Use Radium with Cursor

Connect Radium's Tycho model to Cursor using Cursor's built-in OpenAI-compatible API support. Settings, model ID, verification, and troubleshooting.
Use Radium with Cursor
Cursor has built-in support for OpenAI-compatible APIs, and Radium serves one, so connecting Tycho is four settings and no plugin.
This page covers Tycho specifically. Any Radium model reachable through the OpenAI-compatible endpoint can be added the same way by repeating step three with a different model identifier.
Scope
This connects Radium to Cursor Chat and Cursor Agent. Cursor Tab, the inline autocomplete, keeps using Cursor's own model regardless of what you select here. See About Tab autocomplete.
Before you start
Cursor installed
A Radium API key
Open model settings
In Cursor, go to:
Cursor Settings → Models
Connect the API
Radium's API is OpenAI-compatible, so the connection goes under the OpenAI section of the Models panel, not Anthropic. Under API Keys, set:
Setting
Value
OpenAI API Key
Your Radium API key
Override OpenAI Base URL
https://api.radium.cloud/v1
Enable both toggles, OpenAI API Key and Override OpenAI Base URL. Neither takes effect while off.
The one to get right
This goes in the OpenAI fields even though you are not using OpenAI. Cursor does not have a generic custom-provider slot, and the OpenAI-compatible one is the correct place for any OpenAI-shaped endpoint, Radium included.
Add Tycho
Still in the Models panel, select Add Custom Model and enter the identifier exactly:
tycho-1.0
Enable the model once it is added. It will not appear in the picker otherwise.
Verify the connection
Open a new Cursor Chat or Agent session and select 
tycho-1.0 from the model picker. Send:
Reply with exactly: RADIUM_OK
Expected response:
RADIUM_OK
If you see that, the connection is live and Tycho is answering Chat and Agent requests.
About Tab autocomplete
Cursor Tab, the inline completion as you type, continues to run on Cursor's own model no matter what is selected in the Models panel. Radium serves 
tycho-1.0 for Chat and Agent sessions only. This is a Cursor limitation, not a Radium one, and there is currently no setting that changes it.
When it does not work
Confirm the base URL includes 
/v1. Without it, requests go to the wrong path.
Confirm the model ID is exactly 
tycho-1.0. Cursor does not correct typos in a custom model entry.
Confirm the OpenAI API Key toggle is enabled, not just filled in.
Start a new Cursor session after selecting Tycho. An existing session can keep using whatever model it started with.
Fully quit and restart Cursor if the new settings do not take effect. Cursor does not always pick up a provider change without a restart.
Next
API quickstart, for calling Radium directly
Use Radium with Claude Code
Tool calling