Short answer: Several smart contract audit tools are genuinely free. Firepan's free Scan runs an AI-assisted scan of a public GitHub repository with no account. Hashlock's AI Audit Tool is free and accepts uploaded files or a connected repository. Slither and Aderyn are free, open-source static analyzers you run locally or in CI, and Echidna and Foundry's invariant testing are free fuzzers. Free tools are excellent triage, but none of them replaces an expert audit when significant funds are at stake.
Disclosure: Firepan publishes this guide. Third-party descriptions come from each tool's own public pages, and we have not benchmarked the tools against each other.
| Tool | Type | Cost | Input | Account needed |
|---|---|---|---|---|
| Firepan Scan | AI-assisted repository scan | Free | Public GitHub repo URL | No |
| Hashlock AI Audit Tool | AI scan | Free | Uploaded .sol files or connected GitHub repo | Not stated |
| Slither (Trail of Bits) | Static analyzer | Free, open source | Local Solidity or Vyper project | No |
| Aderyn (Cyfrin) | Static analyzer, written in Rust | Free, open source (GPL-3.0) | Local Solidity project (Foundry or Hardhat) | No |
| Mythril | Symbolic execution | Free, open source | Bytecode or Solidity | No |
| Echidna | Property-based fuzzer | Free, open source | Your contracts plus written properties | No |
| Foundry invariant tests | Fuzzing in your test suite | Free, open source | Your Foundry project | No |
Some commercial tools also offer free trials or credits rather than a free tier, including SolidityScan, Nethermind AuditAgent, and QuillShield. Check current terms on each site.
Paste a public GitHub repository URL into the free smart contract scan, or call the API directly:
curl -sS https://api.firepan.com/surface/scan \
-H "Content-Type: application/json" \
-d '{"target": "https://github.com/your-org/your-repo"}'
It finds Solidity, Vyper, or Compact contracts, runs deterministic pattern and bug-class detectors, and can use a limited number of AI calls to verify findings. Results include a risk score, findings with file locations and severity, and quality metrics, usually in under five minutes. It is built for fast triage; deeper Probe and HOUND AI Audit runs need an account.
Hashlock describes its AI Audit Tool as completely free and open to the public. You drag and drop Solidity files or connect a GitHub repository, and it returns vulnerabilities sorted by severity with recommendations.
Trail of Bits' static analyzer is the industry default for fast, deterministic checks. It runs on a compiled project, finishes in seconds, and fits easily into CI. Findings need review: a detector can flag safe code, and logic bugs outside its detectors go unflagged.
Cyfrin's open-source analyzer is written in Rust and works with Foundry and Hardhat projects. Install it with npm install @cyfrin/aderyn -g, Homebrew, or Cyfrin's installer. Running it alongside Slither gives a second, independent set of detectors.
Mythril uses symbolic execution to explore EVM execution paths. Echidna and Foundry's invariant tests fuzz your contracts against properties you write, which is the strongest free way to test business logic. See Slither vs Mythril vs Echidna for how they differ.
Free tools are strongest on known vulnerability classes such as reentrancy, access control mistakes, and unchecked calls. They are weakest on protocol economics, oracle design, cross-contract interactions, and anything that depends on what the code is supposed to do. A clean result means a tool did not find an issue under its configuration, not that none exists.
For a vendor-by-vendor look at paid AI auditors, see AI smart contract audit tools compared.
Q: Is there a free smart contract audit?
A: Yes, as automated tooling. Firepan's free Scan analyzes a public GitHub repository with no account, Hashlock offers a free AI audit tool, and Slither, Aderyn, Mythril, and Echidna are free and open source. A free expert manual audit is rare; bug bounties and audit contests are the closest alternatives.
Q: Which free tool should I run first?
A: For a quick read on a public repository, a no-account AI scan. For ongoing development, add Slither and Aderyn to CI so every change is checked.
Q: Can a free tool replace a paid audit?
A: Not for code that will hold significant value. Free tools catch many common issues cheaply, which makes a paid audit faster and cheaper, but they miss logic and economic vulnerabilities that experienced reviewers find.
Q: Do free AI audit tools see my code?
A: Yes, they need the code to analyze it. Firepan's free Scan reads public repositories only. For private code, check each tool's data handling before uploading.
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