The September 1, 2026 update holds Fable 5.1 at the same per-token price, cuts cache-read costs sharply, and adds mid-conversation effort control. Mythos 5.1 stays gated to trusted programs.
2026-09-01 · by Moe Ameen
Published 2026-09-01 · Updated
Short answer
Anthropic released Claude Fable 5.1 and Mythos 5.1 on September 1, 2026. They are the same model with different safeguards. Fable 5.1 is generally available, and Mythos 5.1 is limited to vetted organizations. Fable 5.1 keeps the $10 and $50 per million token price but cuts cache reads by 75% to $0.25. It adds mid-conversation effort control and content provenance tracking.
Key takeaways
Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026, an update to the Fable 5 and Mythos 5 line the company first shipped on June 9, 2026. As with the earlier release, the two are the same underlying model at different safeguard levels: Fable 5.1 is generally available, while Mythos 5.1 — with restrictions relaxed for cybersecurity and life-sciences work — is limited to vetted organizations through Anthropic's trusted-access programs, including its Cyber Verification and Life Sciences Verification programs. Fable 5.1 is available on Claude.ai and through the API (model id `claude-fable-5-1`) on AWS, Google Cloud, and Microsoft Azure.
On price, Fable 5.1 holds at $10 per million input tokens and $50 per million output — unchanged from Fable 5 — but Anthropic cut cache-read costs by roughly 75% (to about $0.25 per million tokens) and says typical workloads land around 25% cheaper overall, rising toward 45% for highly agentic tasks. The context window stays at one million input tokens with up to 128,000 tokens of output per response.
Two capabilities are new. The first is mid-conversation effort control, which lets you dial how hard the model works on a given step up or down without starting a fresh session. The second is content provenance tracking, which tags outputs so downstream systems can verify where generated content originated. On benchmarks, Anthropic reports gains over Fable 5 in coding and agentic work; for example, Terminal-Bench 4.0 scores of about 55.8% for Fable 5.1 and 60.9% for Mythos 5.1. Jane Street said Fable 5.1 solves more of its coding problems than Fable 5 or Opus 5.
Treat the exact benchmark figures and pricing as launch-day snapshots that can move, and verify current numbers on Anthropic's page before relying on them. One thing to keep straight: Fable 5.1 is a text-and-reasoning model. It writes, analyzes, codes, and reasons — it does not generate images, video, or audio.
Fable 5.1 is a model you call. Kompozy runs its text generation server-side on managed Claude models, so you never handle keys, migrations, or cache pricing yourself to get on-brand drafts. Picking and adopting models is Kompozy's job.
The launch itself is also content. Drop the facts into Kompozy as a source and the engine fans one take into a package — a Blog Article on what changed for builders, a Carousel comparing Fable 5.1 to Fable 5 slide by slide, a Listicle Video of the five takeaways over a portrait clip, and platform-native Text Posts — then schedules and publishes the set across the nine social platforms plus your blog on autopilot. You ship the explainer during the peak-interest window instead of assembling six formats by hand after the moment has cooled.
A listicle video made in Kompozy, the kind of takeaway video it can build from launch news.
Best for:
Not ideal for:
They are Anthropic's September 1, 2026 update to its Fable 5 / Mythos 5 model line. Fable 5.1 and Mythos 5.1 are the same underlying model at different safeguard levels: Fable 5.1 is generally available, while Mythos 5.1 relaxes restrictions for cybersecurity and life-sciences work and is limited to vetted organizations through trusted-access programs.
Per-token pricing is unchanged at $10 per million input tokens and $50 per million output. What dropped is cache reads, cut roughly 75% to about $0.25 per million tokens; Anthropic says typical workloads run around 25% cheaper overall and up to 45% cheaper for highly agentic tasks. Confirm current rates on Anthropic's page.
Alongside reported gains in coding and agentic computer use, Fable 5.1 adds mid-conversation effort control (dial how hard the model works on a step without a new session) and content provenance tracking that tags outputs so their origin can be verified. Context stays at one million input tokens with up to 128,000 output tokens.
No. Fable 5.1 is a text and reasoning model — it writes, analyzes, codes, and reasons but produces no images, video, or audio. To turn its writing into published video, carousels, and posts, pair it with a content engine like Kompozy that renders the media and publishes across platforms.
We checked every figure, date, quote and product claim on this page against the sources listed below on 2026-10-23, and removed or rewrote anything we could not confirm. Pricing, benchmarks and access come from Anthropic's page and model docs. The release date, the June 9 Fable 5 date and the two new features come from Crypto Briefing's launch report. Kompozy platform and format details come from kompozy.io.
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