Model selection in UI is ignored - reverts to default model instead #9824

Bug Description

When selecting a model in the OpenCode UI model selector, the selection is ignored when submitting a prompt. The system reverts to using a default model instead of the user-selected model.

Expected Behavior

User selects a model in the UI → Submit prompt → System uses the selected model

Actual Behavior

User selects a model in the UI → Submit prompt → System ignores selection and uses default model (e.g., Gemini 3 Pro Preview)

Steps to Reproduce

1. Open OpenCode in a directory with existing auth credentials for multiple providers
2. Open the model selector (Ctrl+O or via UI)
3. Select a different model than the default
4. Submit a prompt
5. Observe that the system uses a different model than selected

Environment

• OS: macOS (darwin)
• OpenCode version: Latest (as of 2026-01-21)
• Providers configured: anthropic (oauth), google (oauth), zai-coding-plan (api)

Troubleshooting Already Done

1. Plugin interference testing

• Disabled oh-my-opencode plugin → Issue persists
• Disabled opencode-antigravity-auth plugin → Issue persists
• Result: Issue is in OpenCode core, not plugins

2. Log analysis

Logs show the system auto-selecting models:

3. Configuration files checked

• ~/.config/opencode/opencode.json - No global model set
• ~/.local/share/opencode/auth.json - Auth credentials present
• ~/.config/opencode/oh-my-opencode.json - Has agent configs
• No config.json in ~/.local/share/opencode/

4. Environment

• No OPENCODE_MODEL env var set
• No model-specific env vars that would override

What Should Happen

The model selected in the UI should be respected and used for the session.

Additional Notes

• The issue occurs consistently across sessions
• Both primary providers (anthropic, google) show this behavior
• This appears to be a regression or design flaw in how OpenCode handles model selection persistence