Anthropic Releases Fable 5.1 and Mythos 5.1 with 75% Cheaper Caching

On September 2, Anthropic released Claude Fable 5.1, the latest upgrade to its strongest model family, specialized in complex, long-running tasks and scientific research, alongside Claude Mythos 5.1, available only through trusted-access programs for cyber defense and life sciences. Both share the same underlying model with different safety tiers; Mythos adds enterprise-grade zero-retention protections. Anthropic also reset all users' 5-hour and weekly usage limits, disclosed changes to thinking-block management in the Messages API to prevent distillation, and published an official intro video.

Confirmed

• Pricing: input/output unchanged at $10/$50 per million tokens; Prompt Cache reads down 75% (to $0.25); typical workloads expected 25% cheaper, heavy-context agentic workflows up to 45%
• Benchmarks: Terminal-Bench-Science 0.1 at 52.6% (double the previous generation); kimmonismus reports big jumps on Terminal-Bench 4.0, Science-Bench and HLE; results posted by Anthropic engineer Felix Rieseberg show incremental gains, and Justwalkingthru3 argues the improvement is modest and OpenAI's upcoming Astra may soon surpass it
• Hands-on: WorldofAI demonstrated Fable 5.1 building a full game from a single prompt, creating Three.js experiences, and replicating complex physics engines, benchmarked against Claude Opus 5
• Value: kimmonismus notes Fable 5.1 High clearly beats Sol 5.6 Max on Cursor Bench at lower cost; official claims it beats Fable 5 at lower cost on lower effort tiers
• Contested costs: MineBench maintainer ENTAlam found total cost for 15 builds rose from $54.93 to $147.55 (nearly 3x) with doubled reasoning time despite unchanged API prices; WonderFactory showed a $3.69 single-task cost exceeding Fable 5; developer thispageleftblank, with cache reads at 78% of spend, estimates 57% total savings
• UX: a Reddit user (relayed by KrayeBaby) found multi-turn output more coherent than Opus 5 and Fable 5, while acknowledging remaining flaws

Why it matters

Fable 5.1 is a positioning move ahead of OpenAI's Astra; Wes Roth noted the timing, and kimmonismus calls it a strong release OpenAI must answer. The deep cache price cut reshapes costs for heavy users, but MineBench-style tests suggest the model may burn far more tokens through longer thinking—whether you actually save depends heavily on your workload.

2026-09-02 ~ 2026-09-03 · 174 related posts

Full story(3 episodes)→

• Episode 1: Claude Fable 5.1 Surfaces on Bedrock and in Official Docs, Launch Imminent(2026-09-01, 5 posts)
• Episode 2: Anthropic Releases Fable 5.1 and Mythos 5.1 with 75% Cheaper Caching(2026-09-02, 174 posts)
• Episode 3: Claude rate limits and caching controversies erupt after Fable 5.1 launch(2026-09-02, 7 posts)

• Anthropic releases Fable 5.1, updates API thought block handling — Icecream_monday · 2026-09-02
• Fable 5.1 model released with unchanged pricing — scaling01 · 2026-09-02
• Fable 5.1 now available in Claude API — Angaisb_ · 2026-09-02
• Fable 5.1 benchmarks released — scaling01 · 2026-09-02
• Claude Fable 5.1 Launches with Better Benchmarks and 75% Cheaper Cache — Anthropic · 2026-09-02
• Anthropic: Fable 5.1 excels at long-running tasks and early scientific research — claudeai · 2026-09-02
• Anthropic releases Claude 5.1: 25% cheaper with zero-retention enterprise safeguards — claudeai · 2026-09-02
• Claude Fable 5.1 Launches: High Usage Hints at Major Compute Jump — VraserX · 2026-09-02
• Anthropic Launches Claude Fable 5.1 and Mythos 5.1 Models — AI_Andrew · 2026-09-02
• Anthropic releases Fable 5.1 and Mythos 5.1 with major cost cuts — ClaudeOfficial · 2026-09-02
• Anthropic Fable 5.1 is now available in Claude Code v2.1 — BLUECOW009 · 2026-09-02
• Fable 5.1 benchmarks show insane jumps on Terminal/Science — kimmonismus · 2026-09-02
• Every's Vibe Check: Anthropic's Fable 5.1 Reclaims the Coding Crown — every · 2026-09-02
• Anthropic Is Back? Fable 5.1 Hands-On: Major Gains in Coding and Writing — every · 2026-09-02
• Claude Fable 5.1 Scores Leak: 77.9% on OSWorld, Beating GPT-5.6 Sol Across Benchmarks — Scobleizer · 2026-09-02
• Benchmarks for Claude Fable 5.1 and Claude Mythos 5.1 leaked — minxio_ · 2026-09-02
• Anthropic quietly launches Claude Fable 5.1 for "hardest challenges" — External-Grape-2978 · 2026-09-02
• Claude new model benchmark leak: 52.6% on TBS-Science — eyishazyer · 2026-09-02
• Fable 5.1 release: Doubles scientific agent scores, cuts costs by 25% — mark_k · 2026-09-02
• Fable 5.1 improves cybersecurity capabilities with significant reduction in fallbacks — ClaudeDevs · 2026-09-02
• Fable 5.1 Released: Amazing Progress in Agentic Scientific Research — Dr_Singularity · 2026-09-02
• Mazebench Launches to Stress-Test Fable 5.1 with Billions of Tokens — patience_cave · 2026-09-02
• Claude Code 2.1.257: Defaults to Fable 5.1, Hardens Auto Mode Security — ClaudeCodeLog · 2026-09-02
• Claude Code 2.1.257 Released: Defaults to Fable 5.1 — ClaudeCodeLog · 2026-09-02
• Claude Fable 5.1 Doubles Scores on Science Benchmarks — testingcatalog · 2026-09-02
• Claude Fable 5.1: Costs Drop 45%, Agent Benchmarks Double — kimmonismus · 2026-09-02
• Anthropic releases official introduction video for Claude Fable 5.1 — avilacjf · 2026-09-02
• Anthropic Launches Claude Fable 5.1 with Better Coding and Cheaper Cache — Scobleizer · 2026-09-02
• Anthropic's Fable 5.1 Leaks: 2x Faster, Half Cost — Yuchenj_UW · 2026-09-02
• Fable 5.1 scores 52.6% on Terminal-Bench-Science, doubling previous performance — Dr_Singularity · 2026-09-02
• Fable 5.1 beats Opus 5 in coding benchmarks — firstadopter · 2026-09-02
• Dev Calc: Fable 5.1 Price Cut Could Reduce Total Costs by 57% — _thispageleftblank · 2026-09-02
• Fable 5.1 Pricing Details: 75% Cheaper Cache — ThunderStorm420 · 2026-09-02
• FrontierFinance eval: Fable 5.1 leads with 55.9% score, 1.7x cost increase — maithra_raghu · 2026-09-02
• Fable 5.1 Review: More Powerful, Faster, and Half the Cost — danshipper · 2026-09-02
• Tips for Claude Fable 5.1: Low-Effort Mode and Cost Optimization — RLanceMartin · 2026-09-02
• User Feedback: Fable 5.1 Outputs are Clearer, No More Neuralese Loops — Justin_Halford_ · 2026-09-02
• Claude Fable 5.1 Launches: Doubles Science Benchmarks, Solves Data Retention — dotey · 2026-09-02
• Fable 5.1 Launches: Doubled Performance at Half the Token Cost — danshipper · 2026-09-02
• Anthropic Releases Fable 5.1 with Massive Gains — AGI Hunt · 2026-09-02
• Anthropic Launches Claude 5.1 Series: Focused on Coding and Knowledge Work — aziz4ai · 2026-09-02
• Fable 5.1 released: Agentic coding tasks solved at half the cost — airesearch12 · 2026-09-02
• Anthropic Releases Claude Fable 5.1 and Resets Usage Limits — thesaraharminta · 2026-09-02
• Anthropic Releases Claude Fable with Doubled Terminal-Bench Scores — BenBlaiszik · 2026-09-02
• Fable 5.1 launches with 75% cheaper cache reads — EricBuess · 2026-09-02
• Fable 5.1 cuts cache read pricing by 75% — EricBuess · 2026-09-02

4 near-duplicate retellings: scaling01 · TFenrir · Scobleizer · prasenx