Isn't this the whole reason they became viable in the last 6 months? The system prompt and harness is improving. It's less and less essential every day to roll your own.
The models also have far more intelligence built in. For example, the pi.dev agent harness has a system prompt which fits on a single page, and includes only 4 or 5 tools. Running with a small coding model like Qwen3.6 27B, this setup is completely capable of agentic coding.
They still aren't viable. Nothing changed within the last 6 months.
I don't think there is a single reason. Models are improving, so are the harnesses, prompts and we who use them a lot also get more proficient and learn where they can be used effectively vs not, so lots of improvements all over the ecosystem, brought together.
Latest big change is probably how feasible local models are becoming, like Qwen 3.6 and Gemma 4, they're no longer easily getting stuck in loops and repetition, although on lower quantizations they still pretty much suck for agentic usage.