Anthropic brings Mythos to the masses with Claude Fable 5, its most powerful generally available model ever

Anthropic today launched two new AI models — Claude Fable 5 and Claude Mythos 5 — marking the company’s first broad release of the powerful “Mythos-class” AI capabilities it previously made available only to participating organizations in its restricted cybersecurity program, Project Glasswing, which it announced two months ago.

The company says Fable 5, which is the version most users and developers will get starting today, exceeds every Claude model it has previously made generally available — featuring stronger performance across software engineering, knowledge work, vision, scientific research and long-running tasks.

It smashes the existing benchmarks and comes atop on nearly all of them, though the prior Claude Mythos Preview version of the model still takes the top spots on computer use and multidisciplinary reasoning (see benchmark chart below and here).

The new Claude Mythos 5, by contrast, is less restricted in its capabilities, but more restricted in its availability. It is an upgraded version of the prior, similarly capable but limited release Mythos Preview model. As such, it has certain safeguards lifted — but it’s only officially accessible to Anthropic-approved users, including Anthropic’s cybersecurity partners in its Project Glasswing effort, and select biology researchers.

The key difference is that the general purpose Fable 5 wraps the same underlying Mythos-class capability in new safeguards. Anthropic says requests involving certain high-risk areas — including cybersecurity, biology and chemistry, and model distillation — are automatically routed to Claude Opus 4.8, Anthropic’s previously flagship general model, instead, with users notified when that happens. That is not the case on Mythos 5.

The company says more than 95% of Fable 5 sessions run entirely on Fable 5’s own responses, with no fallback, and that internal and external red-teaming efforts found no “universal jailbreaks” after more than 1,000 hours of testing.

Anthropic says Fable 5 is available to the general public today through its website, apps, and API, but that Mythos 5 will initially only be made available to users who already have access to the older Claude Mythos Preview.

Pricing, access and a tricky rollout

Anthropic is pricing both Fable 5 and Mythos 5 at $10 per million input tokens and $50 per million output tokens. The company says that is less than half the price of Claude Mythos Preview, but still ranks as the most expensive of major AI models available globally.

For developers, Fable 5 is available through the Claude API as claude-fable-5. Anthropic says Fable 5 is fully available today on the Claude API and on consumption-based Enterprise plans.

For subscription users, the rollout is more complicated. Anthropic says Fable 5 will be included on Pro, Max, Team and seat-based Enterprise plans at no extra cost from today through June 22.

On June 23, the company plans to remove Fable 5 from those plans, after which using it will require usage credits. Anthropic says it aims to restore Fable 5 as a standard part of subscription plans as quickly as possible.

The difference between Fable 5 and Mythos 5

Anthropic is not presenting Fable 5 and Mythos 5 as two separate models in the usual “small versus large” sense. Instead, they appear to share the same base capability level. The difference is access control — that is, how easily it will be for users to get their hands on the models, and the guardrails embedded in each.

As previously mentioned Fable 5 includes a new safeguard layer that detects certain high-risk requests — including cybersecurity, biology and chemistry, and attempts to distill the model’s capabilities into other systems — and routes those requests to Claude Opus 4.8.

Mythos 5 lifts some of those restrictions for trusted users working in approved domains.

In practical terms, Mythos 5 is more powerful for sensitive cyber and biology work because it can answer in areas where Fable 5 falls back.

For most ordinary enterprise and developer tasks, however, Anthropic says Fable 5 performs effectively the same as Mythos 5.

The launch also signals how Anthropic plans to bring frontier models with dangerous dual-use capabilities into the market: not by releasing all capabilities to everyone, and not by simply refusing risky questions, but by routing some requests to a less capable model while keeping the stronger model available for the majority of everyday work.

A major improvement in autonomous coding

For enterprise buyers, the most immediate use case is likely software engineering. Anthropic says Fable 5 can work unattended for longer and with more independence than previous Claude models, which is exactly the capability enterprises need if they want AI agents to do more than autocomplete code or answer developer questions.

On SWE-bench Pro, which measures a model’s ability to complete difficult software engineering tasks, Anthropic says Fable 5 and Mythos 5 reach 80.3%, vastly outperforming OpenAI’s latest and greatest general model GPT-5.5, which scored 58.6%.

On Cognition’s FrontierCode Diamond benchmark, which tests high-quality, maintainable agentic coding, the models score 29.3%, compared with 13.4% for Claude Opus 4.8 and 5.7% for GPT-5.5, according to the benchmark table included in Anthropic’s materials.

Anthropic also says Fable 5 scores highest among frontier models on FrontierCode even at medium reasoning effort, suggesting the model may deliver stronger coding results without always needing maximum compute.

The most striking customer example comes from Stripe. Anthropic says Stripe tested Fable 5 in a 50-million-line Ruby codebase and found that the model completed a codebase-wide migration in one day that otherwise would have taken a team more than two months by hand. Stripe said, “Fable 5 compresses months of engineering into days. In our 50-million-line Ruby codebase, it did in a day what would’ve taken us more than two months by hand.”

Other early users describe the model as especially useful for long-horizon development tasks. Cursor said, “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.” Replit said Fable 5 is the highest-performing model it has tested on ViBench, its end-to-end “vibe-coding” benchmark, and that it builds apps in less time with fewer tokens. Figma said Fable 5 is “a clear step forward on agentic coding and prototyping.”

This is the enterprise shift Anthropic is trying to sell: AI coding systems that can take on larger units of work, not just individual tickets. That could include codebase migrations, app prototyping, pull request review, test generation, debugging across unfamiliar tools, user interface design and multi-step internal software projects.

Base44 said, “Fable 5 is much deeper and better at one-shotting full apps, and its tool calling is excellent.” Genspark said, “Fable 5 came out #1 on our evals, winning head-to-head against every model we tested. It was significantly stronger on the hardest tasks in the set — UI design and game coding.” Rakuten said, “At the highest effort, 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.”

For CTOs and engineering leaders, that suggests the model’s value may come less from raw code generation and more from sustained execution: understanding an intent, planning steps, calling tools, checking its own work and continuing through a task without constant human steering.

Knowledge work, finance, legal and operations

Anthropic is also positioning Fable 5 as a stronger model for enterprise knowledge work. On GDPval-AA, Anthropic reports a score of 1932 for Fable 5 and Mythos 5, compared with 1890 for Claude Opus 4.8, 1769 for GPT-5.5 and 1314 for Gemini 3.1 Pro.

On GDPpdf, a benchmark focused on visual document reasoning, Fable 5 and Mythos 5 score 29.8% without tools, compared with 22.5% for Opus 4.8, 24.9% for GPT-5.5 and 16.7% for Gemini 3.1 Pro.

That matters for enterprises because much of corporate work still lives in messy documents: PDFs, spreadsheets, charts, reports, contracts, filings, slide decks and screenshots. Anthropic says Fable 5 shows gains in document-based reasoning, chart and table interpretation and complex problem solving.

Hex said, “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.” Hebbia said Fable 5 was the highest-scoring model on its Finance Benchmark for senior-level reasoning, with double-digit gains in document reasoning, chart and table interpretation, and problem solving.

The finance examples are notable because they point to AI agents moving beyond summarization into higher-stakes analytical workflows.

IMC said Fable 5 “aced our trading-analysis evaluations nearly across the board: factual lookup, conceptual reasoning, root-cause analysis, expected-value analysis.” Optiver said the model was stronger than Opus 4.8 on its trading benchmark and “remarkably consistent,” scoring identically across repeated runs. Balyasny Asset Management said Fable 5 was the strongest finance-first model it had tested.

Legal and operations teams may also see immediate impact. Crosby Legal said, “Fable 5 feels materially different. In blind review, our lawyers found its redlines matched or beat our current model every time.” Notion said the model can take work “you’d chip away at all afternoon” and turn messy notes into a functioning project plan. Zapier said Fable 5 is the new leader on AutomationBench and is more autonomous than Opus 4.8: “Where Opus stops to ask, Fable 5 keeps looking.”

For enterprise software vendors, that points toward more capable embedded agents in workflow products: agents that can review a contract, update a project plan, assemble a spreadsheet, inspect a chart, file a ticket, run a query, call an internal API and keep going until the work is complete.

Vision and interface understanding

Anthropic says Fable 5 is also its strongest vision model. In its launch materials, the company says the model can extract precise numbers from detailed scientific figures and complete vision-based tasks such as rebuilding a web app’s source code from screenshots alone.

That has immediate implications for enterprise automation. Many business processes still depend on visual interfaces that are not cleanly exposed through APIs: dashboards, PDFs, forms, legacy apps, screenshots, scans and image-heavy reports. A stronger vision model could help agents operate across those environments with less custom integration work.

Anthropic also says Fable 5 needs less scaffolding than previous Claude models. As an example, the company says earlier Claude models struggled to play Pokémon FireRed even with extra tools, while Fable 5 impressively beat the game using a minimal vision-only harness. Anthropic posted a fast forwarded video of its playthrough to YouTube and in its blog post:

The point is not gaming itself, but the broader agentic skill: reading a visual environment, remembering progress, deciding what to do next and executing over a long horizon.

In another internal test, Anthropic says it had the model play the deck-building game Slay the Spire with access to persistent file-based memory. The company says persistent memory improved Fable 5’s performance three times more than it improved Opus 4.8’s, and that Fable reached the game’s final act three times more often. For enterprise users, this suggests Fable 5 may make better use of notes, logs and stored context during multi-step work.

That could matter for internal agents that operate over days or weeks: sales operations agents that track account research, engineering agents that manage migrations, finance agents that update models, or support agents that remember what they tried across many turns.

From restricted cyber model to general-purpose enterprise AI

The announcement follows Anthropic’s April 2025 rollout of Claude Mythos Preview through Project Glasswing, a restricted program for cyber defenders, critical infrastructure providers and major software maintainers. Anthropic created Glasswing after internal evaluations showed Mythos-class models could find and exploit software vulnerabilities at a level that raised meaningful misuse concerns.

Following the debut of Glasswing and Mythos, U.S. officials and intelligence agencies began weighing how such models could reshape both cyber defense and offensive operations, while Sen. Mark Warner warned that AI-assisted vulnerability discovery should force industry to “accelerate and reprioritize patching.” Financial regulators also took notice: The Guardian reported that Mythos entered discussions among senior banking officials and regulators in the U.S. and U.K. because of fears that AI-accelerated cyberattacks could threaten payment systems and broader financial stability.

The reaction has not been limited to alarm. Governments also want access: Reuters reported that South Korea’s national internet security agency had secured Mythos access through Project Glasswing, reflecting a broader geopolitical race to use frontier AI for national cyber defense. At the same time, Anthropic has faced scrutiny over whether it can safely gate the very capabilities it says are too risky for general release. The Verge reported that unauthorized users accessed Mythos after its limited rollout, calling the incident damaging for a company that has built its brand around responsible AI.

Critics have also questioned whether Anthropic’s warning-heavy framing risks becoming a form of market positioning, since it casts the company as both the source of the new capability and the gatekeeper deciding which governments, companies and researchers get to use it.

With Fable 5, Anthropic is leaning into its gatekeeper role, attempting to separate the general enterprise value of a Mythos-class model from the riskiest parts of its capability profile. The company says Fable 5 can handle software engineering, research, visual reasoning, document analysis and long-running agentic workflows, while classifiers block or reroute requests that could provide what Anthropic calls “uplift” to malicious actors.

Those classifiers cover three main areas.

1. Cybersecurity, where Anthropic says Mythos-class models can discover and exploit vulnerabilities and perform broader “agentic hacking” tasks such as reconnaissance, discovery and lateral movement.

2. Biology and chemistry, where the company says the same reasoning that can help researchers design therapies could also help well-resourced malicious actors pursue dangerous biological work.

3. Model distillation, where Anthropic says users may try to extract Claude’s capabilities to train competing models, including models that could be released without similar safeguards.

When Fable 5’s classifiers detect one of those categories, the response is automatically handled by Claude Opus 4.8. Anthropic says users will be told when this happens. That is a notable product decision: rather than declining those requests outright, Anthropic is trying to keep the user experience functional while reducing access to the most capable version of the model in sensitive areas.

Anthropic says it red-teamed the new classifier system internally and externally. The company says an internal bug bounty produced no universal jailbreaks after more than 1,000 hours of testing, and external red-teaming organizations also failed to find a universal jailbreak. One external partner found that Fable 5 complied with zero harmful single-turn cyber requests related to planning cyberattacks, exploit development or defense evasion, even when prompts used any of 30 public jailbreak techniques, according to Anthropic.

The company is still acknowledging tradeoffs. Anthropic says the safeguards are deliberately cautious and may sometimes trigger on benign requests. That could frustrate security professionals, biology researchers and advanced enterprise users whose legitimate work overlaps with the blocked categories. The company says it plans to reduce false positives over time.

Mythos 5 and the restricted frontier

While Fable 5 is the broad commercial launch, Mythos 5 is the model to watch for enterprises operating in security, critical infrastructure and life sciences.

The company says all users with Claude Mythos Preview access can upgrade to Mythos 5 beginning today. It plans to expand access through a trusted access program, in collaboration with the U.S. government.

The distinction is important for sectors where the blocked capabilities are not edge cases but core workflows. A security team may need to reproduce vulnerabilities, test exploitability, analyze lateral movement or simulate attacker behavior in a controlled environment. A biology research team may need to reason through molecular design workflows that would trigger general-use safeguards. Fable 5 is not designed to give every user unrestricted access to those capabilities; Mythos 5 is designed for vetted users who need them.

Anthropic says Mythos 5 has the strongest cybersecurity capabilities of any model in the world. In the company’s benchmark table, the model family scores 78.0% on ExploitBench, compared with 69.0% for Claude Mythos Preview, 40.0% for Opus 4.8 and 34.0% for GPT-5.5. On CyberGym, Anthropic’s chart shows Mythos 5 at 83.8%, slightly ahead of Mythos Preview at 83.1% and far above Opus 4.8 with default safeguards.

The company is making a similar argument in biology. Anthropic says Mythos-class models outperform dedicated protein language models on a task involving adeno-associated viruses, a delivery mechanism used in gene therapies. The company frames that as both promising and risky: the same capability that could help gene therapy research could also be misused in dangerous biological work.

Anthropic says its internal protein design experts used Mythos 5 to accelerate parts of the drug design process by about tenfold. In one example, the company says Mythos 5, using protein design and bioinformatics tools without human assistance, matched or beat skilled human operators by choosing binding sites, selecting and running tools, and recovering from failures. Anthropic says nine of 14 protein targets in the study produced strong candidates for drug design that it is now investigating.

The company also says Mythos 5 produced novel molecular biology hypotheses that Anthropic scientists preferred over Opus-class model hypotheses about 80% of the time in blinded comparisons. Anthropic says several of those ideas have advanced to experimental evaluation, and one hypothesis involving an E. coli protein was later corroborated by an independent lab working on the same problem.

Those claims are potentially significant, but they should be treated carefully until more details are published. Anthropic says it intends to publish additional results in the coming months. For now, the strongest enterprise implication is directional: the company believes its highest-end models can already perform parts of scientific research workflows with less human intervention than prior systems.

New, longer data retention requirement

The company also introduced a new data-retention policy for Mythos-class models. Anthropic says it will require 30-day retention for all traffic on Fable 5, Mythos 5 and future models with similar or higher capability levels, across both first-party and third-party surfaces. The company says it will not use that data to train new Claude models or for non-safety purposes, and says it has added privacy protections including logging human access and deleting the data after 30 days in almost all cases.

That policy may become one of the most important enterprise buying questions around Fable 5. Many businesses want frontier AI capability but also want strict control over data retention, especially in regulated sectors. Anthropic’s position is that stronger monitoring is necessary for models with this level of capability. Enterprise customers will have to decide whether the capability gain justifies the retention requirement.

Enterprise implications

The broader enterprise significance of Fable 5 is that Anthropic is trying to commercialize a more autonomous class of AI model without exposing all of its capabilities to every user. That could become a template for how frontier labs release increasingly powerful systems: one model family, multiple access tiers, and domain-specific restrictions depending on user trust and risk.

If Fable 5 performs as Anthropic and early customers describe, developers may hand off larger tasks: code migrations, refactors, UI builds, test writing, bug fixing, documentation, internal tooling and multi-step app creation.

For knowledge-work-heavy enterprises, Fable 5 could make AI more useful in workflows where earlier models were too brittle: finance research, spreadsheet analysis, legal redlines, procurement review, board materials, market research, sales operations and project planning. The main gain is not just better answers; it is fewer turns, fewer corrections and more ability to keep working through ambiguity.

For security teams, the launch is more complicated. Most organizations will get Fable 5, not unrestricted Mythos 5. That means they may see stronger general coding and analysis, but not full access to the cyber capabilities Anthropic considers risky. Trusted defenders inside Project Glasswing will get Mythos 5, giving them a more direct way to use the model for vulnerability discovery and defensive testing.

For life sciences companies, the pattern is similar. Fable 5 may help with general research, literature analysis, data interpretation and scientific reasoning, but the more sensitive biological capabilities will be restricted. Anthropic is effectively creating a separate access path for vetted researchers whose work requires capabilities that could be dangerous in the wrong hands.

The launch also raises competitive pressure across the AI industry. Anthropic is claiming state-of-the-art results across agentic coding, knowledge work, vision, cybersecurity, legal reasoning, spatial reasoning and health benchmarks. But the more strategically important claim may be that it has found a workable release mechanism for models above its Opus class. If Fable 5’s safeguards hold up under real-world use, Anthropic will argue it can bring more powerful models to market sooner without fully opening the riskiest capabilities.

That is still a large “if.” The enterprise market will test not only Fable 5’s benchmark performance, but also its reliability, false-positive rate, data-retention tradeoffs and cost at scale. A model that can complete more work autonomously can also burn more tokens, trigger more governance questions and create new review burdens for teams that must verify its output.

Still, today’s launch marks a clear shift in the Claude lineup. Opus is no longer Anthropic’s top commercial capability tier. Mythos-class models now sit above it. Fable 5 is the first version of that tier for general users; Mythos 5 is the restricted version for trusted high-risk work. Together, they show how Anthropic plans to push frontier AI deeper into enterprise workflows while trying to keep the most dangerous capabilities gated.