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.

Claude Fable 5 vs Opus 4.8: Benchmarks, Pricing & When to Use Each

Anthropic now offers two frontier tiers: Mythos-class Fable 5 and Opus-class Opus 4.8. They are closely linked — Fable's safeguards fall back to Opus — but priced and positioned very differently. Short answer: Fable 5 for hard, long, autonomous jobs; Opus 4.8 as the cost-and-latency default.

At a glance

Class: Mythos vs Opus
Best for: long-running autonomous work vs strong everyday frontier work
SWE-Bench Pro: 80.3% vs 69.2%
FrontierCode: 29.3% vs 13.4%
Input/output: $10/$50 vs $5/$25 per million tokens
Context: 1M tokens (Fable) · API ID: claude-fable-5 vs claude-opus-4-8

How big is the gap?

The longer and more complex the task, the larger Fable 5's lead. On short, well-scoped work the two are much closer — why Opus 4.8 remains sensible for high-volume production traffic.

Pricing nuances

Fable is ~2× Opus on all token types including cache writes and hits
Token efficiency: Fable often uses fewer turns — net cost gap can shrink on the right tasks
Safeguard reroutes to Opus are not billed at Fable rates

The fallback relationship

When Fable classifiers flag cyber, bio/chem, or distillation content, Opus 4.8 answers instead (under 5% of sessions). API customers configure this via the Fallback API. Treat the pair as a system, not either/or.

When to use which

Reach for Fable 5 when

Tasks are long-running or multi-stage (migrations, multi-day agents)
Quality on hard problems matters more than per-token cost
Opus has plateaued on complex analysis or high-fidelity coding

Stick with Opus 4.8 when

Tasks are routine and well-scoped
Latency or cost per request is the priority
High volume where 2× pricing compounds quickly

Best pattern: route by task complexity behind one gateway endpoint — hard jobs to Fable, everything else to Opus — with budgets and automatic fallback on errors or capacity limits.