[BUG] Korean output: valid Hangul but wrong/missing syllables and hallucinated non-words (not U+FFFD)

Summary

This is distinct from existing CJK corruption issues (#40396, #45508, #46863, #41358),
which all produce U+FFFD (�) replacement characters from UTF-8 byte truncation in
the terminal/streaming layer.

This bug produces syntactically valid Hangul syllables that are semantically wrong —
missing 종성 (final consonants), syllables dropped, and most concerning, hallucinated
non-words that do not exist in Korean appearing in the model's output. Zero � are
involved.

Examples (from a single Plan-mode AskUserQuestion prompt, screenshot to be attached)

| Expected | Actual | Pattern |
|---------------|-----------|------------------------------------------------------|
| 캠페인 | 캐페인 | Final consonant ㅁ dropped (1 jamo missing) |
| 혼란 / 이슈 | 와멸 | Hallucinated non-word — 와멸 is not a Korean word |
| 방향 / 방안 | 팡 | 2 syllables collapsed to 1 |

The string "와멸" does not exist in Korean — not in dictionaries, not in colloquial
usage, not as a loanword. The model emitted it twice in the same prompt as if it were
a real word meaning \"issue/disruption.\" This is the dangerous signature: a native
reader sees valid-looking Korean that is nonsense; an encoding-based check sees nothing
wrong.

Why this is NOT the U+FFFD bug

• Output contains zero U+FFFD replacement characters
• All bytes form valid UTF-8 and valid composed Hangul syllables
• Corruption is at the token-selection / semantic layer, not the byte layer
• File writes show the same corruption (existing U+FFFD bug spares file writes;

this one does not)

This means existing UTF-8 boundary-fix proposals (.floor_char_boundary() etc.)
will not address it. The bug appears to be upstream in inference, not in CLI rendering.

Environment

• Claude Code 2.1.145
• Model: Opus 4.7, 1M context
• macOS 15.6 (build 24G84), default Claude Code TUI
• Heaviest in Plan mode with large system prompt (CLAUDE.md + memory + skills +

multi-project context)

• Onset: roughly early May 2026; progressive — minor at session start, increasingly

frequent after context compaction

Suspected root cause

Not a CLI / terminal bug. Candidates:

1. Opus 4.7 1M-context decoder regression for low-resource-language tokens
2. Context compaction corrupting Korean BPE token sequence integrity
3. New tokenizer (claimed 20–35% efficiency improvement for CJK in 4.7)

introducing a decoding-side regression

Impact

More dangerous than the U+FFFD bug for Korean users:

• No visual indicator the output is corrupted (no \�\)
• Corruption propagates through tool calls, file writes, and git commits

undetected by any encoding-based linter

• Plan-mode \.md\ outputs persist the corruption into the codebase, where it

may be acted on later

(screenshot to follow as a comment)