How I Built Lazarus: Autonomous Legacy Resurrection with Kiro Specs, Custom MCP, and Differential Fuzzing

In the world of software development, legacy JavaScript modules often pose challenges due to their difficult-to-maintain nature. Kiroween 2026, a hackathon category known as Frankenstein, inspired engineer Anjarkar to create Lazarus, an autonomous legacy resurrection engine.

Lazarus is a sophisticated system that combines an isolated execution sandbox with a 4-agent Kiro Crew and fast-check property-based differential fuzzing. Live demonstration of Lazarus can be found at https://lazarus-yf0a.onrender.com, while the source code and related artifacts are available on GitHub at https://github.com/anjarkar25-max/lazarus.

One significant issue with legacy code is the potential for silent edge-case regressions during manual rewrites. For example, the expression (n / 2) | 0 works for positive integers but not for negative odd integers like -3. Traditional unit tests might not catch such discrepancies, making Lazarus a promising solution.

To ensure behavioral parity between the original and modernized code, Lazarus employs an isolated sandbox and a custom MCP server. The sandbox, located at src/sandbox/, executes untrusted legacy code in a locked-down Node child process with strict memory, timeout, filesystem, and network restrictions. The custom MCP server, found in lazarus-sandbox/, allows agents to safely probe legacy expressions via stdio.

At the heart of Lazarus lies a 4-Agent Kiro Crew, consisting of Archaeologist, Architect, Necromancer, and Exorcist, which are defined in .kiro/agents/. The Archaeologist probes boundary inputs to capture baseline runtime traces, the Architect synthesizes observed behavior into formal Kiro specifications, the Necromancer modernizes the code to ES2022+, and the Exorcist runs fast-check differential fuzzing to check for behavior divergence.

To ensure the system's reliability, Lazarus is engineered using Kiro Spec-Driven Development. Each subsystem is designed with formal requirements, architecture designs, and implementation task lists outlined in .kiro/specs/. Additionally, repository-wide steering guidelines, established at .kiro/steering/, and an automated on-save Vitest hook ( .kiro/hooks/run-tests-on-save.json ) verify 86 passing tests across 10 suites, ensuring Lazarus's robustness.