AI models can multitask/use parallel subagents just fine; the issue is with harnesses that don't make it a priority via the default system prompt.

I run an LLM server with Qwen 3.6 in the office, and OpenCode, which the OP mentioned, usually defaults to sequential TODO lists, and it works fine with our little LLM server with 3-4 parallel users. But I noticed that once in a while the LLM got overloaded with requests in the queue, and you couldn't do anything for 20-30 minutes. My investigation led me to an employee who used QwenCode. I tried it myself then, and indeed, it immediately launched something like 6 parallel subagents, where OpenCode would have sequential TODOs with the same model by default.

So in the end, I had to detect QwenCode on the server side and serialize all its parallel requests into a single request queue, because it made life miserable for other OpenCode users :)