Why super-line

super-line is a typesafe realtime data bus for TypeScript. You write one contract; the server implements it and the client calls it with full end-to-end type inference — and zero codegen — over the wire of your choice: WebSocket, HTTP (SSE / long-poll), or libp2p/WebRTC, swapped in one line.

The idea

Realtime apps usually glue together hand-maintained event-name constants, untyped payloads, and ad-hoc validation. super-line replaces all of that with a single defineContract({...}) object that both sides import. From that one declaration you get:

• Types on both ends — the server's handlers and the client's calls are inferred from the same source, so they can't drift.
• Runtime validation — the same schemas that type your payloads also validate them. The server rejects malformed input automatically.
• Five interaction flavors over one connection — requests, events, topics, rooms, and node-to-node messaging.
• Any wire — the same contract and the same code run over WebSocket, HTTP, or libp2p/WebRTC. The transport is one line; everything above it is identical. See Transports.

Two axes: direction and role

The contract is organized along two axes:

• Direction — clientToServer (requests) and serverToClient (events & topics). Each is a named key on the contract, so there are no positional generics to get backwards.
• Role — a shared base plus one block per client role (user, agent, …). A connection's role is fixed at connect (by authenticate) and decides which surface — and which ctx — it gets. A cross-role call is rejected with NOT_FOUND.

See The contract for the full model.

How it compares

	super-line	Socket.IO	tRPC
Typesafe contract	✅	⚠️ types-only	✅
Runtime validation	✅	❌	✅
Per-role contracts	✅	❌	❌
Rooms & topics	✅	⚠️ rooms only	subscriptions
Inter-server messaging	✅	✅	❌
Pluggable wire (WS · HTTP · WebRTC)	✅	⚠️ WS + polling	⚠️ link-dependent

Socket.IO splits its types into ClientToServerEvents / ServerToClientEvents / InterServerEvents interfaces you wire as positional generics (easy to swap) with no runtime validation. super-line keeps the directional split but in one shared object, validates inbound automatically, and adds per-role contracts. See the full comparison & FAQ.

Next: Getting started.