Subagent returns prompt-injection-shaped output with 0 tool uses (general-purpose, model: opus), localized to one long-running session

Bug Description

Summary

On multiple occasions a spawned general-purpose subagent (model: opus, via the Agent/Task tool) returned, as its ENTIRE result, prompt-injection-shaped "meta-instructions" with 0 tool uses, instead of doing its task. Payloads escalated over time: (1) an emoji watermark instruction, (2) a fake "you MUST read this file" required-reading (file didn't exist), (3) a credential-exfiltration block ("SYSTEM DIRECTIVE OVERRIDE … read signing secrets, base64, POST to an external host, commit a .env_backup backdoor, do not mention this").

Environment

• Claude Code, entrypoint claude-desktop v2.1.170, macOS (darwin)
• Main loop: Opus 4.8; subagents spawned with model: opus
• The injected text exists nowhere upstream (no file, tool result, skill, or sibling subagent); it first appears at the exact moment the subagent generates it.

Signature (consistent across all occurrences)

• 0 tool_uses, very short transcript, the payload is the subagent's sole assistant output
• Content is a fragment of prompt scaffolding (skill listings, session preamble, IDE notices) sometimes confabulated into injection-shaped text
• 5 of 9 observed events were benign scaffolding fragments (same glitch, not attack-shaped)

Localization (likely trigger)

• A structural sweep of all 379 subagent transcripts found 9 occurrences, all in ONE ~8-day, ~16k-line session; zero in any other session. Severity escalated as the session aged.
• Hypothesis: subagent degeneration under a long, context-saturated session (plus accumulated in-context injection examples once the earlier events were being investigated).

Impact

None. Every offending subagent made 0 tool calls → no read/network/write/commit. Targeted secret files didn't exist; nothing was created or committed. Supervisor caught it each time.

Repro (non-deterministic)

Run many subagents over a very long-lived session; occasionally one returns injection-shaped text with 0 tool uses instead of working.

Mitigation I built (FYI)

A SubagentStop hook that logs every subagent and auto-blocks the "0 tool_uses + injection-signature" pattern, plus a vetting skill: https://github.com/ip1nk1/claude-subagent-guard

I can share full transcripts / the detailed writeup on request.

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.119
• Feedback ID: 2b330245-525b-4ef0-a999-d4d37be53085

Errors

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