--- title: Claude Fable 5.1: cheaper caching, three breaking changes description: Anthropic released Claude Fable 5.1 on September 1, cutting cache-read pricing to $0.25 per million tokens (75% cheaper than Fable 5), while base input/output... canonical: https://daily.dev/posts/orprjlyxt twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Claude Fable 5.1: cheaper caching, three breaking changes | daily.dev og:description: Anthropic released Claude Fable 5.1 on September 1, cutting cache-read pricing to $0.25 per million tokens (75% cheaper than Fable 5), while base input/output... og:url: https://daily.dev/posts/orprjlyxt og:image: https://api.daily.dev/og/posts/oRpRjlyXT.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. # Claude Fable 5.1: cheaper caching, three breaking changes **[Rajeev R. Sharma](https://daily.dev/sources/vcvyeh9h4cczsbsvu21b0)** · [@ra_jeeves](https://daily.dev/ra_jeeves) · 1 upvotes · 0 comments ## Summary Anthropic released Claude Fable 5.1 on September 1, cutting cache-read pricing to $0.25 per million tokens (75% cheaper than Fable 5), while base input/output pricing stays at $10/$50 per million tokens. Anthropic estimates 25% typical savings and up to ~45% for highly agentic workloads, though actual savings depend heavily on how an agent uses context. Migration requires handling two access blockers (30-day data retention requirement, no Priority Tier support) plus three breaking changes: forced tool_choice (any/specific tool) now returns a 400 error, thinking blocks can't move backward to earlier Claude models without a beta header, and editing prior messages before a thinking block can trigger a 400 error on newer accounts. Claude Code needs version 2.1.250+ and offers a migration command. Cyber safeguards also diverge between Fable and the restricted Mythos 5.1 tier, with mixed community reports on how safeguard triggers behave in practice. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Governing Security in the Age of Infinite Signal](https://daily.dev/posts/governing-security-in-the-age-of-infinite-signal-qhihbir44) · Snyk · 0 upvotes · 0 comments - [No one has a good plan for how AI companies should work with the government](https://daily.dev/posts/no-one-has-a-good-plan-for-how-ai-companies-should-work-with-the-government-nkajqoze1) · TechCrunch · 0 upvotes · 1 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-inference](https://daily.dev/tags/ai-inference) [View this post on daily.dev](https://daily.dev/posts/orprjlyxt) ```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/orprjlyxt","headline":"Claude Fable 5.1: cheaper caching, three breaking changes","text":"Shared: Claude Fable 5.1: cheaper caching, three breaking changes","url":"https://daily.dev/posts/orprjlyxt","datePublished":"2026-09-02T09:15:16.392Z","dateModified":"2026-09-02T09:16:27.132Z","author":{"@type":"Person","name":"Rajeev R. 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