[Bug] Assistant language randomly switches to Korean/Chinese in Thai-language sessions

Bug Description
Title: Assistant unexpectedly switched to Korean mid-conversation (Thai-language session) What happened: In a session conducted entirely in Thai (working on a Thai hospital's population registry app), the assistant's response after a CLAUDE.md-condensing task was written entirely in Korean instead of Thai. The user then reported this happens intermittently ("sometimes Korean, sometimes Chinese"). What the user said: "ทำไมคุยภาษาเกาหลี" (why are you speaking Korean), then "เป็น bug ของแกไหม บางทีเหลาหลีบางทีจีน" (is this a bug on your end? sometimes Korean, sometimes Chinese). Repro: Long session (post context-compaction, large CLAUDE.md file with heavy Thai content mixed with English identifiers/paths) — one assistant turn's entire output language flipped to Korean unprompted, with no language switch cue in the preceding user message.

Environment Info

• Platform: win32
• Terminal: windows-terminal
• Version: 2.1.278
• Feedback ID: b31021f3-76b8-491d-90ec-396cb6a87493

Errors

[{"error":"TelemetrySafeError: VirtualMessageList: itemKeys/messages length desync (keys=129 messages=128 range=[99,129))\n    at Deo (B:/~BUN/root/chunk-zd9p1yms.js:37:31024)\n    at pve (B:/~BUN/root/chunk-zd9p1yms.js:37:21923)\n    at Xs (B:/~BUN/root/chunk-3k2y9fc2.js:18:20036)\n    at du (B:/~BUN/root/chunk-3k2y9fc2.js:18:38202)\n    at gm (B:/~BUN/root/chunk-3k2y9fc2.js:18:84284)\n    at Ab (B:/~BUN/root/chunk-3k2y9fc2.js:18:83284)\n    at wu (B:/~BUN/root/chunk-3k2y9fc2.js:18:83113)\n    at cm (B:/~BUN/root/chunk-3k2y9fc2.js:18:79637)\n    at dt (B:/~BUN/root/chunk-3k2y9fc2.js:18:6261)\n    at Jn (B:/~BUN/root/chunk-3k2y9fc2.js:18:4792)","timestamp":"2026-09-22T12:07:08.436Z"}]