Anthropic's Claude Opus 5.5 just quietly killed the em dash. Here's what you need to know. Opus 5.5 launched on September 22, 2026 as the first model in Anthropic's new 5.5 family. Anthropic says it leads in agentic coding and knowledge work, runs about 40% cheaper than Opus 5 on typical workloads, and outputs over 30% faster. Arena.ai analyzed how Opus 5.5 writes versus Opus 5 across high reasoning Text Arena outputs. 10 of 12 writing measures moved in a better direction, making Opus 5.5 easier to read than any prior Claude model. Key numbers: - Long content words: down from 41.7% to 38.6%, lowest of any Claude model - Em dashes: down about 95% - Semicolons: down about 73% - Sentence length: down about 17% on average - Answer quality: up about 6% The change went viral after AI consultant Peter Gostev posted "RIP em dash" quoting Arena.ai's thread, prompting writers online to joke they can finally reclaim the punctuation mark for themselves.