Built my own coding agent harness and sharing some highlights
Hi all,
I came into a journey of building a coding harness to learn + experiment and to see if I can adapt to my needs: as a "local AI" user familiar with ollama, llama.cpp and vllm, was thinking about the time I would stop my CC subscription and only play with open weight llms.
So, in order to start from something, I took opencode as a reference (well known for local AI coding) and started learning basics of tool loop, permissions, compaction etc.. So took it aswell as a reference in order to structure a minimum my new project.
But I fastly came into my first real design choice: typescript and TUI (as the tendancy) or python + webui? Choosen the last one because:
- I needed controllability
- I needed to add cool features (see below)
- It's not a problem for my usecase to have vs code separated
During the building, I came into others questions: How to preserve context? Do I keep plan agent? Let the user create its own and how? Which providers, only local or openai compatible or full providers compat? Are subagents really usefull?
So for these questions, I had to do a lot of tests + benchmarking (SWE-verified against opencode) in order to really feel the impact of these stuff with "small" models (Qwen3.6, gemma 4). So I ended up with these choices:
- yes subagents are usefull and I spawn them via the tool calls but they work better when parallel calls are allowed by the inference endpoint
- keep plan agent as these models have tendancy to not surface enough for complex tasks
- openai compatble: do not want to mess with others plans and still local + cloud
- try to reduce as most as possible system prompt + tool schemas footprints in context without loosing quality because instructions really have an impact on the model behaviour (at least on these models) => ended with a total footprint of 3.4k tokens
Once the harness was providing results I was expecting, I then came into the fun parts: a webui + python allows a lot of built-in features (the challenge was to keep the experience simple):
- while not a TUI, a file explorer and possibility to select lines to add them in the llm context + diff viewer files modified/created
- browser autmation: allows web navigation through DOM (accessibilitry tree) and + visual grounding (if conditions are met). The result is cool so included the browser view (periodically screenshotted) inside the UI:
- The natural features following browser automation were obviously the skills and jobs so now I can just guide the agent to navigate on the internet only once then click on Create skill to see a form automatically prefilled by the llm so it will be able to execute "offline" at any time. Can be usefull for daily tasks and project webui tests.
Now I am quite satisfied and plan to improve it in the future. If you want to give a try, please have a look at https://github.com/leflakk/openclose, any feedback or discussion about coding agent tools are welcome!
