{"id":721,"date":"2026-09-01T15:18:20","date_gmt":"2026-09-01T19:18:20","guid":{"rendered":"https:\/\/pratcom.net\/?p=721"},"modified":"2026-09-01T15:21:34","modified_gmt":"2026-09-01T19:21:34","slug":"claude-opus-5-1-vs-opus-5-differences-performance-and-costs","status":"publish","type":"post","link":"https:\/\/pratcom.net\/en\/claude-opus-5-1-vs-opus-5-differences-performance-and-costs\/","title":{"rendered":"Claude Opus 5.1 vs Opus 5: Differences, Performance and Costs"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Anthropic has just unveiled two new artificial intelligence models: Claude Fable 5.1 and Claude Mythos 5.1. These models represent a significant advance over the previous generation, particularly in coding, multidisciplinary reasoning, and scientific research. This article offers a comprehensive overview of the differences between Fable 5 and Fable 5.1, performance gains, pricing changes, and new safety measures.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Fable 5.1 and Mythos 5.1: two models, one architecture<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The first thing to understand is that Claude Fable 5.1 and Claude Mythos 5.1 are built on the same underlying model. The difference between the two lies not in their raw capabilities, but in the level of protection and guardrails applied to each.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Claude Fable 5.1 is available to the general public and businesses via Anthropic&#8217;s standard API.<\/li>\n\n\n\n<li>Claude Mythos 5.1 is reserved for <a href=\"https:\/\/pratcom.net\/en\/canada-and-american-ai-when-washington-cuts-access-to-fable-5-and-mythos-5-who-really-pays-the-price\/\">trusted partners<\/a>, particularly in the fields of cybersecurity and life sciences, through controlled access programs developed in partnership with the US government.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">This distinction is important: the performance differences observed between the two models on certain benchmarks do not reflect a difference in intrinsic capability, but rather the impact of older, less precise guardrails that sometimes incorrectly intervened on legitimate tasks. With the new safety measures introduced in Fable 5.1, this gap should narrow considerably.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">What are the concrete differences between Fable 5 and Fable 5.1?<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">Significantly superior performance<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Fable 5.1 sets a new standard on several reference benchmarks. Here are some key figures that illustrate the scale of progress made compared to Fable 5.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Terminal-Bench-Science 0.1 (agentic scientific research): Fable 5.1 reaches 52.6% compared to only 24.7% for Fable 5, more than double.<\/li>\n\n\n\n<li>Terminal-Bench 4.0 (agentic terminal coding): Fable 5.1 scores 55.8% compared to 42.0% for Fable 5. Mythos 5.1 goes even higher at 60.9%.<\/li>\n\n\n\n<li>Humanity&#8217;s Last Exam (advanced multidisciplinary reasoning): Fable 5.1 outperforms Fable 5 at all effort levels, with and without tools.<\/li>\n\n\n\n<li>CursorBench 3.2.0 (assisted coding): Fable 5.1 consistently outperforms Fable 5 across all effort levels tested.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">These results show that Fable 5.1 is not merely a marginal improvement. It represents a significant qualitative leap, particularly in long and complex tasks requiring autonomous problem-solving.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Deeper intelligence, not just faster<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Beyond the numbers, Fable 5.1 stands out for its ability to get to the root of problems rather than resorting to shortcuts. A concrete example illustrates this evolution well: during tests conducted by investment firm Millennium, Fable 5.1 successfully identified the cause of a rare crash in internal systems, a problem that neither the company&#8217;s engineers nor any other AI model had been able to explain after years of investigation.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This ability to diagnose complex problems deeply embedded in software systems represents a major advance for technical teams and companies using AI in their <a href=\"https:\/\/pratcom.net\/en\/context-engineering-for-claude-5-new-rules-for-smarter-agents\/\">development workflows<\/a>.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Effort levels: a new flexibility<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Fable 5.1 introduces several configurable effort levels (Low, Medium, High, Extra High, Max). This granularity allows the model&#8217;s behavior to be adapted according to the desired quality\/cost ratio. For example, a Fable 5.1 set to Low or Medium effort achieves results comparable to or better than Fable 5 at maximum effort, but at a much lower cost. By default, Fable 5.1 operates at High effort in Claude Code, and at Medium effort in Claude Cowork and on Claude.ai.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">A significant reduction in costs<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">One of the most anticipated points for professional users concerns pricing. Anthropic has listened to customer feedback and responds with concrete announcements.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Overall 25% reduction: for typical workloads billed by token consumption, Fable 5.1 will cost approximately 25% less than Fable 5. This decrease is primarily due to a reduction in cache read pricing, meaning when the model reuses previously processed and stored inputs.<\/li>\n\n\n\n<li>Greater savings for agentic tasks: for highly automated and autonomous workflows, savings can reach approximately 45%, as these use cases involve many repeated cache reads.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">This pricing strategy is particularly interesting for companies using Claude in complex pipelines, autonomous agents, or large-scale document processing systems.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">New privacy and security guarantees<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">Enterprise Frontier Safeguards: total confidentiality for businesses<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Anthropic is launching a new system called Enterprise Frontier Safeguards (EFS). This system allows enterprise customers to benefit from complete confidentiality, equivalent to a zero data retention policy, while maintaining a high level of protection against malicious use.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The key to this system lies in its architecture: data is stored in a cloud infrastructure controlled entirely by the customer, not by Anthropic. This directly addresses the concerns of large organizations that were hesitant to adopt AI models due to regulatory constraints or internal data privacy policies.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">EFS will be rolled out gradually to enterprise customers starting in the fall. In the meantime, eligible customers will be able to use Fable 5.1 with a zero data retention policy.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Fewer <a href=\"https:\/\/pratcom.net\/en\/why-the-us-forced-the-withdrawal-of-fable-5-and-mythos-5-what-we-know\/\">false positives in cybersecurity<\/a><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">A recurring problem with previous models was the high rate of false positives, meaning situations where the system blocked perfectly legitimate content or requests. With Fable 5.1, Anthropic announces a 60% reduction in false positives in the cybersecurity domain.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This improvement comes with an important nuance: Fable 5.1 can now be used to discover software vulnerabilities, but not to develop exploits to take advantage of them. This distinction opens up legitimate use cases for offensive and defensive security teams, while maintaining clear ethical boundaries.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Controlled access for advanced biology<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">For the advanced biological capabilities of Claude Mythos 5.1, Anthropic has established an access program developed in partnership with the US government. This program is intended for scientists working in legitimate and regulated contexts. Registration is expected to open soon.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Agentic scientific research: a glimpse of the future<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">One of the most promising dimensions of Fable 5.1 and Mythos 5.1 concerns their ability to contribute to scientific research autonomously. The scores achieved on the Terminal-Bench-Science 0.1 benchmark give a glimpse of what these models can accomplish in long and complex research tasks.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Fable 5.1 scores 52.6% on this benchmark, compared to 24.7% for Fable 5. It also outperforms its direct competitors such as GPT-5.6 Sol (22.4%) and Opus 5 (29.0%). These results hint at a new era where AI models no longer simply assist researchers, but actively participate in solving complex scientific problems.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">In summary: key takeaways<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Fable 5.1 and Mythos 5.1 share the same architecture, but with different levels of guardrails depending on the use case.<\/li>\n\n\n\n<li>Fable 5.1&#8217;s performance far surpasses that of Fable 5, particularly in agentic scientific research and advanced coding.<\/li>\n\n\n\n<li>Cost decreases by approximately 25% for typical use cases, and up to 45% for intensive agentic workflows.<\/li>\n\n\n\n<li>The new EFS system guarantees complete data confidentiality for businesses.<\/li>\n\n\n\n<li>False positives in cybersecurity are reduced by 60%, opening up new legitimate use cases.<\/li>\n\n\n\n<li>A controlled access program allows scientists to leverage the advanced biological capabilities of Mythos 5.1.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">With Fable 5.1, Anthropic takes a decisive step forward: not only is the model more powerful and less expensive, but it also meets the concrete expectations of businesses in terms of privacy and control. An update that deserves the attention of everyone integrating AI into their professional processes.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Discover everything you need to know about Claude Opus 5.1 and Opus 5.1: new capabilities, comparison with the previous version, cost reductions and security improvements.<\/p>\n","protected":false},"author":1,"featured_media":718,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[15],"tags":[],"class_list":["post-721","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-artificial-intelligence"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.1 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Claude Opus 5.1 vs Opus 5: Differences and Performance<\/title>\n<meta name=\"description\" content=\"Compare Claude Opus 5.1 and Opus 5: performance, costs, effort levels and new security measures. 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