I'd guess it's because they don't want people to reverse engineer it.

Note that they're the only provider which doesn't make their tokenizer available offline as a library (i.e. the only provider whose tokenizer is secret).

Anthropic is somewhat becoming the Apple of AI in terms of closed ecosystem. Not saying I blame them, I just don't like it as a customer.

The fact that it's impossible to get the actual thinking tokens anymore, but we have to do with a rewritten summary, is extremely off-putting. I understand that it's necessary for users, but when writing agentic applications yourself, it's super annoying not to have the actual reasoning of the agent to understand failure modes.