Claude Fable 5.1
Claude model for demanding reasoning and long-horizon agentic work
ECI STANDING#2 of 266
SCORE164.47161.36–168.28 CI
FRONTIER LAGLaunched in the leadNothing released before it had scored higher.
SCORES LIKE THESE MODELSGPT-6 AstraOpenAI · Sep 2026166.31Claude Fable 5Anthropic · Jun 2026163.27Claude Opus 5Anthropic · Jul 2026162.3GPT-5.5 ProOpenAI · Apr 2026162.25
Full Epoch Capabilities Index ranking ↗From the lab
Launch announcement · Sep 1, 2026Released Claude Fable 5.1 and Claude Mythos 5.1RELEASE DATESep 1, 2026
WEIGHTSClosed weights
CONTEXT WINDOW1M
MAX OUTPUT128K
REASONINGYes
TOOL CALLINGYes
INPUTStext, image, pdf
OUTPUTStext
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