[Windows] UTF-8/GBK mojibake in model thinking chain and assistant output

Summary

On Chinese Windows (zh-CN locale, console code page 936/GBK), CJK text in the model's thinking chain (reasoning block) and in regular assistant output frequently renders garbled — a UTF-8 ↔ GBK mismatch that is visible live in the UI, not only in exports.

Environment

• OS: Windows 10/11, Chinese (zh-CN) system locale, default console code page 936 (GBK/CP936)
• App: OpenCode Desktop 1.18.15 (also reproduced in the terminal/TUI build)
• Shell: PowerShell 5.1 (default ANSI/GBK on this machine)
• Model: any (reproduced with DeepSeek via proxy)

Observed

• The thinking/reasoning block frequently shows garbled Chinese characters.
• Assistant messages containing CJK are garbled the same way.
• Typical mojibake seen:
  • UTF-8 bytes decoded as GBK → classic 锟斤拷-style garbage
  • Replacement characters �
  • Text that is ASCII-correct but CJK-corrupt within the same message, implying an inconsistent decode path
• Force-switching the console to UTF-8 (chcp 65001) or setting [Console]::OutputEncoding = UTF8 avoids the issue, which points at the console code page (CP936) being picked up somewhere in the render/stream pipeline.

Expected

All text — thinking chain and output — should be rendered consistently as UTF-8 regardless of the Windows console code page.

Reproduction steps

1. On a zh-CN Windows machine (console code page 936), launch OpenCode Desktop or the TUI.
2. Ask a question that produces a long Chinese response with a visible reasoning/thinking block.
3. Observe mojibake in the thinking block and in the assistant reply whenever CJK is present.

Root-cause hypothesis

Something in the TUI/desktop stream pipeline decodes the model's UTF-8 bytes using the console's GBK code page (or transcodes UTF-16 → CP936 before writing, like the known process.stdout behavior), producing the mismatch. This mirrors the export bug in #36050 but affects live rendering.

Related issues

• #36050 — opencode export garbled CJK on Windows due to process.stdout transcoding to console code page (same class of bug, export path only)
• #36985, #23636 — PowerShell output encoding / console UTF-8 workarounds
• #31470 — Chinese text garbled when pasting in TUI input
• #31187 (closed) — garbled CJK due to Windows default ACP (GBK/CP936)
• #30055 (closed) — CLI shell uses GBK by default, Chinese garbled