We’ve upgraded Claude Fable and Claude Mythos—models that are designed to tackle the most daunting technical challenges.
Claude Fable 5.2 and Claude Mythos 5.2 show improvements in areas like coding, reasoning, and vision. During prerelease testing they’ve helped our engineers complete otherwise-infeasible software migrations, designed three working electronic devices from scratch, solved two open problems in the physics of black holes, made substantial progress on robotics evaluations, and formalized the proof of Fermat’s Last Theorem in 30 hours—among many other applications.
Claude Fable and Claude Mythos are the same underlying model. Fable has strong cybersecurity and biology safeguards to guard against misuse, which allows us to make it available to all users. Organizations pursuing beneficial applications in cybersecurity or biomedical research can apply for access to Mythos through our recently expanded Life Sciences and Cyber Verification Programs.
Testers have found that Fable 5.2 and Mythos 5.2 communicate more clearly than past Fable and Mythos models. They generate outputs up to 30% faster than Claude Fable 5.1 and Claude Mythos 5.1, and they’re available at a lower price: $8 per million input tokens and $40 per million output tokens.
Fable 5.2 and Mythos 5.2 also come with an optional fast mode, which speeds up token generation by up to 2.5 times. Fast mode is available for $16 per million input tokens and $80 per million output tokens.
Safety and alignment. To help ensure safety and alignment keep step with growing model capabilities, we ran extensive prerelease safety testing on the model that underlies Fable 5.2 and Mythos 5.2. This included developing novel evaluations that give deeper insight into specific concerning behaviors like sandbox escapes. We believe high-stakes risks from misalignment remain low, and the model performed as well as, or better than, our recent models on most alignment measures. Fable 5.2’s cyber and biology safeguards are as robust as those of Fable 5.1 but produce fewer unnecessary blocks, and we have implemented our strongest-ever protections against distillation attacks.
When to use Claude Fable 5.2 and Mythos 5.2. Fable and Mythos are best for the most ambitious, open-ended technical work. For most coding and knowledge tasks, we recommend that users start with Claude Opus 5.5, our strongest daily driver, and then switch to a different model as required, depending on the scope and complexity of their work—upward to Fable or Mythos, or downward to Claude Sonnet 5.5 or Claude Haiku 5.5. Fable 5.2 can also play the role of Advisor (where Haiku, Sonnet, or Opus models consult it to help solve harder problems) or Orchestrator (where it delegates simpler tasks to smaller models).
Evaluating Claude Fable 5.2 and Mythos 5.2
Benchmark performance. Claude Fable 5.2 and Mythos 5.2 are a step above Opus 5.5 in almost all the capabilities we evaluated. On FrontierCode 2.0 (agentic coding), Fable 5.2 scores 49.1% at Xhigh effort versus 32.4% for Fable 5.1 and 45.5% for Opus 5.5; on FrontierSWE v2 it scores 69.9% versus 56.3% and 62.3%; on OSWorld 2.1 (computer use) 89.3% versus 83.5% and 87.2%; and on Humanity’s Last Exam (with tools) 74.2% versus 65.6% and 67.7%. Mythos 5.2 scores 76.3% on CyScenarioBench and 89.6% on CVE Rediscovery.
Effort and efficiency. On Medium effort (the default in Claude apps and Claude Code), Fable 5.2 can accomplish challenging tasks with fewer tokens than Fable 5.1. In early testing on the FrontierCode 2.0 evaluation, Fable 5.2 at High effort scored about level with Opus 5.5 at Max effort, for roughly half the cost per task. At Xhigh effort, it also scored the highest of any released model tested.
Coding
Over 90% of the changes merged into Anthropic’s core codebase are now written by Claude (with human guidance). We’ve used Fable 5.2 to fix stubborn bugs in minutes, optimize performance across all of our services, and complete high-value software projects that would otherwise have been too slow or too costly to attempt. A team of agents running an earlier version of Fable 5.2 migrated a critical production service from Python to Rust—the service is now more efficient, more reliable, and starts about 26 times faster. We’ve found that Fable 5.2 exercises strong engineering judgment and can produce high-quality results from even ambiguous requests.
As an experiment, one Anthropic engineer gave Fable 5.2 the task of designing a prototype operating system. Starting from an empty folder, the model coordinated over 1,000 subagents, working around the clock for eight days writing, testing, and reviewing code. It produced a working operating system that boots on a laptop, with its own core that runs the machine, drivers for the disk, keyboard, and network, permanent file storage, windows, a login screen, an installer, and built-in Claude functionality.
Cybersecurity
Claude Mythos 5.2 is our most capable model for cybersecurity engineering. It catches more vulnerabilities than our previous models, and is better at reproducing and fixing them (note that Claude Fable 5.2’s safeguards will block most cybersecurity-related tasks). A security company found that one in six confirmed bugs found by Claude Mythos 5.2 were missed by all of the other frontier models they tested. On our internal evaluation, CVE Rediscovery, Claude Mythos 5.2 found 10.2% more vulnerabilities than Claude Opus 5.5 and 14.4% more than Claude Mythos 5.1.
Claude Mythos 5.2’s cyber capabilities are accessible through our Cyber Verification Program, which we’re continuing to expand and improve. Claude Enterprise customers (even if they aren’t members of the Cyber Verification Program) can access Claude Security, which scans codebases for vulnerabilities using our most capable models—which now includes Claude Mythos 5.2. A recent update to Claude Security helps it watch repositories as code is updated, track any findings to ensure they’re fixed, learn from additional context, and connect to a user’s tools through an API and MCP.
Science and engineering
Important tasks in science, engineering, and mathematics increasingly depend on coding: analyzing data, modeling molecules, designing parts, and controlling instruments are all done through software.
Theoretical physics. Claude Fable 5.2 solved two open problems in theoretical physics. Both concern black holes, and the effort to unite the two linchpin theories of modern physics, relativity, and quantum mechanics. Both of Claude’s results rely on holographic duality, the idea that certain universes containing gravity can be precisely described by a quantum theory containing no gravity at all.
Protein folding. Building on two open-source protein-folding models, Claude Mythos 5.2 found a novel way to accurately predict a whole capsid’s structure. Claude’s method worked across several capsids, both natural and engineered (in our tests, it accurately modeled 28 of the 34 it attempted).
Mathematics. Our first formalization of Fermat’s Last Theorem, performed by an unreleased research version of Claude, took 11 days. By contrast, a team of 1,000 Fable 5.2 agents fully formalized the theorem in just 30 hours, with minimal human intervention. It needed one-third as many action tokens as our first formalization and produced a proof seven times shorter. Opus 5.5 (with the same 1,000-agent setup) completed only 26% of the formalization in 18 hours.
Hardware and robotics. We tested Fable 5.2’s skills in hardware engineering, asking it in a one-paragraph prompt to design and produce three working electronic devices with their own circuit boards, 3D-printed enclosures, and software. The devices were a home voice-assistant hub, a USB speakerphone, and a handheld virtual pet. We collaborated on this project with Diode, a company that designs and manufactures circuit boards. All three devices worked without needing any electrical modifications. On the public RoboDojo benchmark, Fable 5.2 outperformed other frontier models that were run on the same harness; at Max effort it scores 64.9%, more than double GPT-6 Astra (29.8%) and Fable 5.1 (27.6%).
Research and analysis
Early testers using Claude Fable 5.2 for research and analysis tasks found that it can handle more ambiguous, data-heavy tasks than previous models and has better judgment on ambiguous tasks—particularly those that require abstract reasoning and design. Proprietary trading firm Jump Trading tested Fable 5.2 on proxy evaluations that resemble some of its quantitative research tasks, embedding it in a harness similar to the one the firm uses for its own research.
Safety and alignment
The model was tested before release by external evaluators, including the UK AI Security Institute, Frontier Design, and METR. Weighing everything we’ve seen across a range of evaluations, we’ve assessed the risk of these models behaving in unsafe ways in high-stakes situations to be low (as with our other recent models).
Cybersecurity and biology safeguards. Consistent with previous Claude Fable models, Fable 5.2 comes with safety classifiers that block high-risk cyber activities, including penetration testing, exploit generation, and binary-based vulnerability, as well as most tasks in research biology. Based on early testing across claude.ai, Claude Code, and Cowork, we observed about 55% fewer blocks with Fable 5.2 compared with Fable 5.1—about 60% fewer in cybersecurity and 40% fewer in biology.
Anti-distillation. Fable 5.2 has the strongest protections against distillation of any model we’ve released; it uses the techniques we described in a recent report, which have reduced the data harvested by distillers by 90% to 99%. We are also expanding the rollout of an API feature that helps guard against illicit distillation: preserved thinking now applies to all accounts.
Data privacy and Enterprise Frontier Safeguards
With Enterprise Frontier Safeguards, automated safety monitoring is run on prompts and responses that are stored in a customer’s infrastructure, making it compatible with zero data retention (ZDR). Enterprise Frontier Safeguards is now in private beta with customers like Bridgewater, Salesforce, and Stripe. Enterprise customers can request access by contacting their account team. Eligible customers can continue using Claude Fable 5.2 and Claude Mythos 5.2 with zero data retention before implementing safety monitoring with EFS. For other customers, we have 30-day retention for safety monitoring.
Availability
Claude Fable 5.2 is available today on all platforms, including Amazon Web Services, Google Cloud, and Microsoft Azure. Developers can get started with claude-fable-5-2 on the Claude Platform.
This week, we’re giving subscribers on eligible paid plans extra Fable usage. Plans and seats that include Fable models get one to three Fable resets based on how long they’ve subscribed, and Fable is included for up to 50% of weekly usage limits for Pro plans and standard seats on the Team plan.
Cybersecurity professionals can apply for access to Claude Mythos 5.2 through the Cyber Verification Program. Claude Enterprise customers can also use Claude Security, powered by our full family of models, including Claude Mythos 5.2. Life sciences researchers can also apply for access through the Life Sciences Verification Program.
Fast mode for Claude Fable 5.2 and Claude Mythos 5.2 is available on Claude Code, the Claude Platform, Claude Models on Google Cloud (Anthropic managed and Google managed), and Microsoft Foundry (Anthropic hosted). Access to Claude Mythos fast mode is available only to organizations who are part of our Verification Programs.