# Startup Context > Validate and diagnose startup context files — CLAUDE.md, AGENTS.md, rules, and .cursorrules — that inject instructions into the model context at session start across Claude Code, Cursor, and Codex. Use when this capability is needed. - Skill: `tomevault-io/startup-context` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/startup-context` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/startup-context/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/startup-context --- Use this skill when working with files that inject instructions at startup — validating format, diagnosing why instructions aren't being read, or understanding load order and precedence per vendor. **What startup context is:** Files that are automatically loaded into the model's context before any user prompt. Each vendor uses different file names and locations. All serve the same purpose: priming the model with project-specific or harness-level instructions. **Vendor mapping:** | Vendor | Primary file | Rules format | Legacy | |--------|-------------|--------------|--------| | Claude Code | CLAUDE.md | .claude/rules/*.md | — | | Cursor | .cursor/rules/*.mdc | .cursor/rules/*.mdc | .cursorrules (root) | | Codex | AGENTS.md | NOT SUPPORTED in plugins | — | **Load order (Claude Code):** User CLAUDE.md (~/.claude/CLAUDE.md) → Project CLAUDE.md → Plugin CLAUDE.md (via @-import). Rules in .claude/rules/ are loaded after CLAUDE.md. **The @AGENTS.md workaround (Claude Code):** Claude Code does not natively read AGENTS.md. When ynh exports for Claude, it writes a CLAUDE.md containing only `@AGENTS.md` — using Claude's @-import syntax to pull in the cross-vendor instructions file without duplicating content. **Diagnostic checklist:** 1. Is the file in the right location for the vendor? 2. Claude Code: does the plugin CLAUDE.md use @-import correctly? 3. Cursor: is the rules file .mdc (not .md)? Does it have required frontmatter (description, globs, alwaysApply)? 4. Codex: rules are not supported in plugin format — use AGENTS.md only 5. Check file encoding and line endings — bare CR can cause silent failures --- > Source: [eyelock/assistants](https://github.com/eyelock/assistants) — distributed by [TomeVault](https://tomevault.io).