TPI: Codex Harness in Agent Host using ChatGPT subscription

[Giuspepe]

Refs: #329668, #327290, #328461, #329139, #329156, #329396, #329957, #330455

• macOS Christof Marti (@chrmarti)
• windows Vijay Upadya (@vijayupadya)
• linux TylerLeonhardt

Complexity: 3

Authors: Giuseppe Cianci (@Giuspepe)

Create Issue

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Context

The Codex Harness in the Agent Host supports models provided through either a GitHub Copilot or ChatGPT subscription. When both accounts are signed in, models from both providers should be available and clearly identified in the Agents window.

This test requires a ChatGPT account. A free ChatGPT account is sufficient.

Test Steps

1. Open the latest desktop VS Code Insiders and make sure these settings are enabled
   • chat.agentHost.codexAgent.enabled
   • chat.agentHost.allowSignedOutWhenUsable
   • Both settings should normally be enabled by experiment, and Codex should be available in the Agents window

2. In the Agents window, sign in to GitHub Copilot and make sure ChatGPT is signed out
   • The account menu should show the Copilot account and offer Sign in to ChatGPT

3. Create a Codex session, select a Copilot-backed model, and send a message
   • The model should respond successfully using the Copilot subscription
4. Sign out of GitHub Copilot, sign in to ChatGPT from the account menu, then create a Codex session using a ChatGPT-backed model and send a message
   • The Agents window should remain usable while signed out of GitHub Copilot
   • The model should respond successfully using the ChatGPT subscription
5. Sign back in to GitHub Copilot while remaining signed in to ChatGPT
   • The account menu should show both accounts
6. Inspect both the Codex model picker and the Manage Language Models dialog
   • Models from both Copilot and ChatGPT should be available under clearly identified providers
   • Models with the same display name should remain distinguishable by provider

7. Select a model from each provider and send a message with each
   • Both models should respond successfully and remain clearly associated with the selected provider

[TylerLeonhardt]

I think we already have an issue somewhere on the duplicate "Copilot" entries in Manage Models so I didn't open a new one for that.