While you can specify 1 token and get 1 token back, which is good for single token answers, and also evaluating that token’s probability, there is another limit you’ll find: functions.
When the AI emits functions, it needs enough tokens to write to the tool recipient with a well-formed beginning of a response that the internals of the API can parse. Otherwise you’ll get a 500 server error. About 12 tokens depending on the tool and function name length.