Can I use an Anthropic-compatible server or gateway instead of Anthropic itself?

Yes, from version 1.9.16. Open Settings, "AI engines", click "Claude", and fill in "Base URL" with the address of the server, for example a provider's Anthropic-compatible endpoint or your company's gateway. Leave it empty to use Anthropic itself. Save the key for that address, type or pick the model name your server uses under "Model", and click Test. On the official Anthropic address nothing changes: the same key, the same model list.