I use a custom harness, but I constantly hear good things about Pi.
EDIT: Checked out Pi and Oh-My-Pi. 250k LoC and 1.5M LoC respectively. Sheesh. How's that for minimalism...
Mine's 50. (Lines.)
https://gist.github.com/a-n-d-a-i/bd50aaa4bdb15f9a4cc8176ee3...
Vendored in my own LLM micro library (100 LoC) so it's 150.[0]
There's no parallelism or anything, but I use it for surgical edits and it's much faster and cheaper than the official harnesses for some reason. (Absence of sysprompt bigger than my repo probably helps there...)
[0] This one uses OpenRouter so you can use it with any model, but jerry rigged Codex sub version available on demand :)
---
I also have a ultra turbo bloated version (500 lines... need to strip it down a bit!) which has autorun files (for grep-based context injection), notification (frog croak when agent done, etc.) Watch this space!