Claude Fable 5: historical model reference
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Okou no longer runs Claude Fable 5. This page is kept as a reference for its specs, pricing and benchmarks. For the same kind of work, use Claude Fable 5.1.
See Claude Fable 5.1Claude Fable 5 is Anthropic's most capable widely released model, sitting at the top of the lineup above Opus 4.8. It targets the hardest reasoning problems and long-horizon autonomous agentic work — complex refactors, overnight coding runs, and end-to-end deliverables that complete without human correction. It leads Anthropic's coding lineup on the harder SWE-bench Pro set and posts the company's strongest reasoning scores.
What is Claude Fable 5?
Released June 9, 2026
Claude Fable 5 launched on June 9, 2026 as Anthropic's most capable widely released model. It targets the top of the difficulty range: multi-step reasoning, long-horizon autonomous agents, first-shot implementations of well-specified systems, and end-to-end enterprise deliverables (financial analysis, spreadsheets, slides, documents). Its stablemate claude-mythos-5 offers the same capabilities through Anthropic's Project Glasswing partner program.
Two API behaviors set Fable 5 apart from the Opus tier. Thinking is always on — the thinking parameter is not configurable and an explicit disable is rejected — and the raw chain of thought is never returned, only a readable summary. Fable 5 also runs safety classifiers targeting research biology and most cybersecurity content, and requires 30-day data retention. It uses the same tokenizer as Opus 4.8, so token counts are roughly unchanged when moving between the two.
What's notable about Claude Fable 5
Headline architecture and capability features.
Fable 5 keeps the 1M-token context window (the maximum is also the default) and up to 128K output tokens. Thinking is always on and controlled through effort levels — low, medium, high, xhigh, and max — rather than a token budget. The raw chain of thought is never exposed; display: "summarized" returns a readable summary. Assistant-message prefill is not supported, and the model requires 30-day data retention (it is not available under zero data retention). Multimodal inputs span text, high-resolution vision, and code.
Specs at a glance
Claude Fable 5 benchmarks
Vendor-reported scores. Third-party trackers report different figures for the same benchmarks depending on dataset, context settings, and agent scaffold, so treat the headline numbers as configuration-dependent. Anthropic highlights the harder SWE-bench Pro set as the more durable coding signal.
Claude Fable 5 pricing
Historical provider list prices, per 1M tokens. These reference figures are not current Okou charges.
How Claude Fable 5 behaves in practice
Observed behaviour from production agent runs.
Long-horizon autonomy
Fable 5 is state-of-the-art at long, autonomous agentic work — complex refactors and overnight coding runs that complete without human correction. Give it the full task specification up front in one well-specified turn and run at high or xhigh effort for the best results.
Always-on thinking
There is no thinking configuration to tune — the model always reasons before acting, and you control depth with the effort parameter. The raw chain of thought is never returned; you get a readable summary instead.
Parallel sub-agent delegation
Fable 5 reliably sustains ongoing communication with long-running sub-agents and peer agents. Asynchronous delegation — where sub-agents keep their context instead of re-establishing it per subtask — outperforms spawn-and-block patterns.
Safety classifiers and refusals
Fable 5 runs classifiers targeting research biology and most cybersecurity content, and can return a refusal on those requests. Benign adjacent work (security tooling, life-sciences tasks) can occasionally trip a false positive, so production pipelines should handle the refusal path and fall back to Opus 4.8 where appropriate.
Best agent tasks for Claude Fable 5
The overnight refactor that finishes on its own
Hand Fable 5 a large, well-specified migration and let it run autonomously across many minutes and hundreds of files. Its long-horizon coherence and leading SWE-bench Pro score are what pay off on runs that used to need a human babysitting each step.
The hardest reasoning problem you have
The teams with the best results gave Fable 5 their hardest unsolved problems first — let it scope the problem, ask clarifying questions, then execute. Its Humanity's Last Exam result is the strongest Anthropic has shipped.
The end-to-end enterprise deliverable
Financial analysis, spreadsheets, slide decks, and long documents produced in one run, with the model verifying its own output as it goes. Reserve it for deliverables where correctness matters more than token cost.
When to skip Claude Fable 5
Skip Fable 5 on routine, high-volume, or latency-sensitive work — its premium pricing ($10/$50) and always-on thinking make it expensive overkill where Sonnet 5 or Opus 4.8 hit the same bar for a fraction of the cost. Avoid it for research-biology or most cybersecurity workloads (its classifiers may refuse), and for organizations that require zero data retention, since Fable 5 mandates 30-day retention.
Claude Fable 5 vs other models
Claude Fable 5 vs Claude Opus 4.7
Opus 4.7 is a previous-generation flagship at Opus-tier pricing. Fable 5 supersedes it on the hardest reasoning and long-horizon work by a wide margin; keep 4.7 only for pinned agents you've already validated and don't want to re-baseline.
Frequently asked questions
Can I turn thinking off on Fable 5?
No. Thinking is always on and the thinking parameter is not configurable — an explicit disable is rejected. You control reasoning depth with the effort parameter (low through max). The raw chain of thought is never returned; you receive a readable summary instead.
Why did a Fable 5 request get refused?
Fable 5 runs safety classifiers targeting research biology and most cybersecurity content, returning a refusal on those requests. Benign adjacent work can occasionally trip a false positive — production pipelines should detect the refusal and fall back to Opus 4.8.
Does Fable 5 work under zero data retention?
No. Fable 5 requires 30-day data retention and is not available under zero data retention. Requests from an organization that doesn't meet the requirement are rejected.
Alternatives
Availability of Claude Fable 5 on Okou
Claude Fable 5 is no longer offered on Okou. See Claude Fable 5.1 for a currently supported alternative. This page retains historical model information, not current Okou pricing.
