/gearboy-romhacking

name: gearboy-romhacking
description: >-
  Hack, modify, and translate Game Boy / Game Boy Color ROMs using the Gearboy
  emulator MCP server. Provides workflows for memory searching, value discovery,
  cheat creation, data modification, sprite/text finding, and translation
  patching. Use when the user wants to create cheats, find game values in
  memory, modify ROM data, translate a Game Boy game, patch game behavior,
  create ROM hacks, discover hidden content, change sprites or graphics, find
  text strings, apply Game Genie or GameShark codes, do infinite lives or health
  hacks, search for score or item counters, or reverse engineer data structures
  in Game Boy or Game Boy Color games. Also use for any ROM hacking, memory
  poking, or game modification task involving Gearboy.
compatibility: >-
  Requires the Gearboy MCP server. Direct tool mode is the default. Before
  installing or configuring, call debug_get_status to check if the server is
  already connected. If --mcp-router is enabled, use get_tool_info and
  execute_tool for routed tools.
metadata:
  author: drhelius
  version: "1.0"

Game Boy / Game Boy Color ROM Hacking with Gearboy

Overview

Hack, modify, and translate Game Boy and Game Boy Color ROMs using the Gearboy emulator as an MCP server. Search memory for game variables, create cheats, find text strings for translation, locate sprite data, and reverse engineer data structures — all through MCP tool calls. Use save states as checkpoints and fast forward to reach specific game states.

MCP Server Prerequisite

**IMPORTANT — Check before installing:** Before attempting any installation or configuration, you MUST first verify if the Gearboy MCP server is already connected in your current session. In the default mode, call `debug_get_status` directly. If Gearboy was intentionally started with `--mcp-router`, call `get_tool_info` with `{"name":"debug_get_status"}`, then call `execute_tool` with `{"name":"debug_get_status","arguments":{}}`. A valid response from either workflow means the server is active and ready.

Only if neither workflow is available or the call fails, you need to help install and configure the Gearboy MCP server:

Installing Gearboy

Run the bundled install script (macOS/Linux):

bash scripts/install.sh

This installs Gearboy via Homebrew on macOS or downloads the latest release on Linux. It prints the binary path on completion. You can also set `INSTALL_DIR` to control where the binary goes (default: `~/.local/bin`).

Alternatively, download from [GitHub Releases](https://github.com/drhelius/Gearboy/releases/latest) or install with `brew install --cask drhelius/geardome/gearboy` on macOS.

Connecting as MCP Server

Configure your AI client to run Gearboy as an MCP server via STDIO transport. Example for Claude Desktop (`~/Library/Application Support/Claude/claude_desktop_config.json`):

{
  "mcpServers": {
    "gearboy": {
      "command": "/path/to/gearboy",
      "args": ["--mcp-stdio"]
    }
  }
}

Replace `/path/to/gearboy` with the actual binary path from the install script. Add `--headless` before `--mcp-stdio` on headless machines.

---

Core Technique: Memory Search

Memory search is the primary tool for ROM hacking. It uses a capture → change → compare cycle to isolate memory addresses holding game values.

The Search Loop

1. memory_search_capture    → snapshot current memory state
2. (change the value in-game using controller_button, fast forward, etc.)
3. memory_search            → compare against snapshot to find changed addresses
4. Repeat 2-3 until only a few candidates remain
5. read_memory / write_memory → verify and modify the found addresses

Search Operators and Types

`memory_search` supports these **operators**: `<`, `>`, `==`, `!=`, `<=`, `>=`

**Compare types**:

• `previous` — compare current value to last captured snapshot (most common)
• `value` — compare current value to a specific number
• `address` — compare current value to value at another address

**Data types**: `hex`, `signed`, `unsigned`

Example: Finding the Lives Counter

1. memory_search_capture                         → snapshot with 3 lives
2. Lose a life in-game (play or use controller_button)
3. memory_search (operator: <, compare: previous) → values that decreased
4. memory_search_capture                         → snapshot with 2 lives
5. Lose another life
6. memory_search (operator: <, compare: previous) → narrow further
7. Or use: memory_search (operator: ==, compare: value, value: 1)
   → find addresses holding exactly 1
8. write_memory on the candidate address to set lives to 99
9. get_screenshot to verify the change took effect

Example: Finding a Score Counter

Score values are often stored as multi-byte (16-bit little-endian on SM83) or BCD-encoded:

1. memory_search_capture                                → snapshot at score 0
2. Score some points in-game
3. memory_search (operator: >, compare: previous)       → values that increased
4. memory_search_capture
5. Score more points
6. memory_search (operator: >, compare: previous)       → narrow down
7. read_memory on candidates — look for values matching current score
8. write_memory to set a custom score

Many Game Boy games store scores as BCD (Binary-Coded Decimal) — each nibble holds a digit 0-9. For example, score 1234 might be stored as bytes $12 $34.

---

Fast Forward for Efficiency

Use fast forward to speed through gameplay when you need to trigger in-game changes:

set_fast_forward_speed (4 = unlimited)
toggle_fast_forward              → enable
(play through the game section)
toggle_fast_forward              → disable

This is essential when you need to reach specific game states without waiting in real-time.

---

Save States as Checkpoints

Save states are critical for ROM hacking — they let you save your position and retry modifications:

select_save_state_slot (1-5)     → pick a sl