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Right, you can’t actually guess a letter (byte) at a time but you can guess a token at a time (I believe the vocabulary is 200000 possible tokens in gpt 5) So you could send each of the 200000 possible tokens, see which is cached, and then send 200000 more tokens to find the next cached token Certainly less efficient but well within the realm of a feasible attack

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It's a good call out re: tokens vs letters, but I think you might have misunderstood my point - you can't do it a token at a time unless the intermediate KV cache is stored after each token is generated.

This won't be the case in any non toy implementation, as it would be unneccessary and slow.

Ah, fair enough. Anthropic caches at a block level (basically a single message) so for non-trivial messages this is really less of a concern, although I definitely understand why they still scope cache to a single tenant

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