Claude Code agent instructions & skills in Gemini CLI, Codex CLI, Copilot CLI — portability check

I, ytyng, use Claude Code as my main development tool. When Claude Code's server responses are unstable I sometimes use other agent development tools too, and I think it would be convenient if the skills, MCP settings and so on that I made for Claude Code could be used there as they are.

So I investigated whether Claude Code's configuration files and command definitions can be used as they are in other tools (Gemini CLI, OpenAI Codex CLI, GitHub Copilot CLI). Ideally, I want to share them just by creating symbolic links.

Tools Covered

• Claude Code (Anthropic)
• Gemini CLI (Google)
• OpenAI Codex CLI (OpenAI)
• GitHub Copilot CLI (GitHub/Microsoft), GA in February 2026

Configuration File Layouts for Each Tool

Project instruction files (Project Instruction)

Each tool reads a Markdown file placed in the project root and decides how to behave in that project from it. Only the file name differs; the content is the same natural-language Markdown.

AGENTS.md is an open standard defined by the Linux Foundation's Agentic AI Foundation (announcement, December 2025). OpenAI, Anthropic, and Block co-founded it. It is adopted by more than 60,000 open source projects, and Claude Code also reads AGENTS.md as a fallback when there is no CLAUDE.md.

Official site: https://agents.md/

Global config directories

CLI help excerpts

These are the configuration-related options, excerpted from each tool's --help.

Claude Code

--system-prompt <prompt>       System prompt for session
--mcp-config <configs...>      Load MCP servers from JSON files
--settings <file-or-json>      Path to settings JSON file
--plugin-dir <path>            Load plugins from a directory
--disable-slash-commands       Disable all skills

Gemini CLI

--policy                       Additional policy files or directories
--extensions / -e              List of extensions to use
--include-directories          Additional workspace directories

Codex CLI

-c, --config <key=value>       Override config from ~/.codex/config.toml
-p, --profile <CONFIG_PROFILE> Configuration profile
-s, --sandbox <SANDBOX_MODE>   Sandbox policy

Conclusion: What Can Be Shared, and What Cannot

What can be shared via symbolic links

1. Project instruction files: fully shareable

All tools use natural-language instructions in Markdown format, so they can be shared just by symlinking from one canonical file.

AGENTS.md                              # Source of truth
CLAUDE.md -> AGENTS.md                 # Claude Code
GEMINI.md -> AGENTS.md                 # Gemini CLI
                                       # Copilot CLI reads AGENTS.md directly

Git tracks symbolic links as they are, so the same structure is kept when team members clone the repository.

2. Global instruction files: fully shareable

In my environment, the global instructions of three tools point to the same file.

~/.claude/CLAUDE.md  -> ~/home-files/config/claude-code/CLAUDE.md
~/.gemini/GEMINI.md  -> ~/home-files/config/claude-code/CLAUDE.md
~/.codex/AGENTS.md   -> ~/home-files/config/claude-code/CLAUDE.md

Claude Code specific instructions (such as "review with /review-stage") are also visible to Gemini and Codex, but a tool without the feature just ignores them, so there is no real harm. The global instructions of Copilot CLI can also be shared by symlink.

3. Slash commands / custom agents

Claude Code and Codex CLI use the same .md + YAML frontmatter format, so they can be shared directly by symlink. Copilot CLI uses its own extension, .agent.md, and the file structure also differs, so direct sharing is not possible.

~/.codex/prompts/ -> ~/home-files/config/claude-code/commands/

In my environment this symlink is already working.

Only Gemini CLI uses the .toml format, so it cannot be shared by a direct symlink.

The Claude Code / Codex CLI command format.

---
description: "Create feature branch, commit, push, and open a pull request."
allowed-tools: Bash(git:*), Bash(gh:*), Bash(open:*)
---

今回の修正内容にあった名前でフィーチャーブランチを作ってください。

The Gemini CLI command format.

description = "Create feature branch, commit, push, and open a pull request."
prompt = """
今回の修正内容にあった名前でフィーチャーブランチを作ってください。
"""

The content is the same, but the file format differs. For Gemini CLI, conversion is needed.

What cannot be shared

Skills (auto-trigger)

Claude Code's skill system (~/.claude/skills/) is a feature of its own that is triggered by automatically matching the user's input based on description. Neither Gemini CLI nor Codex CLI has an equivalent system (Codex has .agents/skills/, but the format and behavior differ).

Copilot CLI has a mechanism called custom agents (.agent.md files), with a feature that automatically delegates to subagents depending on the context. The concept is close to Claude Code's skills, but the file format (.agent.md) and location (.github/agents/ or ~/.copilot/agents/) differ, so sharing by symlink is not possible.

Skills are one of Claude Code's biggest differentiators, but each tool is starting to have its own skill-like feature, and I hope for standardization in the future.

settings.json / config.toml

Not shareable, because each tool's schema is completely different.

• Claude Code: manages permissions, hooks, plugins, sandbox in JSON
• Gemini CLI: manages auth, theme, mcpServers in JSON
• Codex CLI: manages model, trust_level, mcp_servers in TOML

Hooks / Plugins

A Claude Code specific feature. Copilot CLI also has hooks, but the format differs.

MCP server settings

Claude Code and Gemini CLI have a similar structure in JSON format, but tool-specific options (permissions and so on) are mixed in, so automatic sync is difficult. Syncing by hand is the realistic way.

References

• GitHub Copilot CLI GA announcement - Generally available since February 2026
• Creating custom agents for Copilot CLI - Custom agent definitions with .agent.md
• AGENTS.md official site
• Linux Foundation - Agentic AI Foundation announcement
• CLAUDE.md, AGENTS.md, and Every AI Config File Explained - A comprehensive comparison of the config files
• AGENTS.md: One File to Guide Them All - An explanation of using AGENTS.md as a universal standard
• Keep your AGENTS.md in sync - One Source of Truth - A practical guide to the symlink strategy
• Do you symlink your AGENTS.md and skills to .claude? - A discussion on SSW Rules
• Sharing AI Agent Configs Between Cursor and Claude with Symlinks - A hands-on article on sharing by symlink
• Claude Code, Codex, Gemini CLI Converge on Shared Patterns - An analysis of how the three tools are converging
• AGENTS.md Cross-Tool Unified Management Guide - A guide to unified cross-tool management
• awesome-agent-skills - A collection of 1000+ community skills
• ccmanager - A session manager for Claude Code / Gemini CLI / Codex CLI