tui: compaction triggers around 30–35% with gpt-5.6-sol

Summary

In the TUI, automatic compaction consistently triggers when the context indicator reaches only
approximately 30–35% while using openai/gpt-5.6-sol. This causes sessions to be
compacted while most of the reported context window remains available.

Environment

• opencode version: v0.0.0-next-16197
• OS: Darwin 25.4.0, arm64
• Terminal: tmux (TERM=xterm-256color, COLORTERM=truecolor)
• Shell: /bin/zsh
• Install/channel: OpenCode 2 beta/next
• Active plugins: none
• Model: openai/gpt-5.6-sol

Reproduction

1. Launch the OpenCode 2 beta TUI with openai/gpt-5.6-sol as the active model.
2. Continue working in a session until the TUI context indicator reaches approximately 30–35%.
3. Observe that automatic compaction starts at that point.

Expected Behavior

Compaction should occur only when the session approaches the model's usable context
limit. A context indicator showing 30–35% should not cause automatic compaction unless
the displayed percentage or configured model limit is incorrect.

Actual Behavior

Automatic compaction consistently starts around 30–35% reported context usage in the TUI.

Additional Context

The behavior is consistent with openai/gpt-5.6-sol. Other models have not been tested.
No exact error is shown, and no workaround has been identified.

Start by reproducing the issue in the OpenCode 2 beta TUI with openai/gpt-5.6-sol, then trace the context indicator and automatic compaction paths. Compare the displayed percentage with the configured model limit; done means compaction no longer starts around 30–35% unless that limit is actually reached.