“It communicates clearly, it's cheaper per token than Opus 5.0 with the intelligence of Fable 5.1 it's very token efficient and works across every effort level.”

Simon Willison blog · Sep 21 · OpenAI and Anthropic trade blows in AI price war: GPT-6 Sol/Luna vs Claude Opus 5.5

“Starting today in version 2.1.277, if there is no CLAUDE.md in a folder, Claude will check for and use AGENTS.md.”

Simon Willison blog · Sep 17 · Anthropic adds AGENTS.md support to Claude Code

“AGENTS.md support is built off of Claude Code mods, our upcoming way to customize the Claude Code harness.”

Simon Willison blog · Sep 17 · Anthropic adds AGENTS.md support to Claude Code