Deux: Swift → Kotlin — Harness-Driven Cross-Platform Code Conversion

Original: Tweet · Hwang (@hwwaanng) · April 3, 2026 Engagement: 655 likes · 74 retweets · 142K views · 887 bookmarks Category: Practice / Case Study

Overview

Hwang built a harness system that can automatically convert any Swift codebase into a native Kotlin Android project. Users simply start the app and wait a few hours.

"Many people still underestimate the capabilities of today's models. And they underestimate Harness Engineering."

How It Works

Start App
  → AI inspects code, inspects interactions
    → Takes notes, writes tests
      → Continuously spawns Sub Agents to accelerate
        → Self-validates, self-iterates
          → Output: A highly usable Android App

Key features:

Self-validation: The agent actually runs and tests the converted code
Self-iteration: Automatically fixes issues without human intervention
Sub Agent acceleration: Automatically creates sub-agents to process different modules in parallel
Fully unattended: Completes autonomously within hours of starting

Why It Matters

This is one of the benchmark cases for Harness Engineering in practice:

Proves that harness design matters more than model selection — The same model cannot complete cross-platform conversion without this harness
Demonstrates the value of multi-agent in real scenarios — Not a demo, but a deliverable product
Self-validation loops are key — Echoes Anthropic's Generator + Evaluator pattern