LLM-Assisted Codebase Analysis for Migration: Comparing Codex, Claude, and VS Code Agents

Intro Most migrations fail before they start — because nobody actually knows what the system does. Legacy systems rarely fail because of syntax or frameworks. They fail because their behavior is undocumented and poorly understood. This lack of understanding becomes even more critical when development is done with agents. In this series, I explore how LLM tooling can assist in migrating existing systems. The focus is on cross-stack migration, where a system must be moved to a different technology stack due to platform, vendor, or organizational constraints. In these cases, the hardest part is usually incomplete knowledge of the current system. Tools such as Copilot, Codex, Claude Code, and similar agents make it possible to explore a codebase interactively, summarize its structure, and trace important flows instead of relying only on manual reverse engineering. Shift in Engineering Work with LLM Agents With strong LLM agents, the distribution of engineering effort starts to shift. Less