Seems like this may be a delay in the terminal subscribing to the event. Unsure if this is a bug/unintended race, or by design (if intended, I think it should be documented against the API somewhere).

This works:

const emitter = new vs.EventEmitter<string>();
let terminal!: vs.Terminal;
const terminalCreation = new Promise((resolve) => {
	const pseudoterminal: vs.Pseudoterminal = {
		close: () => { },
		onDidWrite: emitter.event,
		open: () => { emitter.fire("Hello world!\r\n"); resolve(); },
	};
	terminal = vs.window.createTerminal({ name: `Danny Test Terminal`, pty: pseudoterminal });
});
await terminalCreation;
terminal.show();
setTimeout(() => emitter.fire("Test 3\r\n"), 0);

I think it's by design, open is the callback when everything is ready, you should only write after open is called.

@Tyriar my suggestion above was that if this was by-design, it was called out in the docs. It's difficult to know when things are bugs or not if they're only documented in GH issues that are really difficult to search and may just result in more people filing issues (or, having bugs).

Will do, thanks