What is Claude Fable 5 and when was it released?

Claude Fable 5 is Anthropic's most capable publicly available AI model, released on June 9, 2026. It is the first model from Anthropic's "Mythos-class" tier to be made generally available to developers and enterprises. The model is designed for long-horizon agentic work, complex software engineering, and demanding knowledge tasks that previous models could not sustain over extended periods. The name "Fable" comes from the Latin fabula, meaning "that which is told" — a deliberate linguistic cousin of the Greek mythos, reflecting the model's shared intelligence with the restricted Mythos 5 model.

What is the difference between Claude Fable 5 and Claude Mythos 5?

Fable 5 and Mythos 5 share the exact same underlying neural network weights and core intelligence. The difference is entirely based on access control and safety guardrails. Fable 5 is the publicly available version with dynamic safety classifiers that block high-risk queries (cybersecurity exploits, biological hazards, model distillation) and route them to Claude Opus 4.8. Mythos 5 is a restricted model with some of those safety classifiers disabled. It is only available to vetted partners through Project Glasswing — primarily US government cyber-defenders and approved biology researchers. For all standard enterprise and developer tasks, Fable 5 performs effectively identically to Mythos 5.

How much does Claude Fable 5 cost? What is the API pricing?

Claude Fable 5 is priced at $10 per million input tokens and $50 per million output tokens. This is exactly double the cost of Claude Opus 4.8 ($5/$25). A 90% prompt caching discount applies to cached input tokens, reducing the effective input cost to $1/M on cache hits. For US-only inference (data residency requirements), pricing is available at a 1.1x multiplier. On Claude.ai subscription plans, Fable 5 counts as 2x usage. Anthropic also offered a free promotional window for Pro, Max, Team, and Enterprise subscribers from June 9–22, 2026.

Is Claude Fable 5 better than GPT-5.5?

Based on Anthropic's published benchmarks and extensive third-party testing, Claude Fable 5 significantly outperforms GPT-5.5 on complex reasoning and software engineering tasks. On SWE-Bench Pro, Fable 5 scored 80.3% compared to GPT-5.5's 58.6% — a gap of over 21 percentage points. On FrontierCode Diamond, Fable 5 scored 29.3% versus GPT-5.5's 5.7%. GPT-5.5 remains competitive in specific niches: it performs well on Terminal-Bench 2.1 via its Codex CLI harness (83.4%) and is competitive on spatial reasoning (Blueprint-Bench 2: 36.2% vs Fable 5's 38.6%). GPT-5.5 also offers a lower price point ($5/$30 vs $10/$50). For long-horizon autonomous coding and complex knowledge work, however, Fable 5 is the clear leader.

What is Claude Fable 5's context window size?

Claude Fable 5 supports a 1,000,000 token (1M token) context window by default. It also supports generating up to 128,000 output tokens per single request. This massive context window enables Fable 5 to process entire codebases, lengthy legal documents, or extensive research papers in a single session without losing context.

Why did my Fable 5 request get answered by Opus 4.8?

This is the intended behavior of Fable 5's safeguard system. If your prompt contained keywords or concepts related to offensive cybersecurity (e.g., building exploits or malware), creating biological hazards, or model distillation, Fable 5's safety classifiers intercepted the request and automatically routed it to Claude Opus 4.8. Importantly, you are not charged Fable 5 rates when this fallback occurs — you only pay for the Opus 4.8 usage. The API will return a successful HTTP 200 response with stop_reason: "refusal" in the metadata to indicate the fallback occurred. Anthropic reports this happens in fewer than 5% of average sessions.

When should I use Fable 5 instead of Opus 4.8?

You should reach for Claude Fable 5 when dealing with tasks that require sustained autonomy, complex multi-step reasoning, or massive code refactoring. Ideal use cases include: large-scale codebase migrations, multi-day autonomous coding sessions, complex financial document analysis, multi-stage research synthesis, and any task where the AI needs to plan, execute, and self-verify over an extended period. You should stick with Claude Opus 4.8 for routine, high-volume, or latency-sensitive work: classification, summarization, drafting, interactive chat, and most day-to-day agentic tasks. At half the price, Opus 4.8 is the sensible default for the majority of enterprise workloads. The optimal strategy for most organizations is intelligent routing — using Fable 5 for the hardest 20% of tasks and Opus 4.8 for the rest.

Where can I access Claude Fable 5?

Claude Fable 5 is available across multiple platforms as of June 9, 2026. For developers, it is accessible via the Claude API directly (model string: claude-fable-5), as well as through Amazon Bedrock, Google Cloud Vertex AI, and Microsoft Foundry. For enterprise customers, it is available on the consumption-based Enterprise plan. For individual users, Fable 5 is accessible through the Claude.ai web interface and mobile apps on paid plans (Pro, Max, Team, and Enterprise). Note that Fable 5 counts as 2x usage on subscription plans due to its higher compute requirements.

Anthropic Brings Mythos to the Masses with Claude Fable 5

Anthropic launched Claude Fable 5 and Claude Mythos 5 on June 9, 2026 — its first broad release of Mythos-class capabilities previously limited to Project Glasswing cybersecurity partners. Fable 5 is the generally available version with new safeguards; Mythos 5 keeps fewer restrictions but remains limited to approved users.

First broad Mythos-class release

VentureBeat reports Fable 5 exceeds every Claude model Anthropic has previously made generally available, with stronger performance across software engineering, knowledge work, vision, scientific research, and long-running tasks. It tops nearly all shared launch benchmarks, though Mythos Preview still leads on computer use and multidisciplinary reasoning in Anthropic's chart.

Safeguards and fallback routing

Fable 5 wraps the same underlying Mythos-class capability in new safeguards. Requests involving high-risk areas — cybersecurity, biology and chemistry, and model distillation — are automatically routed to Claude Opus 4.8 with user notification. Mythos 5 lifts some restrictions for trusted Glasswing and biology-research partners.

Anthropic says more than 95% of Fable 5 sessions run entirely on Fable's own responses. Internal and external red-teaming found no universal jailbreaks after more than 1,000 hours of testing.

Pricing, access and subscription rollout

API pricing: $10 per million input tokens and $50 per million output tokens for both Fable 5 and Mythos 5 — less than half Mythos Preview pricing but the most expensive major frontier model globally
Developers: Fable 5 available today via Claude API as claude-fable-5 and on consumption-based Enterprise plans
Subscriptions: included at no extra cost on Pro, Max, Team, and seat-based Enterprise through June 22, 2026; from June 23 usage credits required until restored as a standard plan feature

Benchmark leadership

On SWE-bench Pro, Fable 5 and Mythos 5 reach 80.3% versus GPT-5.5 at 58.6%. On Cognition's FrontierCode Diamond benchmark for maintainable agentic coding, they score 29.3% compared with 13.4% for Opus 4.8 and 5.7% for GPT-5.5. Fable 5 also leads frontier models on FrontierCode at medium reasoning effort.

Autonomous coding in production

Stripe tested Fable 5 in a 50-million-line Ruby codebase and completed a codebase-wide migration in one day that would have taken more than two months by hand. Cursor, Replit, Figma, Base44, Genspark, and Rakuten praised long-horizon development, one-shot full apps, UI design, game coding, and self-validating autonomous operations.

Enterprise knowledge work

On GDPval-AA, Fable 5 and Mythos 5 score 1932 versus 1890 for Opus 4.8, 1769 for GPT-5.5, and 1314 for Gemini 3.1 Pro. On GDPpdf for visual document reasoning, they reach 29.8% without tools — gains that matter for PDFs, spreadsheets, contracts, filings, and slide decks in corporate workflows.

Safety testing and data retention

VentureBeat notes Anthropic's strategy for dual-use frontier models: route some high-risk requests to a less capable model while keeping the stronger model for everyday work. Fable 5 and Mythos 5 require 30-day data retention on all traffic — even for enterprises with prior zero-retention agreements — to defend against novel jailbreaks, though data is not used for training.