Sounds reasonable. Thinking more but i guess we can do it by introducing new setup hook and give an interface to it to interact.

by introducing new setup hook and give an interface to it to interact.

did you mean adding a new hook that gets called on init and gives a publisher that can publish to all connections? The user would then need to store the publisher somewhere it can be accessed.

something like this?

export interface Hooks extends Record<string, HookFn<any[], any>> {
  /** Called on init - Publish to all connections */
  publisher: (publisher: WSPublisher) => MaybePromise<void>;  // <- new hook

  /** Upgrading */
  upgrade: (req: WSRequest) => MaybePromise<void | { headers?: HeadersInit }>; 

  /** A message is received */
  message: (peer: Peer, message: Message) => MaybePromise<void>;

  /** A socket is opened */
  open: (peer: Peer) => MaybePromise<void>;

  /** A socket is closed */
  close: (
    peer: Peer,
    details: { code?: number; reason?: string },
  ) => MaybePromise<void>;

  /** An error occurs */
  error: (peer: Peer, error: WSError) => MaybePromise<void>;
}

for node this should be simple since the defineWebSocketAdapter does have access to the WebSocketServer so creating a publisher should be easy

for bun on the other hand we would need access to the Bun.serve() return value.
The only way I could think of would be to add a callback to the return value of the defineWebSocketAdapter

like this:

export interface BunAdapter {
  websocket: WebSocketHandler<ContextData>;
  handleUpgrade(req: Request, server: Server): Promise<boolean>;
  setServer(server: Server): void; // <- new part
}

this would have to be called after the Bun.serve() and would call the publisher hock internally

maybe I also completely misunderstood your comment...