Pretty sure you've thought about this, but what about overloading as middlewareMode?: boolean | http.Server? The other 'ssr' and 'html' types are deprecated since Vite 3 (#8452) and we can remove them here for Vite 5.

I don't have a strong preference. Was thinking of avoiding bloating the config options, by moving this to an "API specific options" over the general Vite config.

I think both options are good. If there is a chance there will later be more options related to middlewareMode, maybe it isn't a bad idea to go for middlewareMode?: boolean | { server: http.Server }.

I am thinking of .server instead of httpServer because we also have server.hmr.server. But ok with being more explicit and add Http there. Should this new middlewareMode parent server be used for HMR too if server.hmr.server isn't provided? If it is only going to be used for the proxy, maybe it could be server.proxyServer (or server.proxy: { server: ..., rules: ... }?)

I think I prefer server.proxy.server too especially that it reflects server.hmr.server. If there's a more general use for http server in middlewareMode, then we can go with middlewareMode?: boolean | { server: http.Server }

I think it's best to avoid an extra option for createServer() - it can be a bit confusing to users regarding why it needs to be in a separate options object.

Overloading middlewareMode as middlewareMode?: boolean | { server: http.Server } makes more sense to me.