The system prompt part is surely cached across all users.

You still pay cache token costs on API calls. Cache cost/token are 90% lower, but you pay it every single turn.

I’m not sure if they let you skip the cache write cost on the first turn. That would imply cross-user caching infrastructure or special casing the default system prompt to give you a discount. Maybe? Away from the computer but you could try a “hello” in a fresh session and see what was billed.

For Anthropic models, yes. But OP was using Claude Code with GPT.

the system prompt still takes up useful space in the context window and steers the model into unnecessary actions and over-thinking patterns