[Bug] Chinese IME input shows underscores in TUI on Windows

[cgartlab]

Bug Report: Chinese IME input shows underscores in TUI on Windows

Environment

Item	Value
OpenCode version	1.15.12
OS	Windows 11 Pro Insider Preview (Build 26220)
Terminal	Windows Terminal
Shell	PowerShell 7.6.2
Node.js	v26.2.0
System code page	65001 (UTF-8)
Input method	Microsoft Pinyin (zh-Hans-CN)
Terminal font	Hack Nerd Font

Description

When typing Chinese characters in the OpenCode TUI chat input using a Chinese IME (Input Method Editor), all composed characters are displayed as underscores _______ instead of the actual Chinese characters.

The IME candidate window appears correctly, and selecting a candidate should input the Chinese character, but the TUI renders it as underscores.

Steps to Reproduce

1. Launch opencode in Windows Terminal
2. Switch to Chinese input method (Microsoft Pinyin)
3. Type pinyin in the chat input (e.g., nihao)
4. The input field shows _______ instead of the composition text
5. Select a candidate from the IME window
6. The confirmed character also appears as underscore or is not displayed

Expected Behavior

• During IME composition: the pinyin text should be visible in the input field
• After IME confirmation: the Chinese character should be displayed correctly
• The input should work seamlessly with Chinese IME, similar to how it works in other TUI applications

Actual Behavior

• All Chinese input (composition and confirmed characters) appears as underscores _______
• The IME candidate window works correctly
• English input works fine
• The issue affects all Chinese IME operations in the TUI

Diagnostic Information

System code page verification:

Active code page: 65001
ACP: 65001
OEMCP: 65001

Chinese character OUTPUT test (bash tool):

$ echo "中文测试：你好世界"
中文测试：你好世界

Output works correctly — the issue is INPUT only.

VT processing:

HKCU:\Console\VirtualTerminalLevel = 1
HKCU:\Console\%wt%\VirtualTerminalLevel = 1

Related Issues

This appears to be related to:

• CJK Input Method (IME) not working in terminal - input bypasses IME and shows raw keystrokes #10946: CJK Input Method (IME) not working in terminal
• [Windows Terminal] Chinese IME input intermittently stops working in TUI (v1.2.10) #14761: [Windows Terminal] Chinese IME input intermittently stops working in TUI
• [BUG] Backspace deletes confirmed Chinese characters instead of composition pinyin during IME input #20818: Backspace deletes confirmed Chinese characters instead of composition pinyin
• TUI no support chinese, input output allways in 'w;w;w;w;w;w;w;w;w;;w;w' #23940: TUI no support chinese
• [Bug] Keyboard shortcuts don't work when Chinese IME is active #24192: Keyboard shortcuts don't work when Chinese IME is active
• opentui: Unable to input Chinese characters #3404: Unable to input Chinese characters

Root Cause Analysis

The issue appears to be in the TUI's input handling layer. The IME composition events (compositionstart, compositionupdate, compositionend) are not properly handled in the Windows terminal environment. The TUI renders the composition buffer as underscores instead of the actual text.

PR #17013 and #17041 attempted to fix IME composition handling, but the issue persists on Windows.

Additional Context

• The issue is Windows-specific — macOS and Linux may handle IME differently
• The issue affects input only — Chinese character output works correctly
• The issue persists with system code page set to 65001 (UTF-8)
• The issue affects all Chinese IME operations (composition, confirmation, editing)

Workaround

Currently, the only workaround is to:

1. Type Chinese text in an external editor (e.g., Notepad, VS Code)
2. Copy the text
3. Paste it into the OpenCode TUI

This is not ideal for users who primarily work with Chinese input.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• TUI no support chinese, input output allways in 'w;w;w;w;w;w;w;w;w;;w;w' #23940: TUI doesn't support Chinese input (same symptom: garbled/incorrect Chinese input in TUI)
• [Windows Terminal] Chinese IME input intermittently stops working in TUI (v1.2.10) #14761: Chinese IME input intermittently stops working in TUI on Windows Terminal (same environment and IME handling issue)
• [BUG] Backspace deletes confirmed Chinese characters instead of composition pinyin during IME input #20818: Backspace deletes confirmed Chinese characters instead of composition pinyin during IME input (related IME composition handling bug)

Please review these issues to see if your problem is already tracked. If it is, consider adding any new information there rather than opening a new report.

[cgartlab]

Closing to resubmit with proper template format.