But is there anything preventing them from putting their own proprietary wolfram alpha/prolog/super duper expert system in there?

I guess... but I think, at its core, a good coding harness usually includes:

- well-crafted system prompt that follows best practices

- good contextual reminder prompts (when an llm got stuck in an infinite loop and times out, forgets how to use tools, or needs recurring best practice reminders, etc)

- well-written ergonomic tools the llm can use (read/write files, read diffs, browse the internet, etc)

I dont think these are anything special. The deepest moat I can think of is, proprietary models can be specifically trained to use their proprietary harnesses, so they are more token-efficient and make less tool call and file editing mistakes.

However in my experience, I'm as comfortable working with my own homemade harness as with Claude Code, so I don't think it's a deep moat...

Only that it would just slow down the model and make it dumber.

You can't tool and harness a weak model into strength and you probably don't improve top models with boondoggles.