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

Engineering teams sit on unmaintained legacy JavaScript modules they are afraid to touch because manual rewrites introduce silent edge-case regressions. For Kiroween 2026 (Frankenstein category), I built Lazarus — an autonomous legacy resurrection engine that stitches an isolated execution sandbox together with a 4-agent Kiro Crew and fast-check property-based differential fuzzing.

• Live Operating Table: https://lazarus-yf0a.onrender.com
• GitHub Repository & .kiro/ Artifacts: https://github.com/anjarkar25-max/lazarus
• Social Blitz Post (#KiroweenHackathon26): https://x.com/ashlesha_25/status/2121502679369421082

Why Unit Tests Miss Legacy Regressions

Consider a classic ES5 32-bit integer truncation expression: (n / 2) | 0. A naive modernization replaces | 0 with Math.floor(n / 2). On positive integers, both match. On negative odd integers like -3, (-3 / 2) | 0 evaluates to -1, whereas Math.floor(-3 / 2) evaluates to -2.

How Lazarus Proves Behavioral Parity

1. Isolated Sandbox (src/sandbox/): Executes untrusted legacy code in a locked-down Node child process with strict memory, timeout, filesystem, and network restrictions.
2. Custom MCP Server (lazarus-sandbox): Exposes run_legacy_probe over stdio in .kiro/settings/mcp.json so agents safely probe legacy expressions.
3. 4-Agent Kiro Crew (.kiro/agents/):
   • Archaeologist probes boundary inputs to capture baseline runtime traces.
   • Architect synthesizes observed behavior into formal Kiro specifications.
   • Necromancer modernizes the code to ES2022+.
   • Exorcist runs fast-check differential fuzzing (Object.is parity checks for -0 vs +0, NaN, and integer truncation). When Exorcist finds a divergence, it shrinks the input to a minimal counterexample and feeds it back to Necromancer to self-heal automatically.

Engineered with Kiro Spec-Driven Development

Every subsystem in Lazarus was designed using formal requirements, architecture designs, and implementation task lists inside .kiro/specs/, paired with repository-wide steering guidelines (.kiro/steering/) and an automated on-save Vitest hook (.kiro/hooks/run-tests-on-save.json) verifying 86 passing tests across 10 suites.