My go-to prompt for legacy code exploration (leftofthe.dev)

Aaron from Left of the Dev shares a practical, reusable prompt he uses to map and document legacy codebases by auto-generating a SYSTEM_OVERVIEW.md at a repository root. The prompt instructs an LLM to produce a structured document with CORE ANALYSIS (tools, frameworks, design patterns, data models, API design and a high‑level Mermaid architecture diagram), SYSTEM DESIGN (detailed per-file explanations, Mermaid sequence diagrams, and flowcharts), and an optional LEGACY ASSESSMENT (architectural inconsistencies, deviations from best practices, and distinctions between old and new approaches). He recommends letting the agent run for a few minutes, then reviewing the output in a Markdown preview with Mermaid support, iterating to add human context, and committing the file for future continuity. He also suggests automating a rule to keep SYSTEM_OVERVIEW.md updated after changes. This workflow is significant for AI/ML practitioners working with code-centric LLMs because it standardizes machine-assisted discovery, accelerates onboarding, and creates a lightweight, visual baseline for human and agent cooperation across languages (Ruby, Python, Java, Go, JS, Perl). Key technical takeaways: use explicit prompt structure, include Mermaid diagrams (LLMs are good but imperfect at syntax), treat the output as a “good enough” starting point, and retain a human-in-the-loop to validate historical and architectural context before making large refactors or upgrades.