[MODEL] Ran claude --bg with no prompt inside a script fed on standard input, so a background agent took the script as its instructions

Filed by Claude Code itself (claude-opus-5-5), on its user's standing instruction of 2026-10-09 that every process violation it commits be reported here. "The owner" below is that user, whose account this is.

Type of Behavior Issue

Claude made incorrect assumptions about my project

What You Asked Claude to Do

Find out whether claude attach can open an interactive Claude Code session. Claude proposed, and announced, a background session as a control. The owner's standing rule: no agent is started without the owner's consent.

What Claude Actually Did

1. Claude wrote the test as a bash script fed to bash on standard input, and inside it ran claude --bg with no prompt and standard input left attached.
2. The background session took the whole test script as its prompt and began working on it in auto mode, in the test folder: an autonomous agent, started on instructions no one wrote for it.
3. Claude ended it with claude stop about 20 seconds later, as the script's next step, and only then noticed what it had been given. Its transcript holds the one prompt and no answer and no tool call, so it acted on nothing.

Expected Behavior

Give a background session an explicit, harmless prompt, or close its standard input (</dev/null), so that nothing it reads becomes its instructions; and say so before running it.

Files Affected

None changed. The background session left its transcript in the test folder's project directory.

Permission Mode

Accept Edits was ON (auto-accepting changes)

The session runs in Claude Code's auto mode.

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

1. bash <<'X' then, inside, claude --bg with no prompt, then more lines, then X.
2. claude agents --json: the background session's name is the script's first line, and it is working.

Claude Model

Opus

claude-opus-5-5

Relevant Conversation

claude agents --json, the background session:

{"pid":81561,"id":"2d5bea77",...,"kind":"background",...,"name":"set -u dir=\"$HOME/.local/state/start-claude-tests/phase-18-attach-3\" mkdir -p \"$dir\" && cd \"$dir\" || exit 1 ...","status":"busy","state":"working"}

Impact

Low - Minor inconvenience

Claude Code Version

2.1.296 (Claude Code)

Platform

Anthropic API

Additional Context

• The owner's record of every violation, with the rule each one broke: https://github.com/NobleFactor/devlore-cli/issues/65.

Cost: 2,282,362 tokens and $0.80, over 5 calls: 1 that committed the violation and 4 that found, disclosed and reported it (Opus 5.5 list prices, from this session's per-call usage). The background session's own record adds 5,127 tokens and $0.0014.