Claude Fable 5 and Claude Mythos 5 are the first models in Anthropic's Claude 5 generation, launched on June 9, 2026. Both belong to a new top tier Anthropic calls “Mythos-class” and share the same base model. The biggest difference is availability: Fable 5 is generally available, while Mythos 5 is offered only in limited release.
Update (September 2, 2026): Anthropic released Claude Fable 5.1 on September 1, 2026, and the official model docs now list Fable 5 as legacy. Pricing ($10/$50) and plan terms are unchanged; cache reads got cheaper. The article below describes Fable 5 — see the Claude Fable 5.1 guide for what changed.
🟢 Claude Fable 5 · model notice · Fable subscription
Fable 5 and 5.1 subscription (updated September 7, 2026): Claude Fable 5.1, released September 1, 2026, is the current Fable model and Fable 5 is now legacy. Plan terms are the same for both — Max and Team Premium plans include Fable at up to 50% of the weekly usage limit; Pro and Team Standard use usage credits (
Update (September 27, 2026): Claude Opus 5.5 was released on September 22, 2026 and is now the current Opus model (API claude-opus-5-5, $4 per million input tokens and $20 per million output tokens, down from Opus 5's $5/$25). Anthropic says it performs at the level of Fable 5.1 on most work and costs 40% less to run than Opus 5 on typical workloads. Opus 5 remains available but is marked legacy in the official model docs. Opus 5 prices and comparisons below reflect the pre-launch state and will be updated. Details: Claude Opus 5.5 complete guide.
Anthropic first revealed Claude Mythos Preview in April but limited its rollout because its strong ability to find software vulnerabilities posed misuse risks. Fable 5 is the first model to open this Mythos-class capability to the public safely, by adding safeguards that block responses in high-risk areas.
How Fable 5 and Mythos 5 differ
The two models share the same base model, specs, and pricing. There are two practical differences.
- Safety classifiers: Fable 5 includes safety classifiers that can decline certain requests. Mythos 5 does not include these classifiers.
- Availability: Fable 5 is generally available on the Claude API, apps, and web. Mythos 5 is not generally available and is offered only to approved customers in Project Glasswing, a cybersecurity program. Customers without Mythos 5 access can use Fable 5, which offers the same capabilities.
Key specs
| Item | Claude Fable 5 / Mythos 5 |
|---|---|
| Model ID | claude-fable-5 / claude-mythos-5 |
| Context window | 1M tokens by default |
| Max output | Up to 128k tokens per request |
| Pricing | $10 per 1M input / $50 per 1M output tokens |
| Thinking | Adaptive thinking, always on |
| Data retention | 30-day retention (no ZDR; Covered Model) |
| Launch date | June 9, 2026 |
Both models keep the existing 90% prompt-caching discount on input tokens. US-only inference is available at 1.1x input/output pricing for workloads that must run in the US (per Anthropic).
Safeguards and the Opus fallback
The biggest operational change with Fable 5 is that its safety classifiers can decline a request. Questions in high-risk areas such as cybersecurity, biology, and chemistry are not answered by Fable 5; an Opus-tier model answers instead. Per the official help center the target depends on the request type — biology, chemistry and life-sciences requests fall back to Claude Opus 5, while offensive cybersecurity technique requests fall back to Claude Opus 4.8.
API behavior is as follows (per the Claude API docs).
- Refusal: When Fable 5 declines a request, the Messages API returns
stop_reason: "refusal"as a successful HTTP 200 response, not an error, and reports which classifier declined it. - Fallback: A refused request can usually be retried on another Claude model in three ways: server-side (the
fallbacksparameter, in beta), client-side (SDK middleware), or manual retry. - Billing: You are not billed for a request refused before any output is generated. When you retry on another model, fallback credit refunds the prompt-cache cost of switching, so you do not pay it twice.
Performance
Anthropic says Fable 5 exceeds every model it has previously made generally available and is state-of-the-art on nearly all tested benchmarks, with the lead widening on longer, more complex tasks. According to Anthropic's published benchmarks, on SWE-Bench Pro (solving real GitHub issues) it scores about 80%, ahead of Opus 4.8 (about 69%). These figures are Anthropic's own and are not independently verified.
As a real-world example, Anthropic noted that during early testing Stripe ran a codebase-wide migration across a 50-million-line Ruby codebase in a single day — work that would have taken a team over two months by hand.
How to use it
- Subscribers: At launch, Anthropic said Fable 5 would be included on Pro, Max, Team, and seat-based Enterprise plans at no extra cost from June 9 through June 22, 2026 — a window largely cut short by the June 12 suspension. After the July 1 redeployment, it is included for up to 50% of weekly usage limits — a time-limited window whose end date has been officially extended several times. For the terms currently in effect, see the notice at the top of this article; after the window, it is available via usage credits at standard API rates, separate from plan limits.
- Developers: Fable 5 is available on the Claude API as
claude-fable-5, and is generally available on Claude Platform on AWS, Amazon Bedrock, Vertex AI, and Microsoft Foundry. - Supported features: At launch it supports the effort parameter, task budgets (beta), the memory tool, code execution, programmatic tool calling, context editing (beta), compaction, and vision.
API migration checklist
When moving from an earlier model (e.g. Opus 4.8) to Fable 5, check the following (per the Claude API docs).
- Refusal handling: Add logic for
stop_reason: "refusal"(HTTP 200). - Fallback path: Configure a server-side, client-side, or manual fallback to retry on another model such as Opus 4.8.
- Adaptive thinking: It is the only thinking mode and is always on.
thinking: {"type": "disabled"}is not supported; control depth with theeffortparameter. - Thinking output: Raw chain of thought is never returned. Use
thinking.displayset to"summarized"or the default"omitted", and pass thinking blocks back unchanged on the same model in multi-turn conversations. - Data retention: Both models carry 30-day retention and are not available under zero data retention.
Summary
Fable 5 is the most capable model Anthropic has made generally available, opening Mythos-class performance to everyone behind safeguards. Mythos 5 offers the same capability without those safeguards but is limited to approved Project Glasswing customers. Developers should review the refusal/fallback/billing behavior, always-on adaptive thinking, and 30-day retention requirement before migrating.
Sources: Anthropic announcement (anthropic.com/news/claude-fable-5-mythos-5), product page (anthropic.com/claude/fable), Claude API docs (platform.claude.com). Benchmark figures are Anthropic's own.
Related articles
- Claude Fable 5.1 guide: what changed from Fable 5 — pricing, plan terms, and migration
- Claude Opus 4.7: The Complete Guide to New Features and Specs
- Claude Opus 4.8: The Complete Guide to What's New and How to Use It
- Claude Model Selection Guide: Opus 5 vs Sonnet 5 vs Haiku 4.5
- Claude Fable 5 Is Back: Restored Access, Usage Terms & What Changed
- Claude Sonnet 5 Complete Guide: What's New, Pricing, and Migration Gotchas