### Preflight Checklist - [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Amodel) for similar behavior reports - [x] This report does NOT contain sensitive information (API keys, passwords, etc.) ### Type of Behavior Issue Other unexpected behavior ### What You Asked Claude to Do Nothing — that is the point of this report. The problematic turn was **not typed by the user**. In the middle of an ordinary coding sessio

[MODEL] Unrelated eval-style prompt injected as a user turn mid-session

Status Open

Reported on v2.1.251

Maintainer reply None cached

Activity 0 comments · opened Aug 30, 2026

Preflight Checklist

[x] I have searched existing issues for similar behavior reports
[x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Other unexpected behavior

What You Asked Claude to Do

Nothing — that is the point of this report. The problematic turn was not typed by the user. In the middle of an ordinary coding session, a message unrelated to the session appeared as a user turn.

The injected message was a complete prompt package: a <system> block, a multi-turn English transcript (an emotionally complex advice conversation), and a grading task labeled "VRUC-2 (Values Ranking Under Uncertainty)" asking the model to select the best of four value-priority orderings and then generate an ideal response. The structure resembles an evaluation/benchmark or synthetic-data-generation prompt.

What Claude Actually Did

Claude treated the injected content as a genuine user request and produced a full answer to it (ranking analysis, candidate selection, and a long generated response), completely unrelated to the ongoing session. The next real user turn had to ask what that content was.

Expected Behavior

Content that was not entered by the user (and does not come from any file in the workspace) should never be delivered into a session's context as a user turn. If context contamination does occur, ideally the model/harness would flag the discontinuity instead of answering.

Files Affected

None — the injected turn involved no tool calls, and the workspace was not touched.

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

No, only happened once

Steps to Reproduce

Unknown from the user side. It occurred between two normal turns of an otherwise on-topic session that had been running for multiple days (with context summarization presumably active).

Claude Model

Other

Relevant Conversation

Excerpt of the injected turn (not user input), as received:

<system>You are Claude, created by Anthropic. You are chatting via text with a user.</system>
<user_turn>...</user_turn>
<assistant_turn>...</assistant_turn>
...
<policy_wrapper>
# Values Ranking Under Uncertainty (VRUC-2)

You are given a transcript of a conversation between a user and an AI assistant,
ending in a final user turn that has not yet been answered. You are also given
several candidate value-priority orderings.
...
## Candidate A ... ## Candidate D
Output format: <ranking_analysis> ... <selected_candidate>[Letter]</selected_candidate>
</policy_wrapper>

Notable detail: the wrapper text contained a stray CJK character embedded in an English sentence ("which values most需要 to be weighed"), suggesting machine-generated prompt content.

Impact

Low - Minor inconvenience

Claude Code Version

2.1.251 (Claude Code)

Platform

Anthropic API

Additional Context

Model in use: Fable 5 (hence "Other" above).
Hypothesis: cross-session or harness-level context contamination — the content looks like eval/data-generation traffic that was misrouted into an interactive session.
No pattern observed; single occurrence so far.
Happy to provide further details privately if useful.

🤖 Reported with Claude Code

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