These are the same underlying model, released together on September 1, 2026, at the same $10/$50 pricing with the same 1M-token context and the same 75% cache-read discount. The only differences are the safeguard level and who is allowed to use it. Claude Mythos 5.1 has safeguards lifted in high-risk cybersecurity and life-sciences areas and is restricted to vetted US organisations under Anthropic's trusted-access programme; Claude Fable 5.1 is generally available with standard safeguards. That safeguard gap shows up as a benchmark gap on tasks the safe-tuned model declines: Mythos 5.1 scores 60.9% on Terminal-Bench 4.0 against Fable 5.1's 55.8%. For everyone without trusted access — which is nearly everyone — Fable 5.1 is the model.
AnthropicPremium
Claude Mythos 5.1
Fable 5.1 with the safeguards lifted — partner-only, 60.9% Terminal-Bench 4.0.
VS
AnthropicPremium
Claude Fable 5.1
New frontier leader — better than Fable 5 on every published benchmark, and cheaper to run.
Winner
At a glance
Claude Mythos 5.1
Claude Fable 5.1
Input cost / 1M tokens
$$10.00/1M
$$10.00/1M
Output cost / 1M tokens
$$50.00/1M
$$50.00/1M
Context window
1M tokens
1M tokens
Speed
Deliberate
Deliberate
Price tier
Premium
Premium
How they compare
Which model wins for each use case — and why.
AvailabilityClaude Fable 5.1 wins
Fable 5.1 is generally available on the Claude API, Amazon Bedrock, Google Cloud and Microsoft Foundry. Mythos 5.1 requires vetted trusted access.
Raw ceilingClaude Mythos 5.1 wins
Mythos 5.1 scores 60.9% on Terminal-Bench 4.0 against Fable 5.1's 55.8% — the highest agentic-coding figure Anthropic has published.
Frontier security researchClaude Mythos 5.1 wins
Safeguards are lifted in areas where Fable 5.1 declines legitimate vulnerability and life-sciences research.
PriceTie
Both are $10 per 1M input and $50 per 1M output, with cache reads at $0.25 per 1M.
Context & output limitsTie
Identical: 1M-token context, 128K max output, adaptive thinking always enabled.
Everyday workClaude Fable 5.1 wins
For any task that does not trip a safeguard, the two behave the same — and only one of them is something you can sign up for.
Which should you pick?
Pick Claude Mythos 5.1 if…
You hold vetted trusted access under Anthropic's programme
Your cybersecurity or life-sciences research is being blocked by Fable 5.1's safeguards
You need the highest published agentic-coding ceiling and can clear the access review
For most workflows, Claude Fable 5.1 is the stronger choice.
The new frontier leader, and the rare upgrade that costs less than the model it replaces. Base $10/$50 pricing did not move, but a 75% cut to cache reads makes typical workloads ~25% cheaper than Fable 5 and heavily agentic ones ~45% cheaper — while more than doubling Fable 5 on agentic science and beating Opus 5 on agentic coding. If you were already on Fable 5, switching is a straight win. If you were on Opus 5 for the price, it still costs 2x on uncached tokens.
The case for each model
What each one is genuinely good at, where it falls down, and when we would steer you away from it.
The unrestricted twin of Claude Fable 5.1 — the same model with safeguards lifted in high-risk areas, released September 1, 2026 to vetted cybersecurity and life-sciences organisations under Anthropic's trusted-access programme. Scores 60.9% on Terminal-Bench 4.0, the highest agentic-coding result Anthropic has published.
Input
$10.00/1M
Output
$50.00/1M
Context
1M tokens
Speed
Deliberate
What people actually use it for
Vulnerability discovery and exploit reasoning under Anthropic's trusted-access controls
Life-sciences research workflows that the generally available safeguards constrain
Autonomous engineering where Fable 5.1's safety tuning blocks a legitimate response
Where it wins
60.9% on Terminal-Bench 4.0 — above Fable 5.1's 55.8% and the highest figure Anthropic has published
Same 1M-token context, 128K output, and $10/$50 base pricing as Fable 5.1
Inherits the same 75% cache-read reduction to $0.25 per 1M tokens
Safeguards lifted in areas where Fable 5.1 declines legitimate security and bio research work
Where it falls down
Not generally available — restricted to vetted US organisations under Project Glasswing
No advantage for ordinary work; Fable 5.1 is the same model at the same price for everyone else
Same deliberate, high-latency profile and premium pricing as Fable 5.1
Skip it if
You don't hold vetted trusted access, or your work never trips Fable 5.1's safeguards — the two models are otherwise identical.
Our verdict
The highest published agentic-coding score anywhere — 60.9% on Terminal-Bench 4.0 — but you almost certainly cannot get it. Mythos 5.1 is Fable 5.1 with safeguards lifted, gated to vetted cybersecurity and life-sciences organisations. Unless you hold trusted access and hit refusals that block real research, Fable 5.1 is the same model at the same price.
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.
Input
$10.00/1M
Output
$50.00/1M
Context
1M tokens
Speed
Deliberate
What people actually use it for
Agentic scientific research — 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5's 24.7%
Long-running autonomous coding agents that plan, run, and debug across a whole repository
Cache-heavy agent loops where the 75% cache-read cut ($1.00 → $0.25 per 1M) is the real saving
Where it wins
52.6% Terminal-Bench-Science 0.1 — 2.1x Fable 5 (24.7%), well clear of Opus 5 (29.0%) and GPT-5.6 Sol (22.4%)
55.8% Terminal-Bench 4.0 agentic coding, ahead of Opus 5 (52.3%) and Fable 5 (42.0%)
Cache reads cut 75% to $0.25/1M — ~25% cheaper for typical use, ~45% for heavily agentic work
Independent Vals AI evaluation ranks it #1 of 51 on the Vals Index, #1 on LiveCodeBench (90.5%) and MMLU Pro (92.4%)
1M-token context and 128K max output at standard rates, with adaptive thinking always enabled
Where it falls down
Base rates are still $10/$50 per 1M — double Claude Opus 5 for anything that is not cache-heavy
Anthropic published no SWE-bench Verified or SWE-bench Pro figure for 5.1 at launch
On Claude Pro it only runs on pay-as-you-go credits; Max includes it up to 50% of weekly limits
Deliberate, high-latency profile — the wrong choice for interactive, latency-bound apps
Skip it if
You need low latency, or your workload is uncached and cost-sensitive — Claude Opus 5 is half the base price and within a few points on agentic coding.
Our verdict
The new frontier leader, and the rare upgrade that costs less than the model it replaces. Base $10/$50 pricing did not move, but a 75% cut to cache reads makes typical workloads ~25% cheaper than Fable 5 and heavily agentic ones ~45% cheaper — while more than doubling Fable 5 on agentic science and beating Opus 5 on agentic coding. If you were already on Fable 5, switching is a straight win. If you were on Opus 5 for the price, it still costs 2x on uncached 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.
Frequently asked questions
What is the difference between Claude Mythos 5.1 and Fable 5.1?
They are the same underlying model at the same price. Fable 5.1 ships with standard safeguards and is generally available; Mythos 5.1 has those safeguards lifted in high-risk cybersecurity and life-sciences areas and is restricted to vetted US organisations under Anthropic's trusted-access programme.
Why does Mythos 5.1 score higher on Terminal-Bench 4.0?
60.9% against 55.8%. The gap comes from the safeguard level, not from different weights — some benchmark tasks touch material the generally available model is tuned to decline.
Can I get access to Claude Mythos 5.1?
Only through Anthropic's trusted-access programme, which vets US cybersecurity and life-sciences organisations. There is no self-serve path, and no pricing difference if you are approved.
Is Mythos 5.1 worth pursuing access for?
Only if Fable 5.1's safeguards are actually blocking legitimate work. For everything else the two are the same model at the same price, and Fable 5.1 is available today.