Measuring Japanese Read-Aloud Speed with AmiVoice Timestamps — A Coaching App That Doesn't Stop at STT-to-Claude

📝 Originally published in Japanese on Zenn. This is the English version. Canonical: https://zenn.dev/uya0526_design/articles/main_article_reading-speed-meter Introduction — What I Built I built a web app that lets you read a Japanese passage aloud, measures your speed and fluency, and has an AI coach return a one-line piece of feedback. 🌐 Demo: https://reading-speed-meter.vercel.app/ 📦 Repository: https://github.com/uya0526-design/reading-speed-meter The flow is simple. You read a passage aloud into the mic (up to 10 seconds) while looking at the script — I prepared the opening lines of two Japanese classics, The Tale of the Heike and Hōjōki — and when you press "Measure," : AmiVoice API recognizes the audio, the code computes your pure speaking speed (characters/min) and stagnation rate from that result, and Claude Haiku returns coaching as "one compliment + one improvement." (Measurement starts on a button press after recording — it never runs automatically.) This article aims to be a single, self-contained piece covering the whole picture, the design decisions, and the reproduction steps . 💡 Where this sits in my journey I'm an ex-Java engineer learning TypeScript and Python in public. This was my first project where I deliberately adopted "AI-collaborative development" as a clear mode. Throughout, I'll drop in comparisons to Java — hopefully useful for anyone coming from a similar background. What You'll Get From This Article How to design evaluation logic that fully exploits the per-word timestamps AmiVoice returns How to build a BFF (Backend for Frontend) so API keys never reach the browser A "two-stage" design where code does the math and Claude Haiku only does the wording First-hand findings you only learn by verifying — e.g., "I thought I'd optimized it, but it wasn't actually working" I include concrete endpoints, parameters, and environment variables so you can reproduce it yourself. My Learning Style (AI Transparency) 💡 Learning companions & how this art