Claude Fable 5.1 added

Claude Fable 5.1 was released on 2026-09-01 and joined the board the same day. Board row: 86.3% of answers within $1, #2 of 33. 1,984 of 1,984 answers parsed.

The model's API rejects forced tool calls, which is how the board collects most models' answers, so its row answers as a JSON object and reasons at the provider default. The serving-configuration table in the paper records the transport for every row.

Sensitivity: with the answer tool declared under tool_choice auto, Fable 5.1 scores 87.5% (would rank #2). Under the same request Claude Fable 5 scores 86.9%. Fable 5's board row (79.9%) ran chunked under a forced tool call, which switches off its thinking, so the 6.4-point gap between the two board rows is mostly request shape.

The next board version moves every model to tool_choice auto so each provider's default reasoning engages under the recorded request shape (policybench#139).