Two years ago I open-sourced KeyEcho, a small desktop app that plays a mechanical-keyboard sound the instant you press a key. It got 800+ stars. Then I shipped v0.0.5 in July 2024 and went quiet.
The issues never stopped. People asked for sound packs, reported platform bugs, and kept using a thing I had stopped maintaining. This month I came back and shipped 1.0 in a single PR: 130 files, 11,405 lines added and 9,292 removed. The core of it was a rebuild of the audio hot path. The cached-lookup microbenchmark went from 1184.07 ns/op with 66.84 KiB copied per key to 43.50 ns/op with zero sample bytes copied. That is 27x on the average slice and 38x on the largest.
I built it with an AI agent writing a lot of the code. This post is how the rework works, why Rust made that safe to do fast, and the one change the agent suggested that looked clean and would have quietly broken a feature.
The hot path
KeyEcho's latency-sensitive path is short. A global keyboard hook fires on every key event. The first key-down of each press is pushed into a bounded queue. An audio thread pulls from the queue, maps the key to a slice of the selected sound pack, and plays it locally.
