Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations and no output schema, the description carries the full burden; it does disclose the exact return set (mean, sample std, quartiles, skew, kurtosis), which is genuinely useful. It says nothing about how the alternative string form of the input is interpreted, how missing values are handled, or what happens with empty/constant samples.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.