---
title: "A FREE MCP READS 212 LEGACY FILES AND WRITES THE SPEC BEFORE ANY AGENT TOUCHES THE CODE"
url: "https://reamcp.dev/en/cases/a-free-mcp-reads-212-legacy-files-and-writes-the-spec-before-831c92"
source: "https://x.com/i/status/2111530936441446417"
source_type: "x"
author: "Primee32"
author_handle: "Primee32"
stars: 0
category: "App feature recreation"
categories: ["app-recreation"]
tags: ["code-rewrite", "legacy-modernization", "mcp", "reverse-engineering", "spec-generation", "verification"]
added: "2026-10-19T02:06:48.267Z"
updated: "2026-10-17T18:53:00.000Z"
locale: "en"
content_locale: "en"
---

# A FREE MCP READS 212 LEGACY FILES AND WRITES THE SPEC BEFORE ANY AGENT TOUCHES THE CODE

An X post describing a workflow where REA (morluto/rea) reverse-engineers a legacy billing system into a spec.md (14 endpoints, 37 rules, 9 edge cases), with twelve Sonnet agents building modules from spec slices and REA VERIFY replaying the new system against the legacy one (148/148 behaviours match). The post links to the MIT-licensed GitHub project and gives the npx rea-agents setup install command, plus a warning about sending tool results to model providers.

## Description

A FREE MCP READS 212 LEGACY FILES AND WRITES THE SPEC BEFORE ANY AGENT TOUCHES THE CODE

most rewrites go wrong in the same place. you dump the whole old system into one model and it drowns

this setup splits the work:

OPUS 5.5 > writes the plan, then sits idle for the rest of the run
REA > reverse-engineers legacy/billing once and turns it into spec.md with 14 endpoints, 37 rules, 9 edge cases
SONNET 5.5 x12 > one module each, built from a ~1.5k token slice of the spec. none of them opens the old code
REA VERIFY > replays the new system against the legacy one, 148 of 148 behaviours match

on screen the swarm finishes in 7m 05s for $0.88. opus $0.22, REA $0.32, twelve sonnets $0.34. Opus 5.5 alone sits at 23% and $1.20, still running

the video marks that comparison as illustrative, so take the numbers as a demo

REA doesn't come from Anthropic. it's morluto/rea on github, MIT licensed and free, you only pay for the model. one line installs it: npx rea-agents setup

before you point it at client code:
> REA analyzes locally, but your agent still sends tool results to the model provider
> decompiled code from a private app ends up with Anthropic or OpenAI, under their data policy

it's free today. save it before someone sells it as a $500 course

## Original README

A FREE MCP READS 212 LEGACY FILES AND WRITES THE SPEC BEFORE ANY AGENT TOUCHES THE CODE

most rewrites go wrong in the same place. you dump the whole old system into one model and it drowns

this setup splits the work:

OPUS 5.5 > writes the plan, then sits idle for the rest of the run
REA > reverse-engineers legacy/billing once and turns it into spec.md with 14 endpoints, 37 rules, 9 edge cases
SONNET 5.5 x12 > one module each, built from a ~1.5k token slice of the spec. none of them opens the old code
REA VERIFY > replays the new system against the legacy one, 148 of 148 behaviours match

on screen the swarm finishes in 7m 05s for $0.88. opus $0.22, REA $0.32, twelve sonnets $0.34. Opus 5.5 alone sits at 23% and $1.20, still running

the video marks that comparison as illustrative, so take the numbers as a demo

REA doesn't come from Anthropic. it's morluto/rea on github, MIT licensed and free, you only pay for the model. one line installs it: npx rea-agents setup

before you point it at client code:
> REA analyzes locally, but your agent still sends tool results to the model provider
> decompiled code from a private app ends up with Anthropic or OpenAI, under their data policy

it's free today. save it before someone sells it as a $500 course