Why AI coding tools ignore your project rules and how to fix them

Popular AI coding assistants like Cursor, Claude Code, and OpenAI Codex each support project rules files, but teams frequently find their instructions being ignored due to how each tool loads and prioritizes those files. Cursor uses .mdc files with frontmatter that controls four distinct attachment modes, meaning vaguely described rules with no file globs may never be loaded at all. Claude Code reads CLAUDE.md at every session start, making bloated or unfocused files a liability that dilutes the model's attention across an entire session. Codex merges AGENTS.md files hierarchically from home config down to subdirectories, where a nested file can silently override root-level rules without any warning. Across all three tools, the core fix is the same: write specific, imperative instructions scoped precisely to where they apply, rather than broad guidance intended for a human reader.