Claude Fable 5.1

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.