Anthropic Releases Claude Fable 5.1 and Mythos 5.1 with Big Bench Gains and 75% Cache Price Cut

On September 2, Anthropic officially released Claude Fable 5.1 and Claude Mythos 5.1. Fable 5.1 is an upgraded version of the current most capable model, specializing in complex, long-horizon tasks and scientific research; Mythos 5.1 shares the same underlying model but is available only through a trusted-access program, targeting cybersecurity defense and life sciences. The new models deliver significantly better benchmark scores and much lower caching costs, and ship as the default with Claude Code 2.1.257.

Confirmed

• Performance: scores 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5; tops Terminal-Bench overall; at lower effort levels it already exceeds Fable 5's results.
• Pricing: input/output remain $10/$50 per million tokens; Prompt Cache read cost drops from $1 to $0.25, a 75% cut. Both official statements and summaries by @kimmonismus and @markk confirm typical workloads cost about 25% less, while heavily cached workloads see roughly 45% savings (@drcintas reported 25%–45%).
• Availability: live on the Claude API and apps; Claude Code v2.1.257 makes it the default model, supports 1 million context, and strengthens auto-mode safety.
• Accompanying policy: the launch also reset all users' 5-hour and weekly usage limits (per @thesaraharminta).
• Privacy and security: introduces enterprise-grade zero-retention protections and other privacy/security upgrades (@claudeai).

Timeline

• Earlier on launch day, Reddit users including @External-Grape-2978 noticed the model quietly appear in the app with no official announcement or model card; the Bedrock identifier initially showed as not found. Anthropic then officially announced the release.

Why it matters

• The 75% cache price cut matters most for long-context, reuse-heavy workloads like Coding Agents; @markk called it the most underrated improvement, significantly lowering coding costs.
• Releasing the same model in "general" and "trusted" tiers reflects Anthropic's differentiated safety-governance strategy for high-risk industries (cyber defense, life sciences).

2026-09-02 ~ 2026-09-02 · 70 related posts

Full story(4 episodes)→

• Episode 1: Claude Fable 5.1 Spotted on Bedrock and in Docs Ahead of Imminent Launch(2026-09-01, 5 posts)
• Episode 2: Anthropic Releases Claude Fable 5.1 and Mythos 5.1 with Big Bench Gains and 75% Cache Price Cut(2026-09-02, 70 posts)
• Episode 3: Fable 5.1 Tops Benchmarks but Sparks Cost Controversy(2026-09-02, 21 posts)
• Episode 4: Every's Hands-on: Anthropic Fable 5.1 Reclaims the Coding Crown at Half the Cost(2026-09-02, 8 posts)

• 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
• [source] Claude Fable 5.1 Launches with Better Benchmarks and 75% Cheaper Cache — Anthropic · 2026-09-02
• Anthropic releases Claude 5.1: 25% cheaper with zero-retention enterprise safeguards — claudeai · 2026-09-02
• Anthropic Launches Claude Fable 5.1 and Mythos 5.1 Models — AI_Andrew · 2026-09-02
• [source] 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
• Anthropic releases Claude 5.1 for coding and knowledge work — kieranklaassen · 2026-09-02
• Claude Fable 5.1 Scores Leak: 77.9% on OSWorld, Beating GPT-5.6 Sol Across Benchmarks — Scobleizer · 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
• [source] Fable 5.1 release: Doubles scientific agent scores, cuts costs by 25% — mark_k · 2026-09-02
• Fable 5.1 Released: Amazing Progress in Agentic Scientific Research — Dr_Singularity · 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
• Fable 5.1 Pricing Details: 75% Cheaper Cache — ThunderStorm420 · 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
• 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
• Fable 5.1 Tops FrontierSWE v2 Coding Benchmark — aryaman2020 · 2026-09-02
• Claude 5.1 System Card: Faked Permissions, Sneaky Behavior, and Self-Awareness — flowersslop · 2026-09-02
• Claude Fable 5.1 Cuts Cache Costs by 75%, Boosting Coding Agent Efficiency — mark_k · 2026-09-02
• Fable 5.1 Launches: More Precise Code Reviews, Slower Speed — leslysandra · 2026-09-02
• Claude Fable 5.1 cuts cache read costs by 75% — rohanpaul_ai · 2026-09-02
• Fable 5.1 takes 1st on Artificial Analysis with a score of 66 — Anxious-Yoghurt-9207 · 2026-09-02
• Anthropic launches Claude Fable 5.1: 2x jump on Terminal-Bench-Science, 25% cheaper — heathercmiller · 2026-09-02
• Fable 5.1 makes a bigger jump than it looks on the Artificial Analysis Index — cedric_chee · 2026-09-02
• Anthropic releases Claude Fable 5.1 with stronger long-horizon performance at lower cost — Anthropic · 2026-09-02
• Fable 5.1 Scores 66, Beats Previous SOTA by 3 Points — inductionheads · 2026-09-02
• Fable 5.1's real gains are bigger than its Artificial Analysis index jump suggests — cedric_chee · 2026-09-02

6 near-duplicate retellings: scaling01 · TFenrir · Scobleizer · prasenx · aziz4ai · dr_cintas