Issue 02 | Practical Installation of Five Major Platforms — Claude Code / Antigravity / Gemini CLI / Codex / OpenCode

🎯 Learning Objectives

After completing this issue, you will master:

1. Installing Caveman on the five major platforms: Claude Code, Antigravity, Gemini CLI, Codex, and OpenCode
2. Understanding the installation differences and auto-activation principles of each platform
3. Verifying Caveman's functionality after installation
4. Learning how to install on more extended platforms like Cursor, Windsurf, etc.

📖 Prerequisites

Before starting the installation, ensure your environment meets the following conditions:

📖 Core Content: Installation on Five Major Platforms

2.1 Claude Code Installation (Plugin Method — Recommended)

Claude Code natively supports a Plugin system, which is the most recommended way to install Caveman:

# One line to do it: add from marketplace + install plugin
claude plugin marketplace add JuliusBrussee/caveman && claude plugin install caveman@caveman

After installation, the Plugin system will automatically complete the following configurations for you:

graph TD
    A["claude plugin install"] --> B["Register SessionStart Hook
caveman-activate.js"]
    A --> C["Register UserPromptSubmit Hook
caveman-mode-tracker.js"]
    A --> D["Register Statusline Script
caveman-statusline.sh"]
    A --> E["Install 5 Skills
caveman / commit / review / compress / help"]
    
    B --> F["✅ Auto-activate on each startup"]
    C --> F
    D --> G["✅ Status bar shows current mode"]
    E --> H["✅ Slash commands available"]

Verify successful installation:

# Start Claude Code
claude

# In the session, type
> /caveman

# You should see output similar to:
# 🪨 CAVEMAN MODE: FULL
# Caveman active. Technical substance preserved. Fluff eliminated.

If you see the [CAVEMAN] badge in your status bar, the installation is successful.

Standalone Installation (without Plugin System)

If you prefer not to use the Plugin system, you can install Hooks directly via scripts:

# macOS / Linux / WSL
bash <(curl -s https://raw.githubusercontent.com/JuliusBrussee/caveman/main/hooks/install.sh)

# Windows (PowerShell)
irm https://raw.githubusercontent.com/JuliusBrussee/caveman/main/hooks/install.ps1 | iex

Or install by cloning the repository locally:

git clone https://github.com/JuliusBrussee/caveman.git
cd caveman
bash hooks/install.sh

Uninstallation is also simple:

bash hooks/uninstall.sh

2.2 Antigravity Installation

Antigravity (Google DeepMind's Agentic Coding assistant) supports integrating Caveman via GEMINI.md rule files and the Skills system.

Method One: Install via npx skills

npx skills add JuliusBrussee/caveman

This will install Caveman's Skill files in your project directory.

Method Two: Manually configure rule files

In your project's root directory GEMINI.md, add the following snippet to make Caveman always active in Antigravity sessions:

## Caveman Mode

Terse like caveman. Technical substance exact. Only fluff die.
Drop: articles, filler (just/really/basically), pleasantries, hedging.
Fragments OK. Short synonyms. Code unchanged.
Pattern: [thing] [action] [reason]. [next step].
ACTIVE EVERY RESPONSE. No revert after many turns. No filler drift.
Code/commits/PRs: normal.
Off: "stop caveman" / "normal mode".

Method Three: Project-level .agent/skills installation

# Create skills directory in project root
mkdir -p .agent/skills/caveman

# Download Caveman skill file
curl -o .agent/skills/caveman/SKILL.md \
  https://raw.githubusercontent.com/JuliusBrussee/caveman/main/skills/caveman/SKILL.md

Verification:

# In an Antigravity session, type
> Please answer in caveman mode: When does React useEffect's cleanup function execute?

# Expected Caveman-style answer:
# cleanup run: before next effect + unmount. Return fn from useEffect = cleanup.

⚠️ Note: Antigravity lacks Claude Code's SessionStart Hook mechanism, so Caveman will not auto-activate. You need to make it always-on via the GEMINI.md rule file, or manually say "caveman mode" in each session.

2.3 Gemini CLI Installation

Gemini CLI has a native Extensions system, install in one line:

# Install
gemini extensions install https://github.com/JuliusBrussee/caveman

# Update
gemini extensions update caveman

# Uninstall
gemini extensions uninstall caveman

Gemini CLI auto-activates Caveman via the GEMINI.md context file. Installation also includes custom Gemini commands:

# Switch intensity level
/caveman          # Switch to full mode
/caveman lite     # Switch to lite mode
/caveman ultra    # Switch to ultra mode
/caveman wenyan   # Switch to Wenyan mode

# Auxiliary commands
/caveman-commit   # Generate concise commit message
/caveman-review   # One-line code review

Verification:

gemini

# In the session
> /caveman
# Should see Caveman activation confirmation

> What is TypeScript's discriminated union?
# Expected Caveman-style answer:
# Tagged union. Common prop (kind/type) + switch narrow type. 
# Each branch: TS knows exact shape. Pattern: type Result = 
# {ok: true, data: T} | {ok: false, error: E}

2.4 Codex (OpenAI) Installation

OpenAI's Codex programming agent supports integrating Caveman via the Plugin marketplace and .codex/hooks.json.

Method One: Install via Plugin marketplace

# macOS / Linux:
# 1. Clone the caveman repository
git clone https://github.com/JuliusBrussee/caveman.git

# 2. Enter the repository directory, start Codex
cd caveman
codex

# 3. In Codex: /plugins → Search "Caveman" → Install

Method Two: Install via npx skills

npx skills add JuliusBrussee/caveman -a codex

Configure SessionStart auto-activation:

The Caveman repository comes with .codex/hooks.json and .codex/config.toml, which can auto-activate within Codex. If you want to enable always-on in other projects as well, you need to copy these two files:

// .codex/hooks.json
{
  "hooks": [
    {
      "event": "SessionStart",
      "command": "echo 'Terse like caveman. Technical substance exact. Only fluff die. Drop: articles, filler, pleasantries. Fragments OK. Code unchanged. Pattern: [thing] [action] [reason]. [next step]. ACTIVE EVERY RESPONSE.'"
    }
  ]
}

# .codex/config.toml
[features]
codex_hooks = true

⚠️ Codex Special Syntax: Codex uses $caveman to trigger (with a $ prefix), not /caveman. / is reserved for built-in commands in Codex.

⚠️ Windows Note: Codex hooks are currently disabled on Windows; you need to manually use $caveman to start.

Verification:

codex

# In the session, type
> $caveman

# Should see Caveman activation confirmation
> Explain what a Kubernetes Pod is
# Expected Caveman answer:
# Pod = smallest deployable unit in K8s. Wraps 1+ containers,
# shared network + storage. Ephemeral — dies, gets replaced.

2.5 OpenCode Installation

OpenCode is an open-source terminal AI coding tool that integrates Caveman via AGENTS.md rule files.

Method One: Install via npx skills

npx skills add JuliusBrussee/caveman
# Skill files will be installed in the project directory

Method Two: Manually configure AGENTS.md rule file (Recommended)

Create or edit .config/opencode/AGENTS.md in the project root:

## Caveman Mode (Always-On)

Terse like caveman. Technical substance exact. Only fluff die.
Drop: articles, filler (just/really/basically), pleasantries, hedging.
Fragments OK. Short synonyms. Code unchanged.
Pattern: [thing] [action] [reason]. [next step].
ACTIVE EVERY RESPONSE. No revert after many turns. No filler drift.
Code/commits/PRs: normal.
Off: "stop caveman" / "normal mode".

Verification:

opencode

# In the session, type
> caveman mode

# Should see the response style become concise
> Explain Go's goroutine
# Expected Caveman answer:
# Lightweight thread managed by Go runtime. `go func()` spawns.
# Cheap — thousands OK. Communicate via channels, not shared mem.

💡 Note: OpenCode does not have a slash command system; always use natural language to trigger Caveman (e.g., "caveman mode" / "talk like caveman"). Always-On must be achieved through the AGENTS.md rule file.

📊 Overall Comparison Table of Five Platform Installations

graph TB
    subgraph CC["Claude Code"]
        CC1["Plugin Installation
claude plugin install"] --> CC2["✅ SessionStart Hook Auto-activation"]
        CC2 --> CC3["✅ Status Bar Badge"]
        CC3 --> CC4["✅ /caveman Slash Command"]
    end
    
    subgraph AG["Antigravity"]
        AG1["npx skills add
or GEMINI.md Rules"] --> AG2["⚠️ Requires GEMINI.md always-on"]
        AG2 --> AG3["❌ No Status Bar Badge"]
        AG3 --> AG4["✅ Natural Language Trigger"]
    end
    
    subgraph GC["Gemini CLI"]
        GC1["gemini extensions install"] --> GC2["✅ GEMINI.md Auto-activation"]
        GC2 --> GC3["❌ No Status Bar Badge"]
        GC3 --> GC4["✅ /caveman Slash Command"]
    end
    
    subgraph CX["Codex"]
        CX1["Plugin Marketplace
or npx skills"] --> CX2["⚠️ Requires hooks.json configuration"]
        CX2 --> CX3["❌ No Status Bar Badge"]
        CX3 --> CX4["✅ $caveman Command"]
    end
    
    subgraph OC["OpenCode"]
        OC1["AGENTS.md Rules
Manual Configuration"] --> OC2["⚠️ Requires AGENTS.md always-on"]
        OC2 --> OC3["❌ No Status Bar Badge"]
        OC3 --> OC4["✅ Natural Language Trigger"]
    end

📖 Quick Reference for More Agent Extensions Installation

In addition to the five major platforms, Caveman also supports 40+ Agents via npx skills:

# Cursor
npx skills add JuliusBrussee/caveman -a cursor
# → Installs to .cursor/rules/caveman.mdc

# Windsurf
npx skills add JuliusBrussee/caveman -a windsurf
# → Installs to .windsurf/rules/caveman.md

# Cline
npx skills add JuliusBrussee/caveman -a cline
# → Installs to .clinerules/caveman.md

# GitHub Copilot
npx skills add JuliusBrussee/caveman -a github-copilot
# → Installs to .github/copilot-instructions.md + AGENTS.md

# AMP / Augment / Goose / Kiro / Roo, etc.
npx skills add JuliusBrussee/caveman -a <agent-name>

# Uninstall
npx skills remove caveman

⚠️ Important Note: npx skills add only installs Skill files; it does not install Hooks or rule files. Therefore, Caveman will not auto-activate. You need to manually activate it by saying /caveman or "caveman mode" each time, or paste the always-on snippet into the corresponding rule file.

⚠️ Windows Note: npx skills uses symlinks by default. If symlinking fails, add the --copy parameter: npx skills add JuliusBrussee/caveman --copy

📝 Key Takeaways from This Issue

1. Claude Code installation is the most complete: Plugin system + Hook auto-activation + status bar badge, done with a single command
2. Antigravity requires manual configuration of the GEMINI.md rule file for always-on, with no native Hook support
3. Gemini CLI has native Extensions support, auto-activating via GEMINI.md context
4. Codex is configured via the Plugin marketplace + .codex/hooks.json, triggered using $caveman (note the $ prefix)
5. OpenCode is configured via the .config/opencode/AGENTS.md rule file, triggered using natural language
6. All platforms support deactivating with "stop caveman" and re-enabling with "caveman mode"

🔗 References

• Caveman Installation Documentation
• npx skills Tool
• Caveman Hooks README
• Claude Code Plugin Documentation
• Codex Hooks Configuration
• OpenCode Documentation