Claude Fable 5 and Mythos 5 Are Here: What a Public Frontier Security Model Means for Smart Contracts ## TL;DR On June 9, 2026, Anthropic released two models in its new Mythos tier, the class that sits above Opus. **Claude Fable 5** is the generally available version, now on the Claude API from day one. **Claude Mythos 5** is the same underlying model with cybersecurity safeguards removed, restricted to a small group of cyberdefenders and infrastructure providers through Project Glasswing. Anthropic describes Mythos 5 as having "the strongest cybersecurity capabilities of any model in the world," able to both discover and exploit software vulnerabilities and to run multi-step agentic attacks. This is the moment the April Mythos preview pointed at. A frontier model that finds and weaponizes vulnerabilities at scale is no longer a limited preview behind twelve enterprise partners. The general-purpose version is on a public API, and the offensive ceiling just moved up for everyone, including the attackers targeting your protocol. For smart contract teams, the takeaway is direct. The underlying intelligence available to both sides has stepped up, and immutable on-chain code has no patch pipeline to absorb the shock. The defensible posture is continuous, specialized, agentic security that runs every time your code changes. That is exactly what Cecuro was built to deliver. [Run a free audit with the #1 agentic smart contract auditor.](https://app.cecuro.ai/auth?mode=signup) ## What Anthropic actually shipped Mythos is a tier, not a version bump. Anthropic introduced it above Haiku, Sonnet, and Opus, and the April preview was the first public glimpse of what that tier could do. The June 9 release puts two production models in your hands. **Fable 5** is the Mythos-class model made safe for general use. It is available globally on the Claude API and consumption-based enterprise plans from launch day, priced at $10 per million input tokens and $50 per million output tokens, less than half the cost of the earlier Mythos preview. On capability benchmarks it is state of the art across software engineering, vision, long-context reasoning, and analytical work, posting the top score among frontier models on Cognition's FrontierCode evaluation and being the first model to break 90% on a set of complex analytical task benchmarks. **Mythos 5** is the same model with the cybersecurity guardrails lifted, deployed to a narrow group of defenders through Project Glasswing in collaboration with the US government. It is the offensive-capable sibling: discovery and exploitation of software vulnerabilities, reconnaissance, lateral movement, and coordination of multi-step attacks. Anthropic's own framing is that this is the most capable cybersecurity model in existence. The two models matter together. Mythos 5 shows the true ceiling of the tier. Fable 5 shows what is now sitting on a public API for anyone with a credit card, guardrails and all. ## The safeguards, and why they do not solve the on-chain problem Anthropic was deliberate about the release. Fable 5 ships with safety classifiers, separate AI systems that watch for misuse and jailbreak attempts. When a request touches cybersecurity, biology, chemistry, or model distillation, the system does not hard-refuse. It quietly falls back to a response from the less capable Claude Opus 4.8. Anthropic reports these classifiers trigger in under 5% of sessions, and that at least 95% of Fable sessions run entirely on the full model. The company also ran a 1,000-plus-hour external bug bounty that produced no universal jailbreaks, and it retains all Mythos-class traffic for 30 days for safety review. These are serious, well-engineered safeguards. They are also built for a world where the defender controls the patch pipeline. That assumption breaks on-chain. When a frontier model finds a zero-day in an operating system, the vendor patches it and pushes the fix to billions of devices. The attacker's window closes. Smart contracts are immutable by default. Upgrade paths run through timelocks, multisigs, and governance votes, measured in days or weeks, often contested. A bug found on Monday can be exploited Tuesday and drained Wednesday, and there is no update to ship. So the safeguard story, however good, addresses the wrong threat model for Web3. It limits what the *public* version will casually help a user do. It does nothing about the fact that the capability frontier moved, that determined attackers route around conservative classifiers, and that the underlying intelligence is now cheaper and more widely deployed than it was a week ago. ## This validates the agentic security thesis It is worth saying plainly: a frontier lab publicly shipping a model it calls the strongest cybersecurity system in the world, while keeping the unrestricted version behind a government-partnered defensive program, is the clearest possible confirmation of where software security is heading. The trajectory has been visible for a while: 1. Billions of dollars are stolen from crypto protocols every year, with DefiLlama tracking more than $1.1B in DeFi losses over the past year alone. 2. AI offensive capability against smart contracts has been compounding on a timescale of months, not years. 3. The gap between a restricted frontier model and a commodity one keeps closing. 4. On June 4, just days before this release, a researcher used a frontier model to surface a critical four-year-old bug in Zcash that repeated audits had missed, a public demonstration that these systems find things humans and prior tooling did not. Mythos 5 and Fable 5 reinforce every point. Anthropic is telling the market that agentic offensive security is mainstream, that the right response is layered defense deployed early with specialized partners, and that capability arrives faster than defenders are ready for. That is the posture Cecuro has taken for smart contracts since day one. The debate this lands in is loud. Earlier this year, OpenZeppelin co-founder Manuel Aráoz argued that all of DeFi is effectively unsafe because coding agents are becoming superhuman at finding vulnerabilities, attackers need only one flaw while defenders must close every one. Others, including Aave's Marc Zeller, pushed back that the large majority of last year's DeFi losses did not trace to codebase flaws. Both can be true. The codebase attack surface is real, it is growing, and the cost to probe it is collapsing. ## Why a raw frontier model is still not a smart contract auditor The obvious question: if Fable 5 is this strong, why not point it at a Solidity repo and call it an audit? Because raw capability is necessary but not sufficient. A general-purpose coding agent, even a Mythos-class one, does not carry the specialization that smart contract security demands out of the box. It needs invariant libraries tuned to DeFi primitives. It needs symbolic execution and fuzzing wired into Foundry, Hardhat, and Echidna. It needs a working memory of historical exploit classes, reentrancy, cross-function reentrancy, price oracle manipulation, signature replay, proxy upgrade traps, ERC-4626 share inflation, and dozens more. It needs to reason about the economic context around a contract, not just its bytecode, and to produce reports an engineering team and a governance council can both act on. That specialization layer is why Cecuro ranks #1 on EVMBench, the public exploit benchmark from OpenAI, Paradigm, and OtterSec, built from real Code4rena vulnerabilities, while the best raw frontier models score far lower on the same targets. Cecuro's agents run for hours across an entire codebase, spawn specialized subagents per vulnerability class, validate each finding against execution with proof-of-concept exploits, and reason about the protocol as a system. Fable 5 makes the underlying engine stronger. It does not replace the specialization that turns a strong model into a reliable auditor. A faster, sharper calculator does not replace the structural engineer. It makes one more accurate. ## Traditional audits, raw Fable 5, and Cecuro A quick comparison of where each approach lands now that the Mythos tier is public. | Dimension | Traditional audits | Raw frontier model (Fable 5 class) | Cecuro | |---|---|---|---| | Speed | 2 to 6 weeks | Minutes, unreliable and unstructured | Hours, reliable | | Cost per audit | $30K to $1M+ | API cost only, no workflow | Free audit trial | | Coverage across chains and languages | Limited by auditor availability | Broad but unspecialized | All chains and smart contract languages | | Exploit benchmark standing | Not benchmarked | Trails specialized systems | #1 on EVMBench | | Validates findings with proof-of-concept | Manual, slow | No | Yes, every finding | | Specialized smart contract tooling | Yes, human-driven | No | Yes, agent-driven | | Fit for continuous defense | One-time snapshot | Weak as a standalone product | Strong as a continuous layer | Running more than one detection method on high-value code is industry best practice for any serious protocol, regardless of which firms or models a team already uses. It is about diverse perspectives catching different edge cases. Cecuro is designed to be the leading agentic layer in that defense-in-depth stack, running continuously alongside every human review, bug bounty, and governance proposal you bring to bear. ## What Web3 builders should do this week The Fable 5 and Mythos 5 release is a present signal, not a future one. Three concrete steps: **First, assume parity on the offensive side.** Anthropic is keeping Mythos 5 narrow, but Fable 5 is public, and the gap between a restricted frontier model and a commodity one is now measured in months. Treat Mythos-class capability as something an attacker targeting meaningful TVL can approximate today. **Second, make security posture part of your investor and user story.** Capital allocators are watching this release. "What is your protocol doing about frontier-model offensive capability?" is becoming a standard diligence question. A clear answer backed by a specialized security partner is table stakes. **Third, run Cecuro on your codebase now.** It takes under a minute to connect a GitHub repository, and a full audit comes back in hours, not weeks. The first audit is free to start, and pricing begins at $799, a fraction of a single human review cycle and a tiny fraction of the cost of an exploit. Every finding is validated against execution before it reaches your report, so you act on issues that are real and reproducible, not model speculation. ## The bigger picture Anthropic released Fable 5 and Mythos 5 days after publicly warning that frontier systems are advancing fast enough to raise serious safety questions. That tension, ship the capability and flag the risk in the same breath, is the defining feature of this moment. The capability is real, it is here, and it is increasingly available to both sides. For traditional software, vendors will absorb a lot of this with patch pipelines. Smart contracts have no such cushion. Immutable code, public mempools, and value that sits in front of the fix rather than behind it make Web3 the most exposed surface in software. A frontier model that finds and exploits bugs at scale is not an abstract concern for protocols, it is a direct one. Cecuro's job is to be the on-chain native answer. We were built for smart contracts first, we validate every finding against execution, and we rank #1 on EVMBench, the public exploit benchmark from OpenAI, Paradigm, and OtterSec. When the frontier shifts, as it just did, our job is to bring the smart contract world along with it faster than attackers can move. If you run a protocol, a launchpad, an exchange, or an L1 and you are thinking about what the Mythos tier means for you, the fastest thing you can do is hand your codebase to an agent that is already fighting this fight. [Start a free Cecuro audit now.](https://app.cecuro.ai/auth?mode=signup) ## Sources * Anthropic, "Claude Fable 5 and Claude Mythos 5," June 9, 2026 (anthropic.com/news/claude-fable-5-mythos-5) * TechCrunch, "Anthropic releases Claude Fable 5, its most powerful model publicly, days after warning AI is getting too dangerous," June 9, 2026 * Help Net Security, "Anthropic's Claude Fable 5 is out for public use, with safeguards for high-risk requests," June 10, 2026 * 9to5Google, "Claude Mythos goes public in new Fable 5 model that's 'safe for general use,'" June 9, 2026 * Zcash forum / Decrypt, critical four-year-old bug disclosed June 4, 2026 * EVMBench, public exploit benchmark from OpenAI, Paradigm, and OtterSec * DefiLlama, DeFi exploit loss data, 2026