It is not at all a small app, at least as far as UX surface area. There are, what, 40ish slash commands? Each one is an opportunity for bugs and feature gaps.
The complex and magic parts are around finding contextual things to include, and I'd be curious how many are that vs "forgot to call clear() in the TUI framework before redirecting to another page".
Cue I could build it in a weekend vibes, I built my own agent TUI using the OpenAI agent SDK and Ink. Of course it’s not as fleshed out as Claude, but it supports git work trees for multi agent, slash commands, human in the loop prompts and etc. If I point it at the Anthropic models it more or less produces results as m good as the real Claude TUI.
I actually “decompiled” the Claude tools and prompts and recreated them. As of 6 months ago Claude was 15 tools, mostly pretty basic (list for, read file, wrote file, bash, etc) with some very clever prompts, especially the task tool it uses to do the quasi planning mode task bullets (even when not in planning mode).
Honestly the idea of bringing this all together with an affordable monthly service and obviously some seriously creative “prompt engineers” is the magic/hard part (and making the model itself, obviously).