feat: add variant support for subagents (#7138) [alt of #7140]

[CasualDeveloper]

Summary

Complete implementation for #7138 (agent-level default variant support) with TUI and desktop support.

Adds variant field to agent config, allowing users to set a default variant per agent:

# .opencode/agent/oracle.md
---
model: anthropic/claude-sonnet-4
variant: "high"
---

Priority order: user --variant selection > agent config > none

Comparison with PR #7140

Both PRs implement the same core feature. This PR adds:

Feature	PR #7140	This PR
Core backend logic	✅	✅
TUI display	✅	✅
Desktop/web app	❌	✅
Message metadata	⚠️ undefined when agent default used	✅ Stores resolved variant
OpenAPI schema	❌	✅
Files changed	9	14

Key differences

1. Desktop support (this PR only)

• packages/app/src/context/local.tsx: Added effective() method
• packages/app/src/components/prompt-input.tsx: Uses effective() for display and API calls, shows variant in agent selector

2. prompt.ts change (this PR only)

Stores the resolved variant in message metadata:

// PR #7140 (stores user input only):
variant: input.variant,

// This PR (stores resolved value):
variant: input.variant ?? agent.variant,

Without this, inspecting message history shows variant: undefined when the agent's default variant was actually used.

Changes

Backend

• Add variant field to agent config schema (config.ts, agent.ts)
• Resolve variant in llm.ts: input.user.variant ?? input.agent.variant
• Store resolved variant in message metadata (prompt.ts)
• Subagents use their own configured variant (task.ts)

TUI

• Add effective() method to local.tsx
• Display effective variant in prompt bar (prompt/index.tsx)
• Show variant in agent picker dialog (dialog-agent.tsx)

Desktop

• Add effective() method to local.tsx
• Display effective variant in prompt bar (prompt-input.tsx)
• Show variant in agent selector dropdown (prompt-input.tsx)

SDK

• Update types.gen.ts and openapi.json

Examples

• Add variant: "high" to triage.md and duplicate-pr.md

Closes #7138

[github-actions]

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

Potential Duplicates Found

PR #7140: Add default variant config per agent
#7140

Why related: This is the primary implementation that the current PR (#7156) is an alternative version of. According to the PR description, #7156 is an alternative implementation for #7138 that includes additional changes not present in #7140, specifically:

• Fallback to agent.variant for message metadata in prompt.ts
• Clarifying comment explaining subagent variant design choice in task.ts

The PR author explicitly states they're "happy to close this if #7140 incorporates these changes," indicating these are addressing the same feature with slightly different approaches.

[rekram1-node]

@CasualDeveloper we can do ur change if u want but I think ti requires updates to both tui AND desktop

[CasualDeveloper]

Thanks @rekram1-node! I've updated this PR to include both TUI and desktop support:

TUI: local.tsx, prompt/index.tsx, dialog-agent.tsx
Desktop: local.tsx, prompt-input.tsx

Both now have:

• effective() method that returns current() ?? agent.current()?.variant
• Variant display in prompt bar
• Variant shown in agent selector

The prompt.ts change is backend-only and works for all clients automatically.

Ready for review when you have time!