--- title: Introducing Claude Fable 5.1 and Claude Mythos 5.1 description: Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, two versions of the same model with different safeguard levels, positioned as its most advanced... canonical: https://daily.dev/posts/pex5p6uxm twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Introducing Claude Fable 5.1 and Claude Mythos 5.1 | daily.dev og:description: Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, two versions of the same model with different safeguard levels, positioned as its most advanced... og:url: https://daily.dev/posts/pex5p6uxm og:image: https://api.daily.dev/og/posts/peX5p6uXM.png og:image:alt: Post cover image og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Introducing Claude Fable 5.1 and Claude Mythos 5.1 **[Devs Tech](https://daily.dev/sources/devstech)** · [@serdarbuyukdereli](https://daily.dev/serdarbuyukdereli) · 43 upvotes · 10 comments ## Summary Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, two versions of the same model with different safeguard levels, positioned as its most advanced models for coding, knowledge work, and scientific research. Fable 5.1 is generally available and costs roughly 25% less than Fable 5 for typical workloads (up to 45% less for agentic work), largely due to a 75% cut in cache-read pricing to $0.25 per million tokens. Mythos 5.1 is restricted to trusted access programs for cybersecurity and life sciences work. The release introduces Enterprise Frontier Safeguards (EFS) for customer-controlled data storage, improved cybersecurity safeguards that cut false positives by 60% and now allow vulnerability discovery (not exploit development), and new anti-distillation mechanisms that block editing Claude's prior thinking context for new API accounts created going forward. Benchmark results show gains over Fable 5, Opus 5, and GPT-5.6 Sol across coding, research, and reasoning tasks, alongside scientific applications like a new Venus elevation map and accelerated computational biology models. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Community discussion Top comments from developers on daily.dev. **@yaireo** · 6 upvotes > Still costs double for output-tokens compared to `Opus 5` which is already very good for everyday tasks, which is already expensive in itself, so I envy whoever can afford using Fable all day long :) good for them. My workplace will not be happy with the bill. **@bits\_and\_bytes** · 1 upvotes > using Fable 5.1, its overhyped and consuming more tokens. **@agustinbarrientos** · 1 upvotes > Try replaying a saved agent session through an account that blocks edits to earlier thinking context. That exposes integrations still rewriting those blocks before a future model applies the rule to existing accounts. **@petecapecod** · 1 upvotes > I knew something was up when they hit the reset midday/midweek yesterday. > > Can't wait to give the new Fable a go! ## Similar posts on daily.dev - [Don’t just attend KubeCon \+ CloudNativeCon, Merge Forward your experience\!](https://daily.dev/posts/don-t-just-attend-kubecon-cloudnativecon-merge-forward-your-experience--l0rpp73x8) · CNCF · 1 upvotes · 0 comments - [Announcing H2 2026 KCDs](https://daily.dev/posts/announcing-h2-2026-kcds-m96goajm1) · CNCF · 1 upvotes · 0 comments - [Two months of Open Community Groups](https://daily.dev/posts/two-months-of-open-community-groups-asf52zhbs) · CNCF · 0 upvotes · 0 comments --- Tags: [#llm](https://daily.dev/tags/llm), [#ai-agents](https://daily.dev/tags/ai-agents), [#claude](https://daily.dev/tags/claude), [#anthropic](https://daily.dev/tags/anthropic), [#ai-safety](https://daily.dev/tags/ai-safety) [View this post on daily.dev](https://daily.dev/posts/pex5p6uxm) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"DiscussionForumPosting","mainEntityOfPage":"https://daily.dev/posts/pex5p6uxm","headline":"Introducing Claude Fable 5.1 and Claude Mythos 5.1","text":"Shared: Introducing Claude Fable 5.1 and Claude Mythos 5.1","url":"https://daily.dev/posts/pex5p6uxm","datePublished":"2026-09-01T18:15:50.853Z","dateModified":"2026-09-01T18:18:46.226Z","author":{"@type":"Person","name":"Serdarcan Buyukdereli","url":"https://daily.dev/serdarbuyukdereli","image":"https://media.daily.dev/image/upload/s--tTV8hAPq--/f_auto/v1778701721/avatars/avatar_Su5HqluAE4wLRb1naHjtv?_a=BAMAMiWQ0","description":"Senior Devops and Cloud Engineer\n","interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"EndorseAction"},"userInteractionCount":21370}},"image":"https://media.daily.dev/image/upload/s--HRgLpUt6--/f_auto/v1722860399/public/Placeholder%2003","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":42},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":10}],"sharedContent":{"@type":"WebPage","url":"https://api.daily.dev/r/CYYPiH7N6"},"comment":[{"@type":"Comment","text":"Still costs double for output-tokens compared to Opus 5 which is already very good for everyday tasks, which is already expensive in itself, so I envy whoever can afford using Fable all day long :) good for them. My workplace will not be happy with the bill.","datePublished":"2026-09-02T10:09:48.289Z","url":"https://daily.dev/posts/peX5p6uXM#c-Ipg7RfZ4N","author":{"@type":"Person","name":"Yair Even Or","url":"https://daily.dev/yaireo","image":"https://avatars.githubusercontent.com/u/845031?v=4"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":6}},{"@type":"Comment","text":"using Fable 5.1, its overhyped and consuming more tokens.","datePublished":"2026-09-03T04:13:02.656Z","url":"https://daily.dev/posts/peX5p6uXM#c-Y4CjYmy2k","author":{"@type":"Person","name":"bits_007","url":"https://daily.dev/bits_and_bytes","image":"https://avatars.githubusercontent.com/u/182549945?v=4"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":1}},{"@type":"Comment","text":"Try replaying a saved agent session through an account that blocks edits to earlier thinking context. That exposes integrations still rewriting those blocks before a future model applies the rule to existing accounts.","datePublished":"2026-09-04T17:52:03.618Z","url":"https://daily.dev/posts/peX5p6uXM#c-zLbMi7YSH","author":{"@type":"Person","name":"Agustin Barrientos","url":"https://daily.dev/agustinbarrientos","image":"https://media.daily.dev/image/upload/s--5ayxQnqn--/f_auto/v1788281802/avatars/avatar_wQYYVe5Tbj0NJ7C7qPoa8?_a=BAMAMicg0"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":1}},{"@type":"Comment","text":"I knew something was up when they hit the reset midday/midweek yesterday.\nCan’t wait to give the new Fable a go!","datePublished":"2026-09-02T12:32:27.991Z","url":"https://daily.dev/posts/peX5p6uXM#c-P3KPWD0oD","author":{"@type":"Person","name":"Peter Cruckshank","url":"https://daily.dev/petecapecod","image":"https://media.daily.dev/image/upload/s--ZJhQyKws--/f_auto/v1721235024/avatars/avatar_A9xh33q0QoxtkGoJRCosp"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":1}}],"isPartOf":{"@type":"WebPage","url":"https://daily.dev/squads/devstech","name":"Devs Tech"}} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Devs Tech","item":"https://daily.dev/squads/devstech"},{"@type":"ListItem","position":3,"name":"Introducing Claude Fable 5.1 and Claude Mythos 5.1"}]} ```