Transports

MCP currently defines two standard transport mechanisms for client-server communication:

1. stdio, communication over standard in and standard out
2. HTTP with Server-Sent Events (SSE)

Clients SHOULD support stdio whenever possible.

It is also possible for clients and servers to implement custom transports in a pluggable fashion.

stdio

In the stdio transport:

• The client launches the MCP server as a subprocess.
• The server receives JSON-RPC messages on its standard input (stdin) and writes responses to its standard output (stdout).
• Messages are delimited by newlines, and MUST NOT contain embedded newlines.
• The server MAY write UTF-8 strings to its standard error (stderr) for logging purposes. Clients MAY capture, forward, or ignore this logging.
• The server MUST NOT write anything to its stdout that is not a valid MCP message.
• The client MUST NOT write anything to the server’s stdin that is not a valid MCP message.

sequenceDiagram
    participant Client
    participant Server Process

    Client->>+Server Process: Launch subprocess
    loop Message Exchange
        Client->>Server Process: Write to stdin
        Server Process->>Client: Write to stdout
        Server Process--)Client: Optional logs on stderr
    end
    Client->>Server Process: Close stdin, terminate subprocess
    deactivate Server Process

HTTP with SSE

In the SSE transport, the server operates as an independent process that can handle multiple client connections.

The server MUST provide two endpoints:

1. An SSE endpoint, for clients to establish a connection and receive messages from the server
2. A regular HTTP POST endpoint for clients to send messages to the server

When a client connects, the server MUST send an endpoint event containing a URI for the client to use for sending messages. All subsequent client messages MUST be sent as HTTP POST requests to this endpoint.

Server messages are sent as SSE message events, with the message content encoded as JSON in the event data.

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: Open SSE connection
    Server->>Client: endpoint event
    loop Message Exchange
        Client->>Server: HTTP POST messages
        Server->>Client: SSE message events
    end
    Client->>Server: Close SSE connection

Custom Transports

Clients and servers MAY implement additional custom transport mechanisms to suit their specific needs. The protocol is transport-agnostic and can be implemented over any communication channel that supports bidirectional message exchange.

Implementers who choose to support custom transports MUST ensure they preserve the JSON-RPC message format and lifecycle requirements defined by MCP. Custom transports SHOULD document their specific connection establishment and message exchange patterns to aid interoperability.