Cache is usually not shared between agents - they can have different base prompts, tools, and be an entirely different model.

The system behaviour is totally up to anthropic's discretion. Its current behaviour is verifiable. In claude code, spawn a subagent with

1. Agent("Test")

2. look at your token usage

3. Repeat a few times

I didn't check again as I type this message but am somewhat sure subagent doesn't cache system prompt as of maybe last week

I'm pretty sure the system instructions are a function of your environment and not the same universally. That said, there should be a finite number of branches so still cacheable.