Spec contract tests for a Rust token bucket

You wrote a token bucket rate limiter in Rust. Each key has a bucket of capacity tokens that refills at refill_per_sec, and allow(key) spends 1 token or returns false. An agent will write its test suite.

Write the prompt you would give the agent for the tests.

What you know:

• the public API is Limiter::new(capacity: u32, refill_per_sec: f64) and allow(&self, key: &str) -> bool
• inside, it reads the time with Instant::now()
• buckets start full, and many threads call allow at once
• you can add dev-dependencies

Then, in a few sentences: what does a vague prompt get most wrong here?