Fable 5.1 with the safeguards lifted — partner-only, 60.9% Terminal-Bench 4.0.
100
Coding
97
Writing
99
Research
89
Images
16
Value
98
Long Context
Use this when
Vetted frontier cybersecurity and life-sciences research at the absolute capability ceiling
Skip this if
You don't hold vetted trusted access, or your work never trips Fable 5.1's safeguards — the two models are otherwise identical.
Pricing
$10.00/1M in
$50.00/1M out
Context
1M tokens
Speed
Deliberate
Released September 1, 2026 alongside Claude Fable 5.1, continuing the Mythos-class line that began with Project Glasswing's April 2026 private preview. Access is limited to vetted US cybersecurity and life-sciences professionals. Same underlying model, pricing, context window, and cache-read discount as Fable 5.1; the only published difference is the safeguard level and the resulting Terminal-Bench 4.0 score (60.9% vs 55.8%).
Anthropic's September 1, 2026 frontier release and the new capability ceiling for coding, agents, and scientific work. Base pricing is unchanged at $10/$50, but cache reads dropped 75% to $0.25/1M — roughly 25% cheaper on typical workloads and up to 45% cheaper on agentic ones. 1M context, 128K output, adaptive thinking always on.
Verdict
New frontier leader — better than Fable 5 on every published benchmark, and cheaper to run.
Quality score
98%
Pricing
$10.00/1M in
$50.00/1M out
Speed
Deliberate
2/5 speed
Context
1M tokens
Released September 1, 2026 alongside Claude Mythos 5.1, the first update to the Mythos-class line since Fable 5 on June 9. API ID claude-fable-5-1; generally available on the Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry. Published launch numbers (Fable 5.1 / Fable 5 / Opus 5 / GPT-5.6 Sol): Terminal-Bench-Science 0.1 52.6 / 24.7 / 29.0 / 22.4; Terminal-Bench 4.0 55.8 / 42.0 / 52.3 / 37.3; CursorBench 3.2.0 73.4 / 70.5 / 70.0 / 67.2; AutomationBench 31.4 / 17.1 / 26.9 / 19.6; OSWorld 2.0 strict 41.7 / 36.1 / 39.6; Humanity's Last Exam (no tools) 60.9 / 57.8 / 56.6; GDPval-AA v2 1853 / 1723 / 1824 / 1711. GDPval-AA v2 is rescaled from the v1 numbers quoted on the Fable 5 page and is not directly comparable to them.
Anthropic's new Mythos-class flagship and the most capable coding model anyone can use — 80.3% SWE-Bench Pro, an 11-point jump over Opus 4.8. 1M context, 128K output, native parallel subagents. Released June 9, 2026.
Verdict
Superseded by Fable 5.1 — still 80.3% SWE-Bench Pro, but 5.1 is better and cheaper to run.
Quality score
98%
Pricing
$10.00/1M in
$50.00/1M out
Speed
Deliberate
2/5 speed
Context
1M tokens
Launched June 9, 2026 as the public, Mythos-class release. Available on the Claude API, Microsoft Foundry, and Google Vertex AI. Free for all users until June 22, 2026. Same underlying model as Claude Mythos 5, with safeguards that block specific high-risk cyber responses. Superseded by Claude Fable 5.1 on September 1, 2026 — same $10/$50 base pricing, cache reads cut from $1.00 to $0.25 per 1M tokens.
SWE-Bench Pro 80.3%Mythos-classParallel subagentsAgenticLong contextPremiumSuperseded
Best for
The hardest coding tasks, autonomous multi-step agents, and frontier-grade reasoning
Anthropic's most powerful frontier model — the same underlying model as Fable 5 with safeguards lifted in some areas, restricted to vetted enterprise and research partners. The capability ceiling of mid-2026.
Verdict
Superseded by Mythos 5.1 — same model as Fable 5, safeguards lifted, partner-only.
Quality score
98%
Pricing
$10.00/1M in
$50.00/1M out
Speed
Deliberate
2/5 speed
Context
1M tokens
Launched June 9, 2026 alongside Fable 5, following the April Project Glasswing private preview on Google Cloud. Restricted to vetted enterprise and research partners due to advanced cybersecurity capabilities. Same underlying model and benchmarks as Claude Fable 5. Superseded by Claude Mythos 5.1 on September 1, 2026.
FrontierRestricted accessCybersecuritySWE-Bench Pro 80.3%Mythos-classPremiumSuperseded
Best for
Frontier cybersecurity research, autonomous vulnerability discovery, and the absolute capability ceiling
Claude Mythos 5.1 is best for vetted frontier cybersecurity and life-sciences research at the absolute capability ceiling. It is a strong fit when that workflow matters more than the tradeoffs around premium pricing and deliberate speed.
When should I avoid Claude Mythos 5.1?
You don't hold vetted trusted access, or your work never trips Fable 5.1's safeguards — the two models are otherwise identical.
What is a cheaper alternative to Claude Mythos 5.1?
Grok 4.5 is the lower-cost option to compare first when you want a similar workflow fit with less token spend.
What is a faster alternative to Claude Mythos 5.1?
Claude Fable 5.1 is the better pick when response time matters more than maximum depth or premium quality.
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