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 & Mythos 5 — Developer Documentation Guide

Anthropic's platform documentation for Claude Fable 5 and Claude Mythos 5 describes two Mythos-class models sharing the same capabilities: Fable 5 is the generally available model with safety classifiers; Mythos 5 is the restricted release without those classifiers, available through Project Glasswing.

Models and API IDs

Claude Fable 5 — API model ID: claude-fable-5. Anthropic's most capable widely released model for demanding reasoning and long-horizon agentic work.
Claude Mythos 5 — API model ID: claude-mythos-5. Same capabilities without safety classifiers; limited release via Project Glasswing. Successor to Claude Mythos Preview.

Both models support a 1M token context window by default and up to 128k output tokens per request. Pricing is $10 per million input tokens and $50 per million output tokens.

Refusals, fallback, and billing

Refusals on Claude Fable 5

When Claude Fable 5 declines a request, the Messages API returns stop_reason: "refusal" as a successful HTTP 200 response — not an error. The response also reports which classifier declined the request.

Fallback

A refused Fable 5 request can usually be served by another Claude model. Pass the fallbacks parameter for server-side retry (beta on Claude API and Claude Platform on AWS), or use SDK middleware for client-side retry on any platform.

Billing

You are not billed for requests refused before any output is generated. When retrying on another model, fallback credit refunds the prompt-cache cost of switching.

Availability

Both models became available June 9, 2026. Claude Fable 5 is GA on the Claude API, Claude Platform on AWS, Amazon Bedrock, Vertex AI, and Microsoft Foundry. Claude Mythos 5 is limited availability for approved Project Glasswing customers.

Fable 5 and Mythos 5 are Covered Models with 30-day data retention and are not available under zero data retention.

Messages API behavior

These behaviors are specific to Fable 5 and Mythos 5; the Messages API is unchanged for Opus, Sonnet, and Haiku.

Adaptive thinking is always on

Adaptive thinking is the only thinking mode on Fable 5 and Mythos 5. It applies when the thinking parameter is unset; thinking: {"type": "disabled"} is not supported. Use the effort parameter to control thinking depth.

Raw thinking content is never returned

Raw chain-of-thought is never returned. thinking.display defaults to "omitted" (empty thinking field). Set display: "summarized" for readable summarized thinking. Pass thinking blocks back unchanged in multi-turn conversations on the same model.

Supported features at launch

Effort — control thinking depth and cost
Task budgets (beta: task-budgets-2026-03-13 header)
Memory tool
Tool result clearing via context editing (beta: context-management-2025-06-27 header)
Compaction
Vision

Migrating from earlier models

Anthropic publishes dedicated migration guides for Claude Mythos Preview → Claude Mythos 5 and Claude Opus 4.8 → Claude Fable 5, covering model string changes, thinking behavior, and fallback configuration.