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In Cursor you can specify a custom OpenAI-compatible endpoint.
1

Settings → Models

Open the model settings.
2

OpenAI API Key → Override Base URL

Enable “Override OpenAI Base URL” and enter:
https://api.ethereal.llc/v1
In the API Key field, paste your eth-....
3

Add a model

Add a custom model named claude-sonnet-4-6 (or claude-opus-4-8 / claude-haiku-4-5) and save.
4

Verify

Click “Verify” — Cursor will make a test request through ethereal.
Some Cursor features (Tab, the Composer agent) may require Cursor’s own models and aren’t routed through a third-party key — chat and manual model selection work.