Anthropic's Opus 5 blows past Fable 5 and GPT-5.6 Sol

What happened

The creators of the ARC-AGI benchmark say Anthropic's Claude Opus 5 owes its massive lead on ARC-AGI-3 to genuinely better reasoning. 2 percent on ARC-AGI-3, making it the new leader. 6 Sol (Max). Opus 5 solved five previously unsolved environments, four of them at or above human level. That also puts it ahead of Anthropic's "Fable-class" models, which hit around 20 percent according to ARC Prize.

Reinforcement learning could then reward exploration, planning, rule discovery, and self-correction. Tests on Witness, Guanghan Ning's private benchmark for interactive puzzle games, point to narrower gains. 8 than it did on ARC-AGI-3. 8 on a game with less familiar mechanics. Ning says that pattern fits training on genre-specific data, though Witness can't identify what data Anthropic used.

Greg Kamradt, one of the researchers behind ARC-AGI-3, said the results don't rule out broader reasoning gains. A game based on familiar mechanics doesn't test adaptation to novelty, while one weak result doesn't outweigh the model's overall improvement without detailed scores for that task. Witness was also designed around ARC-AGI-3-style puzzles, so better performance could reflect real transfer rather than memorization.

Why it matters

" During testing, Opus 5 also showed behavior that researchers hadn't seen from a model before. It translated tasks into algebraic notation and independently formulated reflection equations for the first time. Six of the 25 public demo environments have now been solved. The full results, replays, and benchmarking code are publicly available. 5 percent on ARC-AGI-1.

Both results match previous top scores, though at slightly higher costs, according to ARC Prize. ARC-AGI-3 measures how well AI models solve new tasks they didn't encounter during training, including ones humans can usually handle with ease. The current version works like a game. The model must infer the rules of an interactive environment, plan its actions, and carry them out step by step.

This tests general reasoning rather than stored knowledge. Some AI systems may have already passed the benchmark, but they rely on extra software known as a harness. Official scores count only the language model's own performance. ARC Prize argues that future AGI systems shouldn't need outside help to solve new tasks. Opus 5 would likely score even higher if used within Claude Code.

Anthropic hasn't explained the gain, but targeted data labeling and reinforcement learning are plausible factors. Unlike earlier models, Opus 5 was developed after ARC-AGI-3 and its format became public. That may have let Anthropic target the benchmark's skills and puzzle formats, though it doesn't show the company trained on the exact tasks. Annotators could have labeled reasoning traces, useful actions, failed attempts, and recovery steps from similar puzzles.

What to watch

Ning later clarified that Opus 5 did generalize to Witness, just far less than on ARC-AGI-3. He compared the process to the evolution of coding benchmarks. As a major target for interactive reasoning, ARC-AGI-3 will likely attract the most training effort first. Covering more edge cases could then help models generalize to a wider range of abstract reasoning tasks.

Ning said coding followed a similar path, moving from saturated benchmarks such as HumanEval to frequently updated competitions and today's coding agents.