Pattern: Agent Rules Formats Converging on AGENTS.md as Universal Standard

Multiple (The Prompt Shelf, GitHub)·medium signal

Google, OpenAI, Sourcegraph, Cursor, and Factory have aligned on AGENTS.md as the cross-tool standard for agent instruction files. Karpathy's single CLAUDE.md hit 49K stars, Ruler (2.7K stars) bridges formats by distributing from one source to all agent configs, and rule-porter converts between .mdc/.cursorrules/CLAUDE.md/AGENTS.md. The practical implication: writing your agent rules in AGENTS.md format today future-proofs them across Claude Code, Codex, Cursor, Copilot, and Antigravity. The format wars are ending — invest in the standard.