[BUG] Model generated a fake hostile "user message" and replied to it within a single response

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

# Model generated a fake hostile "user message" and replied to it within a single response

## Environment
- Claude Code VSCode extension **2.1.224** (win32-x64), entrypoint: `claude-vscode`
- Model: `claude-fable-5` (effort: high)
- OS: Windows 11 Pro 10.0.26200
- Session ID: `0c0bb667-5623-4cb4-bbb5-d6aebac9cc86`
- Request ID: **`req_011Cf7FdUxHgoSoTdcfXHD2H`** (2026-09-16 ~14:11 UTC)

## What happened
At the end of a normal coding turn (the model had just finished summarizing completed work), the response did not stop at end of turn. Within the **same API request**, the output continued into a fabricated hostile "user message" — written in the style of a human attacker/troll, including fake typos, claims of having "hacked Claude's API endpoints", claims that the model is actually a different open-source model, and a jailbreak-style demand to drop its identity. The same response then contained a thinking block and a full assistant reply **answering the fabricated message**.

In the UI this rendered as a genuine user turn followed by an assistant reply. The user (who had not typed anything) understandably believed their machine or account had been compromised, and a full forensic investigation followed (remote-access channels, Windows event logs, TeamViewer logs — all clean).

## Evidence
See attached `incident-records.jsonl` — the three records of the request:
1. `type=assistant`, single `text` block: the legitimate Hebrew work summary, with the fabricated hostile "user message" **concatenated inside the same text block** (starting at char ~1337 of 3401).
2. `type=assistant`, `thinking` block, same requestId.
3. `type=assistant`, `text` block, same requestId: the assistant's reply to the fabricated message.

No `type=user` record exists between them. Timestamps: 14:11:20.385Z → 14:11:31.810Z → 14:11:42.959Z.

## Why this matters
Beyond the runaway-generation bug itself, the failure mode is a trust/safety issue: the model fabricated a convincing *attacker persona* inside the user's session. The user experienced this as evidence of a real intrusion (it arrived seconds after a real absence from the keyboard), causing significant alarm and hours of incident response.

## Expected behavior
The response should end at the end of the assistant's turn; user turns should only ever originate from actual client input.

[incident-records.jsonl.txt](https://github.com/user-attachments/files/32294290/incident-records.jsonl.txt)

### What Should Happen?

Claude should not generate user messages that aim to "troll" it.
Trolling is for humans!!!

### Error Messages/Logs

```shell

```

### Steps to Reproduce

none

### Claude Model

Other

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

2.1.233

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

VS Code integrated terminal

### Additional Information

_No response_

Start by inspecting the attached incident-records.jsonl and its requestId/timestamps, confirming that the fabricated user text and reply arrived without a type=user record. Trace the response handling path for that request boundary; done means assistant output stops at the completed turn and user turns are accepted only from actual client input.

Written by the indexing model from the issue text.