You can specify different runners to use. The default images are a compromise to keep size down. There is a very large image that tries to include everything you might want. I would suggest trying that if you don’t mind the very large (15GB IIRC) image.
Anecdotally-speaking, I hear that many companies in the LLM agent space roll their own sandbox solutions - I've heard of both Firecracker- and Kubernetes-based implementations.
Apart from that it's been quite good!
The hacky workaround which has been stably working for a while now was to encode the offending request body and decode it on the destination server.
We expect Engine to be as good as the other code-writing agents out there at the moment - we understand almost everyone in the space to be using very similar base models and agent scaffolding.