What I Actually Put in My Project Context Files

Every AI coding tool has its own context file. Cursor has .cursorrules and .mdc files. Claude Code has CLAUDE.md . Copilot has .github/copilot-instructions.md . They all do roughly the same thing: tell the AI how you want it to write code. Most guides stop at the format. "Here's the YAML frontmatter, here are the fields, good luck." That's not the hard part. The hard part is figuring out which rules the AI actually follows and which ones it quietly ignores. I've spent the last few weeks testing this with Cursor's .mdc format specifically, running the same rules through multiple models and tracking compliance. Some of what I found was obvious. Some of it wasn't. The quick landscape If you're using multiple tools, here's where things stand: Tool File Format Agent support Cursor .cursorrules Plain text Yes (legacy) Cursor .cursor/rules/*.mdc YAML frontmatter + markdown Yes (recommended) Claude Code CLAUDE.md Markdown Yes GitHub Copilot .github/copilot-instructions.md Markdown Yes The big