AI models are improving fast at math, code, and reasoning. Their writing quality, though, has stalled or even gotten worse.

Anthropic employee Jackson Kernion, who works on Claude fine-tuning, recently explained why Opus 4.6 was the last good "writing model" from Anthropic. The answer is partly predictable: newer models have been optimized for math and code.

But they've also been trained to produce technical explanations aimed at other AI models, and that's what really drives Claude's odd phrasing ("Claudeish"). The model has been "adapted to LLM psychology," Kernion says, and learned to write for AI models, not for people.

Kernion compares this to humans who only communicate with other autistic people. They develop a style that works within that group but feels hard to follow for outsiders, he says. LLMs have far more working memory than humans and pick up on details at a much finer level, and so, during training, a writing style emerges that works well for AI models but produces what human readers experience as "overly-dense info dumps."

Optimizing for code comes at a cost to natural language

The issue, according to Kernion, comes down to the reward structure in reinforcement learning. Some rewards optimize for AI model comprehension, others for human comprehension. The more you train on math and code, the more you have to actively push back by rewarding simple explanations that human readers can follow.

With Opus 5.5, Anthropic found a better balance, Kernion says, adding that he hasn't been "as happy about a model's writing since Opus 4.6." That doesn't mean Opus 5.5 surpasses the older model, though. "It's a hard problem to solve, and we'll continue to make improvements," Kernion writes.

Kernion via X