feat: add agent default variant handling in TUI and desktop

[CasualDeveloper]

Issue for this PR

Closes #22065

Type of change

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

What does this PR do?

Respect an agent's configured model variant in the app and TUI when the current model supports it.

The change keeps selected/current/configured variant resolution in a shared helper so both clients agree on:

• user-selected variant wins
• Default clears the active variant override
• no saved choice falls back to the agent-configured variant
• invalid configured variants are ignored for the current model

The PR stays client/local-state scoped; it does not add backend/session/API/SDK behavior.

How did you verify your code works?

• bun turbo typecheck
• From packages/opencode: bun test --timeout 900000 ./test/agent/agent.test.ts (40 pass)
• From packages/core: bun test --timeout 900000 ./test/util/model-variant.test.ts (8 pass)

Screenshots / recordings

Not included; this changes prompt-bar metadata display/state behavior and is covered by the focused tests above.

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:

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!

[CasualDeveloper]

@rekram1-node rebuilt feature on top of latest origin/dev, taking into account your feedback:

• kept core/session variant handling unchanged (no extra llm fallback logic)
• moved variant behavior to TUI/Desktop local state only
• aligned both clients to the same selected/configured/current/cycle helper flow
• agent variant now works across models when that variant key exists on the active model (not locked to one model)
• kept hydration guard so configured defaults don't get persisted as explicit overrides

Also reran typecheck + variant-focused tests locally and they pass.

[Copilot]

Pull request overview

Adds agent-level default model variant handling and surfaces the effective variant in the UI, aligning TUI/Desktop behavior with per-agent configured reasoning effort.

Changes:

• Introduces shared variant resolution helpers for the TUI (configured vs selected vs default sentinel) plus tests.
• Updates TUI local context + prompt bar to compute/show an “effective” variant.
• Adjusts Desktop variant cycling behavior/tests and adds variant to the triage agent config.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/opencode/test/cli/tui/model-variant.test.ts	Adds unit coverage for variant resolution/cycling semantics in the TUI.
packages/opencode/test/agent/agent.test.ts	Verifies agent variant is loaded from config and defaults to undefined.
packages/opencode/src/cli/cmd/tui/context/model-variant.ts	Adds helper functions for configured-variant matching and variant resolution/cycling.
packages/opencode/src/cli/cmd/tui/context/local.tsx	Wires configured + selected variant resolution into the TUI local model context.
packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx	Initializes local agent/model/variant from the last user message; shows effective variant in prompt bar.
packages/app/src/context/model-variant.ts	Updates Desktop cycling logic (no longer advances from configured variant).
packages/app/src/context/model-variant.test.ts	Updates/adds tests to match the new Desktop cycling behavior.
packages/app/src/context/local.tsx	Adds effective() alias for variant display parity.
.opencode/agent/triage.md	Sets triage agent default variant: "high".

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[CasualDeveloper]

Fork updated to sync with dev (as of cb1a500).

[lowlyocean]

Hey everyone, wondering what the latest is on this? Trying to have "Build" agent (with local model) default to enable_thinking: false , and there doesn't seem to be a way to configure that yet: #23995