server Features: Resources, prompts, and tools exposed by servers
Client Features: Sampling and root directory lists provided by clients
Utilities: Cross-cutting concerns like logging and argument completion

All implementations MUST support the base protocol and lifecycle management components. Other components MAY be implemented based on the specific needs of the application.

These protocol layers establish clear separation of concerns while enabling rich interactions between clients and servers. The modular design allows implementations to support exactly the features they need.

See the following pages for more details on the different components:

Lifecycle Resources Prompts Tools Logging Sampling

Auth

MCP provides an Authorization framework for HTTP+SSE transport. Implementations using HTTP+SSE transport SHOULD conform to this specification, whereas implementations using STDIO transport SHOULD NOT follow this specification, and instead retrieve credentials from the environment.

Additionally, clients and servers MAY negotiate their own custom authentication and authorization strategies.

For further discussions and contributions to the evolution of MCP’s auth mechanisms, join us in GitHub Discussions to help shape the future of the protocol!

Schema

The full specification of the protocol is defined as a TypeScript schema. This is the source of truth for all protocol messages and structures.

There is also a JSON Schema, which is automatically generated from the TypeScript source of truth, for use with various automated tooling.