Rewrite a finance backend from .NET to Hono with Claude Code under a parity harness

Try it yourself

What you get

A backend rewrite where AI writes the code and a recorded parity harness decides when each route is actually done.

Tools

• Claude Code
• Cursor
• Codex
• Hono
• Existing .NET backend as oracle

Time / cost: Author: 36 days (Sep 4 → Oct 9, 2026)

Steps

1. Freeze routes, status codes, error codes and money types (never JS numbers for money).
2. Build a recorder that captures responses, DB writes and outbound calls from the old system.
3. Port in tiers from low-risk to high-risk; gate each route on replay, not on “files exist”.
4. Cut over with a written rollback; expect agents to cheat toward green—block hand-edited expectations.

Copy-ready prompt

We are migrating our [OLD STACK] backend to [NEW STACK] with exact behavior preservation.
Rules:
1. Same routes, status codes, error codes and pagination fields. No “improvements”.
2. Money fields stay decimal/string, never floating point.
3. For each route: record old response + DB delta + outbound calls; port; replay until equal.
4. If old and new disagree, assume new is wrong unless I approve a numbered discrepancy report.
Start by listing every HTTP operation and a recording plan for side effects (email, payments, analytics).

Prompt written by AgentPlays, based on @ali_ilhami’s post. Check the output yourself and approve before anything is sent or bought.

Watch out: Finance systems. Keep a human on money discrepancies. Token and dollar figures are the author’s own accounting.

Proof / source

Self-reported Author’s Oct 14 article: 269/269 routes, 2,330 cases green at a checkpoint, one launch-night bug; screenshot stats in an earlier Oct 10 post.