I use Maki[1] harness and GML-5.3, GLM5.3 Flash and Deepseek-4.1 Flash 1017, with a little bit of MiMo-2.6.

The key I've found is a strong skill. At the beginning, sure there were problems. However, I implemented an AGENTS rule that said if a Janet script failed more than twice on the same "gotcha", the correct syntax was added to the skill. The skill is regularly reviewed, consolidated and pruned. The Janet skill is a floor and loaded at the start of every session.

Edit: Also worth mentioning is that I developed a Janet module with runtime type guards and other helpers to encode the "gotchas". https://github.com/jennystats/janet-kit

[1] https://maki.sh

Agents are very good at dealing with less used languages as long as you have skills defined for each. That's really the trick.