DeFiSent: Check-Guided Repair for Financial-Semantic\\ Vulnerabilities in DeFi Smart Contracts

Abstract

DeFi smart-contract vulnerabilities often arise from protocol-level financial semantics rather than syntactic Solidity patterns. Direct LLM prompting can generate plausible patches that compile but leave oracle, vault, lending, or AMM invariants unresolved. We aim to make LLM-based smart-contract repair more reliable by separating patch generation from domain-specific judgment and requiring accepted patches to improve check-visible semantic obligations. We present DeFiSent, a check-guided repair framework. It normalizes contract code, infers protocol roles, routes vulnerability-family checks, emits structured findings and violated invariants, and prompts an LLM with this evidence. Candidate patches are accepted only after compilation, tests, interface preservation, non-increasing semantic severity, and invariant-specific gates. We evaluate DeFiSent on a coverage-aware corpus from DeFiHackLabs, EVMbench, LISA-Bench, SmartBugs Curated, and BCCC-SCsVuls-2024. The DeepSeek campaign contains 40,452 paired tasks and 80,904 model-mode runs, including a 41,332-row code-level shard. On the unified 6,000-task DeepSeek subset, check guidance improves dynamic pass rate from 54.82\% to 79.95\%, semantic-improvement rate from 40.42\% to 53.92\%, and total severity reduction from 8,733 to 24,084. On a 2,000-task SiliconFlow matrix, aggregate semantic-improvement rate increases from 27.0\% to 57.8\%. Structured findings and gate-based validation substantially improve LLM repair behavior for DeFi contracts. DeFiSent does not replace expert auditing, but it provides a reproducible path for aligning LLM-generated patches with financial-semantic proof obligations.