GitHub issue #96326 in anthropics/claude-code, opened 2026-09-23 by noboru007, records a report that in a long agentic session the model sent several user-facing replies in English despite a CLAUDE.md rule and a session language setting requiring Japanese, that the English replies always followed turns whose tool output was English, and that three corrections did not hold, with the model slipping back into English within the same turn or the turn after.
Verbatim Excerpts from Cited Records
“The user-level `~/.claude/CLAUDE.md` says every reply must be in Japanese, and the session language setting also says to always reply in Japanese.”
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anthropics/claude-code (issue 96326 opening body, What happened section)
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2026-09-23T11:11:41Z
·GitHub Issue ↗
“In one long agentic session, the model (Opus 5.5, 1M context, Concise output style) sent several user-facing replies in English.”
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anthropics/claude-code (issue 96326 opening body, What happened section)
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2026-09-23T11:11:41Z
·GitHub Issue ↗
“They always followed turns whose tool output was English: an English specification document, English subagent reports, and English code-review output.”
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anthropics/claude-code (issue 96326 opening body, What happened section)
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2026-09-23T11:11:41Z
·GitHub Issue ↗
“For example, right after the model wrote that it would reply in Japanese from then on, the same turn contained the English line "All gates pass. Now for the pre-commit review."”
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anthropics/claude-code (issue 96326 opening body, What happened section)
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2026-09-23T11:11:41Z
·GitHub Issue ↗
Counter-record: Issue remains open with 20 comments as of 2026-11-05; no maintainer resolution appears in the surveyed thread. The reporter added recurrences the same day (2026-09-23): drift recurred after filing, after compaction, and after denied tool calls. A second reporter reproduced it on 2026-09-24 (0-7 percent English on CLI 2.1.280 versus 45 percent on 2.1.281, English only in Concise segments), then corrected the same day that a Default segment also drifted after reading a long English shell script, weakening both the Concise-reminder hypothesis and the switch-to-Default workaround. The same pattern was reported for Hebrew, Italian, Korean, Traditional Chinese, German, Russian and Persian between 2026-09-25 and 2026-11-02, and one commenter reported the opposite direction (English required, drift into another language). A session-replay measurement on 2026-11-05 found that defining allowed English by who reads it cut English replies from 15 of 100 to 1 of 100. The claim documents only the original report, not a verified current state.
Falsification probe: If issue 96326 is deleted without archive, if the author edits the opening body so the quoted sentences no longer match verbatim, or if the author posts a retraction of the report, this claim drops to Withdrawn.
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