By imposing the use of their harness, they control the system prompt:

> On April 16, we added a system prompt instruction to reduce verbosity. In combination with other prompt changes, it hurt coding quality, and was reverted on April 20. This impacted Sonnet 4.6, Opus 4.6, and Opus 4.7

They can pick the default reasoning effort:

> On March 4, we changed Claude Code's default reasoning effort from high to medium to reduce the very long latency—enough to make the UI appear frozen—some users were seeing in high mode

They can decide what to keep and what to throw out (beyond simple token caching):

> On March 26, we shipped a change to clear Claude's older thinking from sessions that had been idle for over an hour, to reduce latency when users resumed those sessions. A bug caused this to keep happening every turn for the rest of the session instead of just once, which made Claude seem forgetful and repetitive. We fixed it on April 10. This affected Sonnet 4.6 and Opus 4.6

It literally is all in the post.

I don't worry about anything though. It's not my product. I don't work for Anthropic, so I really couldn't care less about anyone else's degraded (or not) experience.

> they control the system prompt

They control the default system prompt. You can change it if you want to.

> They can pick the default reasoning effort

Don't see how it's an obstacle in allowing third party wrappers.

> They can decide what to keep and what to throw out

That's actually a good point. However I still don't think it's an obstacle. If third party wrappers were bad, people simply wouldn't be using them.