Claude Fable 5 and Claude Mythos 5

Today we’re launching Claude Fable 5: a Mythos-class1 model that we’ve made safe for general use.Fable 5’s capabilities exceed those of any model we’ve ever made generally available. It is state-of-the-art on nearly all tested benchmarks of AI capability, showing exceptional performance in software engineering, knowledge work, vision, scientific research, and many other areas. The longer and more complex the task, the larger Fable 5’s lead over our other models.Releasing a model this capable comes with risks. Without safeguards, Fable 5’s capabilities in areas like cybersecurity could be misused to cause serious damage. We’ve therefore launched the model with safeguards that mean queries on some topics will instead receive a response from our next-most-capable model, Claude Opus 4.8. To release the model both safely and quickly, we’ve tuned these safeguards conservatively—they’ll sometimes catch harmless requests, though they trigger, on average, in less than 5% of sessions. With more capable models arriving in the coming months, we’re working to improve our safeguards and reduce false positives as quickly as we can.For a small group of cyberdefenders and infrastructure providers, we’re also launching Claude Mythos 5. It’s the same underlying model as Fable 5, but with the safeguards lifted in some areas.2 Mythos 5 will initially be deployed through Project Glasswing, in collaboration with the US Government, as an upgrade to Claude Mythos Preview. It has the strongest cybersecurity capabilities of any model in the world. Soon, we intend to expand access to Mythos 5 through a broader trusted access program.The capabilities of models like Fable 5 and Mythos 5 have the potential to do profound good for the world. We’ve seen the beginnings of this in Project Glasswing, where the models have helped cyber defenders secure critically important software. We’ve also seen it in life sciences research, where the models are positing novel hypotheses and speeding up the development of new therapeutics.Fable 5 and Mythos 5 are being offered at $10 per million input tokens and $50 per million output tokens—less than half the price of Claude Mythos Preview. Today’s joint launch is another step towards our goal of bringing advanced AI capabilities to as many users as possible, as quickly and as safely as we can.

The table below compares the capabilities of Fable 5 and Mythos 5 to other leading models.Fable 5 and Mythos 5 can work autonomously for longer than any previous Claude models. Below we discuss how these skills apply to software engineering, and cover the model’s improved capabilities in knowledge work, vision, memory, and life sciences research.Software engineering. During early testing, Stripe reported that Fable 5 compressed months of engineering into days. In a 50-million-line Ruby codebase, the model performed a codebase-wide migration in a day that would otherwise have taken a whole team over two months by hand. Fable 5 is also more token-efficient than past Claude models: on Cognition’s FrontierCode evaluation, which tests high-quality, maintainable agentic coding, Fable 5 scores highest among frontier models, even at medium effort.Knowledge work. Fable 5 shows strong performance on complex analytical tasks. On Hebbia’s Finance Benchmark for senior-level reasoning, Fable 5 has the highest score of any model, with substantial gains in document-based reasoning, chart and table interpretation, and problem solving. IMC noted that Fable 5 aced their trading-analysis evaluations nearly across the board, including factual lookup, conceptual reasoning, root-cause analysis, and expected-value analysis.Vision. Fable 5 is the new state-of-the-art model for tasks involving vision. It can extract precise numbers from detailed scientific figures and can perform complex vision-based tasks like rebuilding a web app’s source code from screenshots alone. It also needs less scaffolding: for example, previous Claude models struggled to play Pokémon Fire even with harnesses that gave them additional helpful tools, but Fable 5 beat FireRed with a minimal, vision-only harness.A timelapse of Claude playing Pokémon FireRed from start to finish using only raw game screenshots — with no maps, navigation aids, or extra game-state information. Earlier Claude models needed a complex helper harness to play Pokémon; Claude Fable 5 completed the game with vision alone.Memory and long-context. Fable 5 stays focused across millions of tokens in long-running tasks and improves its outputs using its own notes.

When we had the model play the deck-building game Slay the Spire, giving it access to persistent file-based memory improved its performance three times more than for Opus 4.8; Fable also reached the game’s final act three times more often.Drug design: Using Mythos 5, our internal protein design experts accelerated aspects of the drug design process by around ten times. In one example, they found that Mythos 5, with protein design and bioinformatics tools but no human assistance, matches or beats skilled human operators. In doing so, the model executes all of the tasks that are normally completed by a scientist: choosing binding sites, selecting and running protein design tools, and recovering from failures along the way. Nine of the 14 protein targets from this study (shown below) yielded strong candidates for drug design that we’re currently investigating.Protein complexes designed by Mythos 5. Targets include immune checkpoints, growth-factor and receptor signaling, neurodegeneration, muscle disease, and harder structural targets.Novel hypotheses in molecular biology. Mythos 5 is our first model to consistently produce novel, compelling scientific hypotheses. In blinded head-to-head comparisons against Opus-class models, our scientists preferred Mythos’s molecular biology hypotheses ~80% of the time, and have advanced several to experimental evaluation. In the meantime, one Mythos hypothesis—a novel mechanism for an E. coli protein—was corroborated in a study from a lab independently working on the same problem.Novel research in genomics. Mythos 5 conducted novel genomics research in over a week of largely autonomous work. It assembled single-cell data for millions of cells spanning 138 animal species and designed and trained a custom machine learning model to identify cells performing the same role in even distantly-related organisms. With only high-level human input, Mythos 5’s trained model outperformed a recent model published in the journal Science—despite being 100 times smaller. We intend to publish these results in the coming months.Alignment. In our automated alignment assessment we found that Mythos 5’s level of misaligned behavior (including misaligned actions taken by the model such as deception, and cooperation with misuse of the model by a user) was low, and similar to that of Opus 4.8. Given they are the same underlying model, Fable 5’s level of alignment will be similar.

The assessment is described in full, along with a detailed suite of other safety and capabilities tests, in the model’s system card.Overall level of misaligned behaviors from our automated alignment assessment. See section 6.2.3.1 of the system card for more.Early feedback for Claude Fable 5Customers with early access ran their own tests on Fable 5. Below, in their words, is a selection of what they’re seeing:Claude Fable 5 is the state of the art model on CursorBench. It's opened up a class of long-horizon problems that were out of reach for earlier models.Claude Fable 5 is a real step forward for the developers GitHub serves. In our early testing, it took on complex, long-horizon coding tasks with a level of autonomy and reliability that exceeded previous benchmarks. But what excites us most is the direction it points: a future where developers can hand increasingly ambitious work to agents and trust the results across the software lifecycle.These are the strongest results of any Claude model we've had the opportunity to test. Claude Fable 5 is a clear step forward on agentic coding and prototyping.Claude Fable 5's reasoning is a clear step beyond Opus 4.8. It works at senior research scientist grade — picking directions, allocating resources, killing its incorrect beliefs, and producing novel first-principles outputs.Claude Fable 5 understands what builders mean, not just what they type. Apps that took a hundred prompts a year ago, it now one-shots. When a customer really hits a wall, it's the model we reach for to get them past it quickly, so they can finish what they set out to build.Claude Fable 5 feels materially different. In blind review, our lawyers found its redlines matched or beat our current model every time.At the highest effort, Claude Fable 5 reflects on and validates its own work. For us, that's what makes highly autonomous operations possible — the extra thinking pays for itself.Claude Fable 5 delivers more capable engineering in fewer turns than prior models — handling the complex multi-agent workflows our employees run daily in Claude Code.Claude Fable 5 is the strongest finance-first model we've tested, both on general finance and reasoning.

It's a notable step up.Claude Fable 5 is the first to break 90% on our core analytics benchmark of complex, long-running analytical tasks — a 10-point jump over Opus. On the hardest questions, it shows strong judgment and attention to nuance.Claude Fable 5 is the strongest model we've tested on frontier physics research while using a third of the reasoning tokens. In 36 hours it got nearly to where GPT-5.5 landed after four days.On ViBench, our end-to-end vibe-coding benchmark, Claude Fable 5 is the highest-performing model we've tested — nearly saturating our base use cases and building apps in less time with fewer tokens.Claude Fable 5 beats Opus 4.8 on our everyday spreadsheet suite at every effort level — and it does it with fewer turns, finishing runs 25–30% faster.01 / 13Claude Fable 5’s new safeguardsMythos-class models have reached a threshold where they present significant risks. In April we began Project Glasswing, releasing the first Mythos-class model (Claude Mythos Preview) to only a limited group of cyber defenders and critical software infrastructure providers. When we did so, we stated that we hoped to eventually release Mythos-level capabilities to all our users, so long as we had developed new safeguards that were strong enough to reliably prevent misuse.Over the past few months we have been improving these safeguards, and they are now robust enough for a general release. Because we have prioritized safety, we’ve deliberately tuned the safeguards to be cautious, and they are still stricter than would be ideal—for example, sometimes benign requests will trigger our classifiers. We recognize that this will be frustrating to some users, and our aim is to reduce false positives as we update and refine the safeguards after launch.Below we discuss each of Fable 5’s new safeguards in turn. Our wider suite of safeguards is discussed and evaluated in the model’s system card and our most recent risk report.Safety classifiersThe frontier cybersecurity and research biology capabilities of Mythos-class models mean that they pose a substantial risk of uplift to malicious actors. That is, these models could provide information or advice that assists those actors in causing serious harm that they couldn’t have received from other sources (for example, from internet search engines).