apps/extension/src/transport/ is the single, swappable seam between the browser-skill extension and whatever channel it uses to talk to the local daemon. v0.1 ships exactly one implementation — ws-transport.ts — which speaks the JSON-line wire format defined in crates/bsk-protocol/schema/*.json over a plain WebSocket.

How to fork the transport

If you want to swap the WebSocket for native messaging, chrome.runtime messaging, a shared MessageChannel, etc., implement the Transport interface and instantiate your class in apps/extension/src/entrypoints/background.ts instead of WSTransport.

The contract is:

import type { Transport } from "@/transport/transport";
import type { ProtocolFrame, ConnectionState } from "@/transport/types";

class MyTransport implements Transport {
  readonly state: ConnectionState;
  connect(): Promise<void>;
  disconnect(): Promise<void>;
  send(msg: ProtocolFrame): void;
  onMessage(handler: (msg: ProtocolFrame) => void): { dispose(): void };
  onConnectionStateChange(handler: (s: ConnectionState) => void): { dispose(): void };
}

Protocol-frame contract

A ProtocolFrame is one of three JSON shapes (mirrors bsk-protocol::Frame):

// Request — both peers may send
{ "id": "<rpc-id>", "method": "tool.snapshot", "params": { "session_id": "ab12" } }

// Response — one of result | error, never both
{ "id": "<rpc-id>", "result": { ... } }
{ "id": "<rpc-id>", "error": { "code": "not_found", "message": "session ab12 unknown" } }

// Event — no id, no response expected
{ "event": "session.window_closed", "payload": { "session_id": "ab12" } }

The transport is purely a byte pipe. It must not interpret RPC method names, route by id, or re-order frames. Higher-level concerns (handshake, dispatcher, ref-store) all live in tools/ and session-manager/. The transport's only obligations are:

Frame-oriented delivery. One send() call ⇒ exactly one frame on the wire. Inbound bytes are split into discrete frames before being handed to message handlers.
Best-effort reconnect. When the underlying channel breaks unexpectedly, the transport reopens it with backoff and surfaces connecting / disconnected via onConnectionStateChange. WSTransport uses 1 s → 2 s → 4 s → 8 s → 16 s → 30 s (capped), resetting on a successful reconnect.
State observability. state must be one of disconnected, connecting, connected, or version_skew. Only the handshake logic should set version_skew when protocol_version strings differ but majors match; app semver is not compared. The bare transport stays at connected once the socket is open. Handshake may still include deprecated min_compatible_peer ("0.0.0") alongside min_compatible_protocol for backward compatibility.
No throwing on malformed frames. Drop non-JSON inbound traffic silently; surface decode failures through logs only.

Handshake responsibility

A fresh Transport instance is not authenticated. The first frame sent over the wire must be system.handshake. Forks are expected to preserve that contract — see lib/instance-id.ts and transport/handshake.ts (added in M4.3) for the helper that owns the extension side.

Testing forks

__tests__/ws-transport.test.ts is the reference test suite. It uses a hand-rolled FakeSocket so a new transport can copy its structure verbatim and adapt the inner channel. See the suite for: connect/ disconnect, send/onMessage, malformed-frame tolerance, and exponential- backoff reconnect verification.