feat: Unix Domain Socket transport

[joshrotenberg]

Summary

Add Unix Domain Socket (UDS) transport support, allowing MCP servers to listen on a Unix socket instead of TCP. This is useful for local-only server deployments where network exposure is unnecessary and IPC performance matters.

Motivation

• rmcp has UDS client support in progress
• turbomcp lists UDS as a supported transport
• UDS avoids TCP overhead for local communication (no port allocation, no loopback)
• Common pattern for containerized/sidecar deployments

Implementation

The existing transport architecture makes this straightforward. Both HttpTransport and WebSocketTransport follow the same pattern: bind a listener, build an axum Router, call axum::serve(listener, router). axum's serve() accepts any type implementing the Listener trait, and tokio::net::UnixListener satisfies this.

Approach

1. New module: crates/tower-mcp/src/transport/unix.rs
2. UnixSocketTransport struct parallel to HttpTransport/WebSocketTransport
3. Feature flag: unix (depends on http feature for shared axum machinery)
4. Reuses session management, SSE notifications, and middleware from HTTP transport
5. .layer() support inherited from the same pattern

Scope

• Unix-only (Linux, macOS, BSD) -- #[cfg(unix)] gated
• No Windows Named Pipes support initially
• Streamable HTTP over UDS (same protocol, different listener)
• Example: examples/unix_socket_server.rs

Estimated Effort

3-5 days including tests, docs, and example.

References

• rmcp UDS PR: feat(transport): add Unix domain socket client for streamable HTTP modelcontextprotocol/rust-sdk#749
• axum UnixListener support via tokio