fix(opencode): apply agent variant when no explicit model is configured

[21pounder]

Issue for this PR

Closes #21632

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

In packages/opencode/src/session/prompt.ts:902, the same variable is computed as:

const same = ag.model && model.providerID === ag.model.providerID && model.modelID === ag.model.modelID

When a subagent has no explicit model configured, ag.model is undefined, so same is false. This gates off the ag.variant branch, causing the configured variant to be silently dropped at runtime.

Fix: treat a missing ag.model as "same model" (inherited), so the variant is applied:

const same = !ag.model || (model.providerID === ag.model.providerID && model.modelID === ag.model.modelID)

How did you verify your code works?

Added a temporary debug log at line 908 to trace variant resolution, then ran the subagent before and after the fix.

Before fix: agVariant=high is present but variant is missing (undefined).
After fix: variant=high is correctly resolved.

Screenshots / recordings

Subagent running (configured with variant: "high", no model):

Before fix — agVariant=high but variant missing:

After fix — variant=high correctly resolved:

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found several related PRs that address variant inheritance and agent configuration:

Potentially Related PRs:

1. fix(opencode): Subtasks and task calls don’t inherit selected thinking level #20742 - fix(opencode): Subtasks and task calls don't inherit selected thinking level

   • Directly related: addresses variant/thinking level inheritance for subagents, which is the same core issue

2. feat(task): pass variant to subagent so it inherits parent's thinking level #12567 - feat(task): pass variant to subagent so it inherits parent's thinking level

   • Related: implements variant inheritance for subagents from parent agents

3. fix(acp): sync model and variant when activating session modes #18663 - fix(acp): sync model and variant when activating session modes

   • Related: addresses syncing model and variant configuration

4. fix(acp): session/set_mode now activates agent's configured model #18621 - fix(acp): session/set_mode now activates agent's configured model

   • Related: handles agent model configuration activation

These PRs all touch on similar areas of agent variant/model configuration and inheritance, particularly for subagents. PR #20742 appears most closely related as it specifically addresses thinking level (variant) inheritance for subtasks.

[github-actions]

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