Crate fastmcp_transport 

Transport layer for FastMCP.

This crate provides transport implementations for MCP communication:

• Stdio: Standard input/output (primary transport)
• SSE: Server-Sent Events (HTTP-based streaming)
• WebSocket: Bidirectional web sockets

Transport Design

Transports are designed around asupersync’s principles:

• Cancel-correctness: All operations check cancellation via Cx::checkpoint()
• Two-phase sends: Use reserve/commit pattern to prevent message loss
• Budget awareness: Operations respect the request’s budget constraints

Wire Format

MCP uses newline-delimited JSON (NDJSON) for message framing:

• Each message is a single line of JSON
• Messages are separated by \n
• UTF-8 encoding is required

Role in the System

fastmcp-transport is the I/O boundary for FastMCP. It is deliberately protocol-agnostic: transports move JsonRpcMessage values in and out while the server/client layers handle semantics. This keeps transport implementations small, testable, and reusable.

If you need to add a new transport (for example, QUIC or a custom IPC), this is the crate to extend.

Modules

event_store

Event store for SSE resumability.

http

HTTP transport for FastMCP.

memory

In-memory transport for testing MCP servers without subprocess spawning.

sse

Server-Sent Events (SSE) transport for MCP.

websocket

WebSocket transport for MCP.

Structs

AsyncLineReader

Async line reader with cancellation support.

AsyncStdin

Async wrapper for stdin.

AsyncStdioTransport

Async stdio transport with integrated cancellation support.

AsyncStdout

Async wrapper for stdout.

Codec

Codec for encoding/decoding JSON-RPC messages.

SendPermit

A permit for sending a message via two-phase commit.

StdioTransport

Stdio transport implementation.

Enums

CodecError

Codec error types.

TransportError

Transport error types.

Traits

Transport

Transport trait for cancel-correct message passing.

TwoPhaseTransport

Extension trait for two-phase send operations.