I use routatic/proxy (actually my own fork with various changes/fixes http://github.com/nijave/proxy/ -- fyi the fork is all vibes...)

Some gateways/proxies/hosts also offer multiple endpoints (z.ai has both Anthropic endpoint and Completions endpoint)

Edit: Here's the PoC I made for work although it assumes you're using Cloudflare Access (VPN) with a Cloudflare AI Gateway to hand wave the auth away (auth to the LLM API gets auto injected by Cloudflare Access derived from the client's VPN connection) https://gist.github.com/nijave/6f5e5793a992619808135ce85fa49...