Claude Fable 5.1
Released
Claude Fable 5.1 is an AI model released by Anthropic on Tuesday, Sep 1 2026, 39 days after Claude Opus 5. It has a 1M token context window. Benchmark results (shown below) cover SWE-Bench Pro, Terminal-Bench-Science 0.1, Humanity's Last Exam, AA-Briefcase, SWE-Bench Multilingual, SWE-Bench Multimodal, and 5 more.
API pricing
- InputWhat you pay for everything you send the model — your question, plus any documents or earlier conversation you include with it.
- $10.00
- Cached inputA reduced rate for text you send over and over. If every request starts with the same instructions or the same document, the provider keeps a copy ready and charges less to read it again.
- $0.25
- 5 min cache writeA one-off charge for storing text so later requests can reuse it at the cheaper cached rate. This option keeps it for five minutes.
- $12.50
- 1 hr cache writeA one-off charge for storing text so later requests can reuse it at the cheaper cached rate. This option keeps it for an hour, so it costs more than the five-minute one.
- $20.00
- OutputWhat you pay for the text the model writes back. It is normally the dearer half: producing an answer costs more than reading one.
- $50.00
- Adaptive thinking is always on.
- Cache reads are priced at 0.025x the base input price on this model, against 0.1x on every other Claude model.
Benchmarks
Coding
Terminal & CLI
Agentic & tool use
Reasoning & science
Knowledge work
Healthcare
About
Claude Fable 5.1, released September 1, 2026, succeeded Claude Fable 5 at the top of Anthropic's lineup twelve weeks after the Claude 5 generation opened. It shipped at the same $10 per million input tokens and $50 per million output as its predecessor, with one price cut: cache reads dropped to $0.25 per million, a quarter of Fable 5's rate and 0.025x the base input price where every other Claude model charged 0.1x. Anthropic described it as its most capable widely released model and pitched it narrowly — start with Claude Opus 5, and move up to Fable 5.1 for demanding reasoning and long-horizon agentic work, or when Opus 5 at its highest effort still falls short.
The launch notes concentrated the gains in six areas: agentic coding over sessions that run for hours, knowledge work that carries an analysis through to a finished document, spreadsheet or slide deck, multistep research, reading dense charts and tables inside PDFs, reasoning across the full 1M-token context window, and computer use. Adaptive thinking was always on, with a 128K output limit and a June 2026 knowledge cutoff. It also brought breaking API changes: forced tool use returned an error, and thinking blocks became bound to the model and the conversation that produced them. Every response carried Anthropic's text watermark, and media files came with C2PA Content Credentials. Claude Mythos 5.1 shipped the same day as the Project Glasswing twin. The launch table, run with production safeguards enabled, put Fable 5.1 at 55.8% on Terminal-Bench 4.0 against Fable 5's 42.0% and Opus 5's 52.3% under the same harness, 52.6% on Terminal-Bench-Science 0.1 (more than double Fable 5's 24.7%), a GDPval-AA v2 Elo of 1853, 60.9% on Humanity's Last Exam without tools and 65.0% with, and 31.4% on AutomationBench. Its OSWorld 2.0 figures, 77.9% partial and 41.7% strict, were run on the benchmark's August 2026 task release, which Anthropic said made them not directly comparable to earlier published scores. The system card filled in the coding detail the launch post left out: 81.2% on SWE-bench Pro, 89.1% on SWE-bench Multilingual and 54.7% on SWE-bench Multimodal, alongside an AA-Briefcase Elo of 1694 and 62.1% on HealthBench Professional.
Compare Claude Fable 5.1 with
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Frequently asked questions
Claude Fable 5.1 was released by Anthropic on Tuesday, Sep 1 2026.
Claude Fable 5.1 was built by Anthropic. AI safety company building the Claude family of models. Founded in 2021 by former OpenAI researchers.
Claude Fable 5.1 costs $10.00 per million input tokens and $50.00 per million output tokens through the Anthropic API. Cached input is $0.25 per million tokens. Rates are pay-as-you-go API prices verified against Anthropic's published pricing on September 1, 2026.
Claude Fable 5.1 reports 12 tracked benchmark scores — SWE-Bench Pro: 81.2%; SWE-Bench Multilingual: 89.1%; SWE-Bench Multimodal: 54.7%; Terminal-Bench 4.0: 55.8%; Terminal-Bench-Science 0.1: 52.6%; Humanity's Last Exam (no tools): 60.9%; Humanity's Last Exam (with tools): 65%; ARC-AGI-2: 90%; AutomationBench: 31.4%; HealthBench Professional: 62.1%; GDPval-AA v2: 1853; AA-Briefcase: 1694. Scores are the figures published at release by Anthropic. It holds the best score among all models tracked here on SWE-Bench Pro, Terminal-Bench-Science 0.1, Humanity's Last Exam (no tools), Humanity's Last Exam (with tools) and AA-Briefcase.
Claude Fable 5.1 has a context window of 1M. That is the maximum amount of input plus output the model can hold in a single request.
No. Claude Fable 5.1 is a proprietary model. The weights are not published — it is available only through the provider's own API, apps, or partner platforms.
Anthropic's previous tracked release was Claude Opus 5 on Jul 24 2026, 39 days earlier. It was followed by Claude Mythos 5.1 on Sep 1 2026.