tui: auto-assigned "medium" variant crashes sessions for models with empty variants (e.g. mimo-v2.5-free)

Summary

opencode2 (V2) auto-assigns the medium variant when creating a session with a reasoning model whose catalog entry has variants: [] (empty). Runtime then fails with SessionRunnerModel.VariantUnavailableError and the session silently falls back to another model.

Environment

• opencode version: 0.0.0-next-17444 (next channel)
• OS: Linux 7.0.11-76070011-generic (linux x64)
• Terminal: xterm-256color / truecolor
• Shell: /bin/bash
• Install/channel: next (v2 binary opencode2)
• Active plugins: @warp-dot-dev/opencode-warp

Reproduction

1. Open the opencode2 TUI (next channel).
2. Select opencode/mimo-v2.5-free from the model picker (/models); do not touch any variant setting.
3. Send any prompt.

Expected Behavior

The session runs mimo-v2.5-free with variant default (the catalog entry defines no variants), and the model responds normally.

Actual Behavior

The created session is pinned to variant: "medium" even though the model catalog entry has variants: []. The model resolver throws:

SessionRunnerModel.VariantUnavailableError: Variant unavailable for opencode/mimo-v2.5-free: medium

and the session falls back to opencode/big-pickle.

Root Cause

The TUI defaults the variant to medium for models with reasoning: true, but mimo-v2.5-free's catalog entry has an empty variants list (variants: []). The resolver does not tolerate a variant that does not exist and hard-fails instead of falling back to the default.

Verified via the HTTP API: creating a session with variant: "default" (or omitting the variant) resolves correctly and the model works. Log source: ~/.local/share/opencode/log/opencode.log (Failed to drain Session ... SessionRunnerModel.VariantUnavailableError).

Workaround

After selecting the model with /models, choose the default option in /variants, or switch via API:

opencode2 api post /api/session/<sessionID>/model --data '{"model":{"id":"mimo-v2.5-free","providerID":"opencode","variant":"default"}}'

Additional Context

• opencode V1 (1.17.18) works fine with the same model: it does not use the variant mechanism.
• The models.dev cache entry for mimo-v2.5-free has reasoning: true and reasoning_options: [].
• Related: #40524 (V2 catalog reconciliation), #41868 (variants not selectable for Merge Gateway).