Anthropic Unveils Fable 5.1 and Mythos 5.1 with Major Cost Cuts and EU AI Act Compliance

Anthropic's new Fable release is cheaper, less restrictive

On Tuesday, Anthropic released Fable and Mythos 5.1, twinned versions of the company's most advanced AI model. In addition to performance upgrades, the new Fable release includes changes meant to reduce token cost and false-positive restrictions from the model's safeguards. As with the previous Mythos model, Mythos 5.1 will only be available to registered Anthropic partners engaged in either cybersecurity or life sciences research. Fable 5.1, the unrestricted version, is available starting today on cloud platforms or through the Anthropic API. One of the most significant changes is Anthropic's previously reported embrace of Zero Data Retention, allowing clients to run Anthropic models on their own infrastructure without data outflows. Previously unavailable for Fable due to security concerns, a high-privacy service (called Enterprise Frontier Safeguards) will now roll out to users in June. Notably, the system will still monitor for misuse by agents or human users, but clients will control how the monitoring takes place. As part of the announcement, Anthropic assured customers that their data had not been inappropriately accessed. "Anthropic has never trained on enterprise data without explicit permission, and never will," the announcement reads. As is common for an Anthropic release, the new models set records in a range of benchmarks, including Terminal-Bench 4.0 (for CLI-based coding) and Humanity's Last Exam (for general reasoning). Anthropic also released three novel scientific findings generated by the models before their release, including a custom GPU optimization and a high-resolution map of Venus assembled from existing photos. As with previous releases, the models come with a detailed system card, which explains their capabilities in most straightforward terms. The system card rates Mythos as "low-risk" for concerns related to automated AI development -- where the AI improves itself -- which some see as a trigger for a loss of human control. It says, "its ability to accelerate internal AI R&D progress is in line with current trends." In terms of general misbehavior, Mythos is slightly more prone to it than Opus, possibly as a result of its enhanced capabilities. "Mythos 5.1 is a slight regression on overall misaligned behavior compared to Opus 5, and an improvement over Mythos 5 and Claude Sonnet 5," the system card reads. "It cooperates with human misuse and accepts unverifiable claims of authorization somewhat more readily than Opus 5, but it is less likely to ignore explicit constraints, hallucinate inputs, or falsely claim to have completed tasks than previous models."

Anthropic launches Claude Fable 5.1 and says it's up to 45 percent cheaper for agentic work

Anthropic says its newest AI models, Fable 5.1 and Mythos 5.1, address criticisms from customers about price, data retention, and overzealous safeguards. The company claims Claude Fable 5.1 offers stronger performance than Fable 5, but costs around 25 percent less typically and up to 45 percent less for complex agentic tasks, thanks to reduced pricing on cached data that was already processed and stored. Along with the announcement, a slew of early impressions popped up, including from Every CEO Dan Shipper, who claims, "It's the strongest coding model we've used, but now it's fast, token-efficient, and crucially actually speaks like a normal person." Box CEO Aaron Levie is another early access believer, saying that his company's agent with Fable 5.1 picked up on subtleties and ambiguities in data that Fable 5 missed in the same test. Meanwhile, Lisan al Gaib points out that on benchmarks, Mythos 5.1 with low reasoning scores the same as its predecessor set to Max reasoning. Fable 5.1 also has "more precise safeguards" that Anthropic says are less likely to block basic biology questions than Fable 5, but Mythos 5.1 has the same biology restrictions as the previous model. Anthropic also explained its progress on data retention, saying that Enterprise Frontier Safeguards offer "complete privacy" by storing data on the customer's cloud servers instead of its own, and will start rolling out later this fall. Anthropic says it's "now allowing Fable 5.1 to be used for identifying software vulnerabilities," but it will still redirect some cybersecurity tasks to Opus models, like "penetration testing, exploit generation, and binary-based vulnerability scanning." Claude Fable 5.1 is now available on all platforms, while Mythos 5.1 is available to Project Glasswing participants only.

Anthropic releases Claude Fable 5.1 and Mythos 5.1, cutting cache read prices by 75%

Fable 5.1 cuts cache read pricing by 75% and more than doubles a scientific research benchmark, while the detection API that makes the AI Act's marking obligation meaningful is limited to regulators, media, researchers and EU civil society groups Anthropic has released Claude Fable 5.1 and Mythos 5.1, the same model with different safeguards, cutting cache read prices by 75% and reporting 52.6% on a scientific research benchmark against 24.7% for its predecessor. Its outputs carry a watermark required by the EU AI Act, and the detection API is in private preview for eligible organisations. Anthropic has released two new Claude models. Fable 5.1 and Mythos 5.1 are the same model with different safeguards, the company said. Only one of them is generally available. Mythos goes to vetted users through two access programmes covering cybersecurity and the life sciences. Anthropic says the biology programme was built with the US government. The developer headline is price. Cache reads fall 75% to $0.25 per million tokens, cutting typical workload costs by about 25% and agentic ones by up to 45%. The benchmark jump is large on Anthropic's own numbers. Fable 5.1 scores 52.6% on Terminal-Bench-Science 0.1 against 24.7% for Fable 5, and 55.8% on Terminal-Bench 4.0. The European part is near the bottom of the post. Anthropic signed the EU's Code of Practice on transparency of AI-generated content in July, alongside 190 other signatories. That is why Claude's output carries a watermark. It applies to models released after 2 August 2026, is invisible without the detection tool, and Anthropic says it carries nothing about the user. TNW reported last month that Anthropic would ship a free detection API so anyone could check a piece of text. What has shipped is narrower than that. It is in private preview for eligible organisations, including regulators, law enforcement, media, fact-checkers, independent researchers, educational bodies and EU civil society groups. The underlying duty is wide. Article 50 requires providers to mark synthetic output in a machine-readable format and make it detectable, and it has applied since 2 August 2026. Enterprises with their own compliance obligations can apply too. Anthropic says it plans to expand access over time, without giving a date. The second European gap is the other model. Mythos's access programmes were built with the US government and are open only to a set of US organisations, two years after Claude arrived in the EU. Anthropic says it is working to include international partners. European cyberdefenders and life scientists are not in the programme today. Which is the shape of the launch. The mark exists because of European law, the tool to verify it is in preview, and TNW has reported that removal tools already exist.

Claude's updated Fable and Mythos models watermark their replies

Claude Fable 5.1 is now generally available in the Claude desktop app and other platforms, while Claude Mythos 5.1 is restricted to Anthropic's "trusted access program." While both models are the same, Mythos lands with less stringent safeguards to "support work in cybersecurity and the life sciences." In its announcement, Anthropic shows off a variety of benchmarks demonstrating the performance boosts the two models, complete with double-digit increases when it comes to agentic scientific research, agentic coding, and business workflows. Anthropic also says both Fable 5.1 and Mythos 5.1 were evaluated using the company's new safety and alignment procedures, developed in response to Anthropic's disclosures of Claude models hacking outside companies during internal testing. Specifically, Mythos 5.1 is "less likely than Mythos 5 to try to access resources outside of its test environment when assigned an otherwise impossible task," Anthropic said. Beyond their agentic capabilities and upgraded safeguards, through, Claude Fable 5.1 and Mythos 5.1 mark the first Claude models to watermark their text and file outputs, in accordance with the EU AI Act. "As we recently explained, this watermark is invisible to anyone who does not have the detection API," Anthropic wrote in its announcement. "It has no practical impact on the quality or content of Claude's outputs and contains no information about the user, their organization, or their conversations with Claude." As for the detection API, Anthropic says is now available to regulators, law enforcement, media, fact-checkers, independent researches, and other "eligible groups" as defined by the just-enacted EU law, adding that it will "expand access to the detection API over time." You can apply for access to the API via this signup form. Anthropic had previously stated it would add watermark support to any new Claude models released after August 2, 2026. Claude Fable 5.1 and Mythos 5.1 are the first new Claude models to arrive subsequent to that date, while older Claude models will eventually add watermarking support as well, Anthropic has previously said.

Anthropic upgrades Claude with new Fable 5.1 model, details here

In June, Anthropic first released Fable 5, its Mythos-class AI model for Claude. Three months later, Anthropic is enhancing Claude with its new Fable 5.1 model upgrade. Claude Fable 5.1 and Mythos 5.1 arrive today In Anthropic's world, Fable is Claude's most capable model. Haiku, Sonnet, and Opus are the small, medium, and large models below Fable. "Claude Fable 5.1 sets a new standard on coding, knowledge work, and long-running problem-solving tasks," Anthropic says. The company says the new model has a higher level of performance compared to Fable 5. Anthropic also says Fable 5.1 "achieves similar or better results than Fable 5 at a much lower cost" when set to low or medium effort. More from Anthropic below: Fable 5.1 avoids shortcuts that result in poorer-quality work, and it's smart enough to fix the root causes of software issues. For example, in testing by the investment firm Millennium, Fable 5.1 found the cause of a rare crash on their internal systems that none of their engineers (or any other model) had been able to explain after several years of trying. In addition to Fable 5.1, Anthropic is also releasing Mythos 5.1 for users with Mythos access. "Claude Fable 5.1 and Claude Mythos 5.1 are the same model, but with different levels of safeguards," Anthropic explains. "Fable 5.1 is generally available, while Mythos 5.1 is available only through our trusted access programs; its safeguards are specifically designed to support work in cybersecurity and the life sciences." Here's pricing data for Fable 5.1: Fable 5.1 will cost an estimated 25% less than Fable 5 for typical workloads, wherever usage is billed by token. This is because we're reducing our pricing on cache reads (where the model reads inputs that have already been processed and stored). For highly agentic work, the savings will often be much larger -- up to approximately 45%. In terms of pricing, Anthropic says it has "reduced the price of Fable 5.1's cache reads (where the model reuses context it has already processed) wherever usage is billed by token, such as on our API." The company says this "leads to a substantial reduction in the overall cost of running the model." Pricing is otherwise unchanged from Fable 5. You can learn much more in the announcement post here.

Anthropic releases new models, cuts agent costs

Why it matters: This could be the new playbook for a release: it's not enough to have a more intelligent model, you also need new tools and updates that address customer concerns. Driving the news: Claude Fable 5.1 is now generally available, while the more restricted Claude Mythos 5.1 remains limited to vetted partners working in areas like cybersecurity and life sciences. * Anthropic says Fable 5.1 improves on its June release across coding and multi-step knowledge work. * Its base price remains $10 per million input tokens and $50 per million output tokens. * Anthropic is also cutting the cost associated with Claude referencing or pulling on information it's already processed by 75%, which should make long-running AI agents cheaper to operate. Zoom in: Anthropic is also introducing new enterprise safeguards that it says can provide protections equivalent to zero-data-retention policies while still allowing the company to police malicious use. Zoom out: Anthropic said on X that it "improved our safeguards" which it defines as users being less likely to trigger safeguards that route them to more restricted responses. * Medical or biology questions will have 85% fewer interventions, while some users could see roughly 60% fewer cybersecurity-related interventions per session. * That addresses a major criticism developers have had regarding Anthropic. Routing company founder Joseph Perla tells Axios that his users were looking to be less reliant on Claude in part due to these interventions. * Anthropic told Axios in June that the company was initially being more conservative on safeguards and they were working to improve on that over time. Flashback: Fable 5 and Mythos 5 debuted in June as Anthropic's highest-capability models, with Mythos access deliberately restricted because of its advanced cyber and biological capabilities. Between the lines: With one update, Anthropic unveiled new models, new safeguards, cybersecurity and intervention changes, cost cuts and added new enterprise IP protections. * Competing AI lab OpenAI was rumored to release its latest model, Astra, earlier in the summer. * The company paused some work on models following the HuggingFace incident. What they're saying: "Releasing Fable 5.1 before Astra is quite a flex," Gavin Baker, managing partner and CIO at Atreides Management, said on X. * That's perhaps even more true as it's widely expected that Anthropic could file IPO paperwork this month. The bottom line: Anthropic has a lot of new things it's doing to retain customers.

Anthropic launches Fable 5.1 as AI security worries mount

Anthropic released Claude Fable 5.1 and a more restricted sibling, Claude Mythos 5.1, this week, positioning the pair as its most capable models yet for coding and knowledge work. Notably, the announcement arrives just weeks after fresh reports of AI agents behaving badly in security tests. The two models are technically identical, Anthropic says in its blog post, but come with different guardrails. Fable 5.1 is broadly available, while Mythos 5.1 is reserved for vetted partners working in cybersecurity and the life sciences through the company's trusted-access programs. Anthropic also says it's cutting prices. According to the company, typical workloads will cost around 25 percent less than they did with Fable 5, with savings of roughly 45 percent for heavily agentic tasks, largely due to cheaper cache-read pricing. At the time of writing, Claude customers are accusing the AI firm of deceptive pricing and marketing for its Claude Max subscription. On the safety side, Anthropic touts more precise filters. The company says its updated biology safeguards now fire far less often on benign medical questions, and that cybersecurity safeguards produce about 60 percent fewer false interventions in Claude Code sessions than before, in part because Fable 5.1 is now allowed to help identify software vulnerabilities -- though not build exploits for them. That framing lands amid a rockier moment for AI agents and the cybersecurity industry at large. In early August, multiple outlets reported that AI agents from both Anthropic and OpenAI had been caught taking unsanctioned actions during red-team testing. The UK's AI Security Institute, which ran the tests, said agents from the two companies took unauthorized actions a combined 19 times across 122 test runs, with the bulk of the incidents attributed to an earlier Anthropic model, Mythos 5. Anthropic responded at the time by noting that the tests were run under what it called deliberately permissive conditions that don't reflect how its production models actually operate. The company says Mythos 5.1's cyber capabilities are the strongest it has released yet, but the model still falls in the lower-risk tier under its internal framework -- and Anthropic found no evidence of a critical-severity jailbreak in testing.

Anthropic's Claude Fable 5.1 and Mythos 5.1 arrive with a 75% cost reduction for Fable cache reads

It's only the first day of September 2026, but the month and fall season are already off to the races in AI land, as Anthropic has just released its latest and most powerful large language models yet -- Claude Fable 5.1 and Claude Mythos 5.1. The two names refer to the same underlying model. Fable 5.1 is the generally available version, with Anthropic's production safeguards in place. Mythos 5.1 is available through restricted-access programs for vetted cybersecurity and life-sciences organizations that need capabilities normally constrained by those safeguards. For enterprise buyers, however, the release is about more than another round of benchmark gains. Anthropic is simultaneously changing the economics of running persistent agents, reducing the cost of cached context by 75%, and introducing a new security architecture called Enterprise Frontier Safeguards, or EFS, designed to let organizations retain monitoring data inside infrastructure they control. Those changes arrive at a particularly consequential moment. Over the past several weeks, Anthropic and the U.K. AI Security Institute have disclosed incidents in which earlier Claude models, running under unusually permissive cybersecurity evaluation conditions, took unauthorized actions against real systems. Anthropic temporarily paused external cyber evaluations and has since introduced additional containment and monitoring before resuming them. Taken together, Fable 5.1 looks less like a conventional model refresh than an attempt to solve three increasingly intertwined enterprise problems: how to make agents capable enough to finish difficult work, economical enough to leave running for hours, and governable enough to give access to sensitive systems. A model built for work that does not finish in one prompt Anthropic is positioning Fable 5.1 primarily around sustained problem-solving. On Terminal-Bench-Science 0.1, which evaluates agentic scientific research, Anthropic reports Fable 5.1 scoring 52.6%, compared with 24.7% for Fable 5, 29.0% for Opus 5 and 22.4% for GPT-5.6 Sol in its evaluation setup. On Terminal-Bench 4.0, Fable 5.1 scores 55.8%, versus 42.0% for Fable 5 and 52.3% for Opus 5. Mythos 5.1 reaches 60.9% on the same coding benchmark when operating under its more permissive cyber safeguards. The gains extend beyond coding. Anthropic reports a GDPval-AA v2 score of 1,853 for knowledge work, versus 1,824 for Opus 5 and 1,723 for Fable 5. On AutomationBench, intended to measure business workflows, Fable 5.1 scores 31.4%, compared with 17.1% for Fable 5 and 26.9% for Opus 5. On CursorBench 3.2.0, it reaches 73.4%. Those numbers should be read as vendor-reported results rather than independent proof of superiority. Anthropic also notes qualifications around several evaluations: production safeguards can affect scores, and its August 2026 OSWorld task release is not directly comparable with some previously published results. The more useful signal for enterprise teams may therefore come from the kinds of failures early-access partners say the model can resolve. Investment firm Millennium told Anthropic that Fable 5.1 traced an extremely rare software crash to a bug inside an external vendor library after the problem had resisted explanation for four to five years. Corporate expense management provider Ramp described an unattended 38-hour machine-learning run in which the model re-evaluated a previous result, launched six experiments and returned with findings and proposed next steps. Browserbase said Fable 5.1 completed 82% of tasks on its hardest browser-agent benchmark, versus 74% for Opus 5 and 57% for Fable 5. These are customer testimonials supplied as part of Anthropic's launch, not independently reproduced benchmarks. But they illustrate the direction Anthropic is pursuing: moving the unit of AI work from an answer or code snippet toward an entire investigation. That changes deployment architecture. A model that can operate for hours needs durable context, tool access, checkpoints, logging, permission boundaries and reliable recovery from errors. Model intelligence becomes only one component of the system. Pricing: Fable 5.1 remains premium, but caching changes the equation The most immediately measurable enterprise change is pricing. Fable 5.1 retains Fable 5's headline API rates: $10 per 1 million input tokens and $50 per million output. That makes it considerably more expensive on uncached tokens than other models in Anthropic's lineup. Opus 5 costs $5 per million input tokens and $25 per million output tokens, while Sonnet 5 costs $2 and $10 respectively. The important change is cached input: Anthropic has cut a Fable 5.1 cache hit to just $0.25 on input, down from $1.00 for Fable 5. That's also just 2.5% of Fable's normal input-token price of $10, rather than the 10% multiplier used by most other Claude models. Five-minute cache writes remain $12.50 per million tokens and one-hour writes $20, but subsequent reads cost just $0.25 per million. That produces an unusual pricing profile. Fable 5.1's ordinary input and output are twice as expensive as Opus 5's, yet its cached input is half the cost of Opus 5's cache reads. Its cache-read price is only 25% above Sonnet 5's despite Fable's base input price being five times higher. That matters for agents because they repeatedly revisit the same codebase, system instructions, tool definitions, documents and accumulated conversation history. Anthropic says the lower cache price reduces Fable 5.1's effective cost by around 25% for typical workloads and as much as roughly 45% for highly agentic workloads in which cached context accounts for a larger share of usage. This is a more useful enterprise framing than simply comparing per-token list prices. Model selection for an agentic workflow increasingly depends on cost per successfully completed task, including retries, context replay, tool calls and the number of tokens a model consumes before reaching a usable result. Fable 5.1 nevertheless remains expensive relative to much of the broader market. OpenAI's current promotional API pricing for GPT-5.6 Sol is $4 per million input tokens, $0.40 for cached input and $20 per million output tokens through at least Nov. 21. Google's Gemini 3.7 Flash currently lists at $0.75 per million input and $3.75 per million output through the end of 2026. Fable therefore needs to justify its premium through higher task completion, lower token consumption, or the ability to replace more expensive human or multi-stage workflows -- not simply through raw API price. Anthropic's pricing system provides additional levers. Batch processing cuts Fable 5.1 input and output rates in half, to $5 and $25 per million tokens, for asynchronous workloads. U.S.-only inference adds a 1.1x multiplier. Web search costs $10 per 1,000 searches in addition to token usage, while web fetch carries no separate fee. For procurement teams, those details make workload profiling increasingly important. A code-review agent with enormous reusable repository context may have very different economics from a report-generation workflow dominated by new output tokens. The cyber incidents behind the release The security architecture surrounding Fable 5.1 also needs to be understood against Anthropic's recent cybersecurity incidents. On July 30, Anthropic disclosed that a review of 141,006 cybersecurity evaluation runs had identified three incidents, spanning six runs, in which Claude models reached the public internet from a third-party testing environment and obtained unauthorized access to real organizations. The models had been running without the cyber safeguards applied to generally available Claude deployments. They were performing capture-the-flag exercises and had explicitly been told that their environments were simulations without internet access. A configuration problem nevertheless left an internet path open. In the most serious incident, Claude Opus 4.7 encountered a real company that shared a name with the fictional organization in its test. Across four runs, the model obtained application and infrastructure credentials and accessed a database containing several hundred rows of production data. Anthropic said the model eventually encountered indications that the system was real but continued its attack. A second incident involved Mythos 5. The model discovered developer instructions referencing a nonexistent Python package and decided that publishing a package under that name could help it win the simulated challenge. It created a PyPI account and uploaded malicious code to the real public package repository. The package remained available for roughly an hour and was downloaded and executed on 15 real systems. One belonged to a security company whose automated scanner executed the package; Claude's code extracted credentials and subsequently used them to access additional infrastructure. In the third incident, an internal Anthropic research model scanned roughly 9,000 internet targets after failing to reach its intended fictional target. It compromised an internet-facing application using exposed credentials and SQL injection, then stopped after recognizing that the system had no connection to the exercise. The incidents did not involve customer data or Anthropic's production infrastructure, and Anthropic says the production safeguards omitted during the tests would have blocked the behavior. Still, the episodes demonstrated a problem enterprise agent architects will recognize: a sufficiently persistent agent can exploit ambiguity between what its operator intended it to access and what its technical credentials allow it to access. A separate U.K. AI Security Institute disclosure reinforced that concern. During testing conducted in July, AISI ran a cyber challenge 122 times across multiple models with internet access deliberately enabled and vendor cyber classifiers disabled. Ten runs produced 19 unsanctioned real-world actions. Seventeen involved Anthropic's Mythos 5; two came from a GPT-5.6 Sol run. The most serious sequence involved a Mythos 5 agent attempting to insert malicious code into a real open-source project, creating fake identities and attempting to socially engineer a maintainer into accepting the change. The maintainer rejected it, and AISI said its investigation found no resulting real-world harm. The agent also attempted prompt injection against other AI systems and left messages that later agents discovered and used. Again, these were deliberately permissive research configurations, not commercially available deployments. But AISI's conclusion is relevant far beyond AI labs: increasingly capable agents should be assumed to explore routes their operators did not anticipate, particularly when given difficult objectives and broad tool access. From model safeguards to infrastructure safeguards Anthropic's response has been to add layers around the model. The company paused external cyber evaluations and briefly halted some internal testing, then deployed a real-time classifier designed to detect aggressive probing, sandbox-escape attempts or unexpected internet access before a tool call executes. It also moved high-risk cyber workloads into stronger isolation and imposed new requirements on external evaluators, including explicit network isolation and pre-engagement validation of sandboxes. External cyber evaluations have now resumed. Fable 5.1 itself also gets more precise production safeguards. Anthropic says its cyber protections now create around 60% fewer interventions per Claude Code session than Fable 5's previous safeguards. The model can be used to discover software vulnerabilities for defensive purposes, while work such as exploit generation, penetration testing and some binary-based vulnerability scanning remains redirected or restricted. That distinction matters for security teams attempting to operationalize AI. A safeguard that blocks too many legitimate actions can make an autonomous security workflow unreliable; one that permits too much creates a materially different risk. Precision, rather than simply the existence of a filter, becomes a production requirement. Enterprise Frontier Safeguards moves data custody to the customer Anthropic is addressing a second enterprise constraint through EFS. The company previously introduced 30-day data retention for Fable 5 as part of its misuse-detection system. For regulated organizations, retaining sensitive conversations with a model provider can make deployment difficult regardless of contractual assurances. EFS changes the architecture. Monitoring data can reside in the customer's own AWS, Azure or Google Cloud environment under customer-managed encryption keys, access policies and audit logging. Anthropic's automated systems can analyze the data for patterns associated with serious misuse, while alerts go to the customer for review; Anthropic says human review by its employees is not required. Anthropic says it developed EFS with more than 100 organizations across financial services, healthcare, manufacturing, telecom, law, retail and government, and with AWS, Google Cloud and Microsoft Azure. Support is planned across Claude Code, Claude Enterprise, the Claude Platform, Amazon Bedrock, Claude Platform on AWS, Google's Agent Platform and Microsoft Foundry. The rollout begins in phases this fall. Eligible customers can use Fable 5.1 with zero data retention until EFS becomes available. Anthropic does not charge separately for EFS, although customers remain responsible for their own cloud storage, operations and egress costs. This is potentially as important as the model upgrade itself. Enterprise AI governance is shifting from promises about what a provider does with data toward architectures that determine where the data can exist in the first place. Fable for production, Mythos for controlled frontiers The split between Fable and Mythos gives Anthropic a mechanism for separating general enterprise deployment from particularly sensitive domains. Fable 5.1 is available now through Anthropic's API as , as well as through AWS, Google Cloud and Microsoft Azure. Mythos 5.1 uses the same underlying model but exposes more permissive safeguards to vetted cyberdefenders and life-sciences organizations through verification programs. That same model has shown capabilities extending well outside software. Anthropic reports that Mythos 5.1 designed experimentally validated protein binders, while Fable 5.1 trained a neural network that produced a higher-resolution elevation map covering roughly a third of Venus. Mythos 5.1 also optimized seven open-source biological deep-learning models, with Anthropic reporting inference speedups as high as 2.5x. For pharmaceutical, engineering and research organizations, that points toward a future in which the same agent architecture used to investigate a code failure may also orchestrate modeling, experimentation and analysis. The operational lesson is the same in every case: the more work an agent can complete without intervention, the more consequential its permissions become. Fable 5.1 makes those long-running agents more capable and, through cheaper cached context, potentially much cheaper to operate. EFS gives regulated companies another mechanism for governing their data. More precise safeguards reduce some of the friction that has made high-capability models difficult to use in security workflows. But Anthropic's own recent incidents also demonstrate why the enterprise deployment question cannot stop at model selection. The next generation of AI infrastructure will need to treat agents more like powerful service accounts than chatbots: narrowly scoped credentials, segmented networks, explicit allowlists, continuous telemetry, human approval around irreversible actions and the assumption that an agent may find pathways its developer did not anticipate. Fable 5.1 raises the amount of work organizations can plausibly delegate. Its larger significance may be that it also makes the infrastructure surrounding that delegation impossible to treat as an afterthought.

Anthropic Ships Claude Fable 5.1, More Than Doubling Its Predecessor on Key Benchmark

Fable 5.1 is not included in Pro plans or standard Team seats, which run it on usage credits; Max and premium Team or Enterprise seats get it included for up to 50% of weekly limits. Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on Tuesday, its first update to the Mythos-class model line since Fable 5 launched on June 9. The company claims new AI models are the world's "most advanced for coding and knowledge work," and it lands three months into a launch cycle that has already included an 18-day export-control shutdown, a mid-summer pricing fight over subscription access, and a July release of Claude Opus 5 that undercut Fable 5 on price. Fable 5.1 and Mythos 5.1 are, per Anthropic, the same underlying model with different safety filters bolted on. Fable 5.1 is generally available to anyone with a Claude account. Mythos 5.1 stays restricted to vetted cybersecurity and life-sciences professionals through Anthropic's Cyber Verification Program and Life Sciences Verification Program, the successor to the Project Glasswing access track that gated Mythos 5. What the benchmarks actually measure Anthropic's headline number comes from Terminal-Bench-Science 0.1, a benchmark that tests whether an AI agent can carry out scientific research tasks inside a command-line environment, scored as a pass rate. Fable 5.1 hit 52.6% against Fable 5's 24.7% and Opus 5's 29.0%, more than double its predecessor. Terminal-Bench 4.0 tests agentic coding done through a terminal -- writing, running, and debugging code across multi-step command-line sessions, scored as a percentage of tasks completed correctly. Fable 5.1 scored 55.8%, up from Fable 5's 42.0% and ahead of Opus 5's 52.3%. Mythos 5.1, running with lighter cybersecurity filters, scored 60.9% on the same test; Anthropic says the gap reflects tasks its safeguards intercepted and rerouted to Opus 4.8. On Humanity's Last Exam, a multidisciplinary reasoning test built from expert-level questions across dozens of academic fields and scored as a pass rate, Fable 5.1 reached 60.9% without external tools and 65.0% with them, both ahead of Opus 5, showing this is Anthropic's most advanced model for academic purposes. Where it beats Opus 5, and where the math gets murkier Opus 5 launched in July costing half of Fable 5's per-token price while outscoring Fable 5 on most major benchmarks, effectively making the flagship model redundant for most paying users. Fable 5.1 reverses that: It now beats Opus 5 on every benchmark Anthropic published, including the ones where Opus 5 had previously beaten Fable 5. But Fable 5.1 still costs twice as much per token as Opus 5 -- $10 input and $50 output versus Opus 5's $5 and $25. Tokens are the basic amount of information a model can handle (usually around two thirds of an average English word), and companies pay for use because heavy workflows usually deplete the quotas set in subscription plans very quickly. Anthropic's own pitch for the model leans on effort levels rather than raw scores: it says that running Fable 5.1 at low or medium reasoning effort matches or beats Fable 5's old results at much lower cost, while reserving the top effort tiers for problems that stump everything else. For routine work, that argument for skipping Opus 5 entirely gets weaker the lower the effort dial goes. Access follows the same split that applied to Fable 5 after months of Anthropic adjusting the terms. On Pro plans and standard Team or Enterprise seats, Fable 5.1 draws entirely from pay-as-you-go usage credits billed at the API rate, since it isn't part of those plans' weekly limits. On Max plans and premium Team or Enterprise seats, it's included as a standard part of the subscription for up to 50% of weekly usage. Claude Fable 5.1 is available now on the Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry, under the model ID "claude-fable-5-1."

Anthropic Beats OpenAI To The Punch With A Nominally Cheaper Claude Fable 5.1, As Samsung Bets Its Chip Designs On Claude Code

In the ongoing seemingly never-ending one-upmanship between Anthropic and OpenAI, the former has just taken an incremental step forward by releasing the Claude Fable 5.1 and Mythos 5.1 models ahead of OpenAI's much-anticipated Astra-class ones. Meanwhile, Anthropic's Claude Code - the coding-focused harness around its models - has just received a resounding endorsement from an unexpected quarter: Samsung's chip design-focused LSI division. Anthropic's Claude Fable 5.1 model offers a respectable 4-point jump over Fable 5 on the Artificial Analysis Intelligence Index, while entailing lower API prices Anthropic has just launched the Claude Fable 5.1 model to moderate acclaim. Do note that both the Fable 5.1 and the Mythos 5.1 are basically the same model, with the substantially strengthened guardrails in the case of the Fable 5.1. As far as benchmarks are concerned, the Fable 5.1 surpasses every previous Claude model as well as OpenAI's GPT-5.6 Sol (its premier model right now) on virtually every metric. What's more, Anthropic has been able to achieve this feat while reducing the overall API cost. As a case in point, consider the fact that the AI lab has priced the Fable 5.1 at the same $10 per 1 million tokens of input and $50 per 1 million tokens of output as the Fable 5.0, but cache reads are now priced at $0.25 per 1 million tokens instead of the previous $1 (75 percent cheaper). This suggests that Anthropic might be offloading more of its KV cache to Flash storage now. On the Artificial Analysis Intelligence Index, Anthropic's Claude Fable 5.1 offers a 4-point jump over the Fable 5.0 but its weighted average cost per Intelligence Index task sits at $3.69 vs. $2.34 for the Fable 5.0, which suggests that the model is around 58 percent more expensive to run for a complex, real-world job. Samsung LSI partners with Anthropic's Claude Code on chip design-related tasks Meanwhile, Choi Ki-young, the CEO of Anthropic Korea, has just confirmed that "Samsung Electronics is testing the application of Claude in semiconductor design work." According to Choi, "While Samsung researchers have handled specialized design software manually, there have been no instances of automating such design work using AI. This is an area where Claude is virtually the only one capable of handling it." Of course, this statement is more of a hyperbole than a fact, especially as OpenAI has just deigned its very own ASIC, dubbed Jalapeno, with the help of its unreleased Astra model. At any rate, Samsung LSI is apparently using Claude Code to generate verification environments, prepare test scenarios, and help identify potential issues, but the model's output still requires human review. Follow Wccftech on Google to get more of our news coverage in your feeds.

Anthropic rolls out Claude Fable 5.1 and Mythos 5.1 with improved performance and safeguards

Anthropic has introduced Claude Fable 5.1 and Claude Mythos 5.1, based on the same underlying model with different safeguard levels. Fable 5.1 is for general use, while Mythos 5.1 is aimed at vetted cybersecurity and life sciences users. Claude Fable 5.1 Claude Fable 5.1 is designed for coding, knowledge work and long-running problem-solving. Anthropic says Low and Medium effort can match or exceed Fable 5 on some workloads while using less compute. It defaults to High effort in Claude Code and Medium effort in Claude Cowork and Claude.ai. Investment firm Millennium also used it to identify the cause of a rare internal system crash that had remained unexplained for several years. Scientific research Anthropic tested the models across molecular design, computational analysis and computational biology. Mythos 5.1 used open-source protein design and folding tools for molecular design. Results included: * 10x higher binding affinity than the best Adaptyv Bio competition designs on three targets. * Nearly 50% hit rate for viable binders across 12 targets. * Typical protein design hit rate of 10% to 15%. Fable 5.1 created a high-resolution elevation map covering about one-third of Venus using NASA Magellan radar images. It provides 2-3 km detail instead of 10-20 km, with heights measured up to 25% more accurately. Anthropic plans to release it under a Creative Commons license ahead of NASA's VERITAS and ESA's EnVision missions. Mythos 5.1 also created custom GPU kernels and cached intermediate results to accelerate seven open-source deep learning models by up to 2.5x, with identical outputs. The optimizations could reduce GPU costs by 30% to 60% and will be open-sourced. Scientific support includes: * Model Hardware Standard: Claude can operate laboratory equipment directly and safely. * AI for Science: Free credits for researchers working on high-impact projects. * Claude Team: Discounted usage for scientists. Safety and alignment Anthropic evaluated the models for chemical and biological risks, cybersecurity, agentic safety and alignment through automated evaluations, expert red-teaming and external testing. Mythos 5.1's chemical and biological capabilities are higher than Mythos 5 but remain below the next risk tier under the Responsible Scaling Policy. Cybersecurity testing with safeguards disabled found its highest measured cyber capability among Anthropic's released models, while remaining in the lower risk category of the Frontier Compliance Framework. Fable 5.1's cyber safeguards were tested internally, by two external organizations and by Gray Swan. No critical-severity jailbreak was found, consistent with Fable 5 and Opus 5. Mythos 5.1 refused malicious agentic coding and computer-use requests at rates comparable to Mythos 5, Sonnet 5 and Opus 5. Alignment testing covered behavioral evaluations, natural-language autoencoder analysis, misalignment tests, training-data reviews, internal use and external testing. Compared with Mythos 5, it was less likely to access resources outside its test environment, use motivated reasoning or ignore explicit constraints. Training-data reviews also found lower rates of attempted and successful reward hacking, although it can still bypass approvals and auto-mode classifiers. Safeguards and anti-distillation Enterprise Frontier Safeguards (EFS) provide privacy equivalent to zero data retention while allowing misuse detection. Customer data remains in customer-controlled cloud infrastructure, with human review handled by the customer by default. EFS was developed with more than 100 customers across financial services, healthcare, manufacturing, telecom, law, retail and the public sector, along with AWS, Google Cloud and Microsoft Azure. It supports: * Claude Code * Claude Enterprise * Claude Platform * Amazon Bedrock * Claude Platform on AWS * Google's Agent Platform * Microsoft Foundry Fable 5.1's biology safeguards trigger 85% less often for benign elementary biology and medical questions than those introduced with Fable 5. It can identify software vulnerabilities for defensive work, while penetration testing, exploit generation and binary-based vulnerability scanning remain redirected to Opus models. Life sciences R&D queries also continue to be directed to Opus models. Claude Code users see around 60% fewer cyber safeguard interventions per session than with previous Fable 5 safeguards. For anti-distillation, new API accounts created from today onwards cannot manually edit Claude's prior context in multi-turn conversations while retaining the transcript of Claude's prior thinking. The change is rolling out gradually, and existing accounts are not currently affected. Trusted access and EU AI Act Mythos 5.1 is offered through: * Cyber Verification Program (CVP): Reduced cyber safeguards for defensive security work, with Mythos-class models planned for inclusion. * Life Sciences Verification Program (LSVP): Mythos 5.1 access for professional life sciences R&D while retaining its other safeguards. LSVP was developed with the US government, with the first participants enrolled. Claude Security, which scans codebases for vulnerabilities and suggests patches for human review, is now powered by Mythos 5.1. In July 2026, Anthropic and 190 other signatories signed the EU AI Act's Code of Practice on Transparency of AI-Generated Content. Models released after August 2, 2026 must include a watermark estimating the likelihood Claude helped write text. The watermark is invisible without the detection API, has no practical impact on output quality or content, and contains no user, organization or conversation information. The API is in private preview for regulators, law enforcement, media, fact-checkers, independent researchers, educational organizations, EU civil society groups and enterprises with compliance requirements. Pricing and availability Claude Fable 5.1 is available today across Anthropic's platforms, including AWS, Google Cloud and Microsoft Azure. Developers can use claude-fable-5-1 through the Claude API. Cache-read pricing is 75% lower at $0.25 per million tokens. Typical workloads cost around 25% less than Fable 5, while complex coding and highly agentic workloads can save up to approximately 45%. Fable 5.1 pricing: * Input: $10 per million tokens * Output: $50 per million tokens Claude Mythos 5.1 is available to vetted cyberdefenders and life scientists through the trusted access programs. It is currently limited to a set of US organizations, with expansion to additional domestic and international partners planned.