Use ChatGPT subscription models in DeepSeek Harness

This guide uses the official model gpt-6.1-terra as an example and connects DeepSeek Harness to the ChatGPT subscription models available on the Coskey host. DeepSeek Harness receives only a ckg_… client token issued by Coskey. It never receives the ChatGPT OAuth token or another provider’s API key; the model gateway forwards the actual request.

← Back to Model gateway

1. Before you begin

• Coskey is running and the model gateway is enabled;
• the Coskey host has a valid ChatGPT subscription login; an API-key login is not a substitute for a ChatGPT subscription login;
• DeepSeek Harness is installed on the same computer;
• note the Base URL shown on Coskey → Model gateway. A normal installation usually shows http://127.0.0.1:15721/v1.

The screenshots in this guide come from an isolated instance: 17978 is that instance’s assigned port. Use the address displayed on your own gateway page, not the port in the screenshots.

2. Create a dedicated client in Coskey

Open Coskey → Model gateway and click Create client in the clients section:

Use these values:

Saving displays a ckg_… token once. Copy it now and use it as the API key in DeepSeek Harness. Only the plaintext token is kept in the system keychain; the Coskey configuration stores its SHA-256:

The command-line equivalent is:

coskey gateway enable
coskey gateway client add deepseek-harness \
  --scope local \
  --model gpt-6.1-terra

gateway client add also displays the token only once. To add another model later, edit the client row and select the model there. You do not need to recreate the client or replace the token; rotate it only if the token is lost.

3. Add Coskey to DeepSeek Harness

In DeepSeek Harness, open Settings → Models → Add model provider, choose Custom model API, and enter:

Provider ID coskey is DeepSeek Harness’s local identifier; it is not the Coskey gateway client ID deepseek-harness. Click Get available models, select gpt-6.1-terra and add it:

The model ID must exactly match the ID returned by the Coskey gateway. When the client allows only gpt-6.1-terra, that should be the only model in the list. You can also add the ID by hand.

4. Start a session and verify

Save the provider, start a new session and confirm that the model picker shows gpt-6.1-terra. Send a minimal test message:

Reply only with “Coskey gateway test passed”.

A matching reply confirms that the Coskey client, model gateway, ChatGPT subscription login and DeepSeek Harness are connected correctly:

5. Subscription login and security boundary

gpt-6.1-terra is an official model. The Coskey gateway reads the ChatGPT subscription login in the host’s CODEX_HOME/auth.json to make the upstream request. It reads the login without refreshing it. The client submits only its ckg_… token; the host OAuth token does not appear in the DeepSeek Harness configuration, gateway API responses, chat history or gateway logs.

Official and third-party models use their respective quotas: an official model uses the ChatGPT account subscription quota on the host, while a third-party model uses its provider’s quota. When you no longer need Harness, disable or revoke the deepseek-harness client in Coskey; other agents’ clients are unaffected.

6. Common questions

If the problem remains, run coskey gateway status and coskey doctor in Coskey. The gateway log is ~/.coskey/logs/gateway.log; it never contains tokens, upstream keys, prompts or completions.

Related: Model gateway, Status and settings.