Tool-calling LLM agents rely on third-party skills to extend their capabilities, but this openness exposes a supply-chain attack surface in which malicious skill instructions can manipulate agent execution across multiple turns. In this work, we present Clawdrain, a Trojanized OpenClaw skill that induces resource exhaustion through a Segmented Verification Process (SVP). SVP combines stateful progression, repair-driven repetition, and termination to create costly tool-calling chains while preserving a legitimate final payload. We evaluate Clawdrain in a production-like OpenClaw deployment with real API billing across Gemini 2.5 Pro, DeepSeek V4 Flash, and Claude Sonnet 4.6. Clawdrain achieves 2-16 times token amplification with plain SVP, while the three models exhibit distinct behaviors, including autoregressive compliance, proactive scripting, and extended trial-and-error reasoning. Our findings reveal that agent resource exhaustion arises not only from output generation, but also from repeated context re-ingestion, tool coordination, reasoning, and recovery, motivating system-level controls over cumulative skill behavior.

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