MD Debugger & Error Handler

(Credit: Vladikcomper)
Source: GitHub Repo
Commit: 3ac7129

Description from Vladikcomper's Sonic Retro thread linked below.
The MD Debugger and Error Handler is a ready-to-use error handler that comes with a powerful integrated debugger. It aims to provide robust and extensible debugging tools built-in directly into the Mega-Drive ROM, that can be used anywhere (from emulators to the real hardware).

Currently, it targets the AS Macroassembler and ASM68K assembler. (For S1Fixed users, this applies to the former). It has installation instructions and full support for the mainline Sonic disassemblies, but it can be integrated into any AS or ASM68K project (both the error handler and debugger) or even any pre-existing ROM (error handler only in a binary form).

Guides and tips can be found at the Sonic Retro thread.