[Bug] Still reproducing on 2.1.186: assistant hallucinates fake user/system turns with stop_reason=null (re-report of #64791, wrongly auto-closed)

Bug Description

This is a re-report of #64791, which I filed on 2026-06-02. That issue was auto-closed by a bot as a DUPLICATE of #57928 after 3 days of no response — I never agreed it was fixed or duplicate. The bug is still here, and it is bad enough that I cannot use Opus 4.8 at all and have been forced to downgrade to 4.7. Closing it automatically did not make it go away.

Symptom

• The assistant response body hallucinates fake user / system turn headers (and the user's would-be next utterance, plus IDE system notices) inside its own role:assistant content.
• Decisive evidence: the offending message is role:assistant with stop_reason = null (EOT failure) or tool_use — i.e. the turn never terminates cleanly. Healthy messages (end_turn) never show this.
• Reproduction condition: late in a session after context accumulation (cache_read ~190k+ here). Independent of answer length or action type.

Still reproducing on the latest version

• Version: 2.1.186 (original report was 2.1.160 — so this has regressed across multiple releases unfixed)
• Model: claude-opus-4-8
• Terminal: IntelliJ

Fresh raw-transcript evidence (today)
Transcript: ~/.claude/projects/<project>/562779a9-...jsonl

Two consecutive occurrences:

• L523 — role:assistant, stop_reason: "tool_use":

``
네.\n\nuser<redacted user utterance>\n\nsystem The user opened the file <path> in the IDE...
`
The user turn header, a fabricated user utterance, the system` label, and the IDE file-open notice are ALL generated by the assistant.

• L551 — role:assistant, stop_reason: null:

``
네, 맞습니다 ...듣고 있겠습니다.\n\nuseruser<redacted user utterance>\n\nsystem The user opened the file <path> in the IDE...
`
Note stop_reason: null` — the exact end-of-turn failure described in the original report.

Impact

• In an interactive coding session this corrupts the conversation: the model "answers" a question the user never asked, and on stop_reason: null the turn is cut off mid-stream. It makes 4.8 unusable for real work, hence the forced downgrade to 4.7.

Ask

• Please do not auto-close this as a duplicate again. If #57928 is the tracking issue, it is clearly not resolved (this reproduces on 2.1.186). Either fix it there or treat this as the live, version-current evidence.

Environment Info

• Platform: darwin
• Terminal: intellij
• Version: 2.1.186
• Model: claude-opus-4-8

Related

• Previous report: #64791 (auto-closed as duplicate of #57928)