Agent variant incorrectly applied to all models, causing errors with OpenAI-compatible providers

[neavo]

Bug Description

When configuring an agent with a specific variant (e.g., xhigh for reasoning effort), the variant is incorrectly applied to all models used by that agent, not just the model it was configured for. This causes API errors when switching to models that don't support the variant parameter.

Steps to Reproduce

1. Configure an agent with a model and variant:

   {
     "agent": {
       "plan": {
         "model": "openai/gpt-5.2",
         "variant": "xhigh"
       }
     }
   }

2. Use the agent with its configured model (works fine)
3. Switch to a different model (e.g., opencode/kimi-k2.5-free) while still using the same agent
4. The variant xhigh is still sent to the API, causing:

   Extra inputs are not permitted, field: 'variant', value: 'xhigh'
   

Expected Behavior

The variant should only be applied when using the specific model it was configured for. When switching to a different model, the variant should not be sent.

Root Cause

The variant field is currently treated as an unknown key and gets merged into agent.options, which is then passed to all API calls regardless of which model is actually being used.

Environment

• OpenCode version: latest
• Provider: OpenAI-compatible (opencode/zen)
• Model: kimi-k2.5-free (and other non-OpenAI models)

Related

This issue affects any OpenAI-compatible provider that doesn't support the variant parameter, including Kimi, GLM, and other models accessed through the opencode provider.

[github-actions]

This issue might be a duplicate of existing issues or related to broader parameter handling problems. Please check:

• [FEATURE] Support default variant configuration per agent (#7156) #7138: [FEATURE] Support default variant configuration per agent - Related to variant configuration in agents
• [BUG] Kimi K2 Rejects permissions Field in Agent Frontmatter #11320: [BUG] Kimi K2 Rejects 'permissions' Field in Agent Frontmatter - Similar issue where agent-level fields cause API errors with OpenAI-compatible providers (Kimi K2)
• Custom OpenAI-compatible provider options not being passed to API calls #5674: Custom OpenAI-compatible provider options not being passed to API calls - Related to configuration fields not being properly handled for OpenAI-compatible providers
• Extra inputs are not permitted, field: 'promptCacheKey' #4386: Extra inputs are not permitted, field: 'promptCacheKey' - Similar pattern of extra fields causing errors with providers using OpenAI SDK

The root cause appears to be a broader issue where OpenCode doesn't properly isolate model/provider-specific parameters. Fields like 'variant', 'permissions', and 'promptCacheKey' are being passed to all models/providers instead of being filtered based on provider compatibility.

Feel free to ignore if none of these address your specific case.