## [Samau](https://github.com/AlephAlpha/Samau), 2 bytes

 ▌τ

Yes, it's 2 bytes. Samau uses [CP737](https://en.wikipedia.org/wiki/CP737) as its default character encoding.

 ▌ read a number
 τ test if it is a prime

`τ` uses the [`isCertifiedPrime`](http://hackage.haskell.org/package/arithmoi-0.4.1.3/docs/Math-NumberTheory-Primes-Testing.html#v:isCertifiedPrime) function in the haskell package [`arithmoi`](http://hackage.haskell.org/package/arithmoi). It's not a probabilistic algorithm.

---

There's also a 7-bytes answer if built-ins are not allowed:

 ▌;\│Σ2=

Most mathematical functions in Samau automatically thread over lists.

 ▌ read a number, let's call it n
 ; duplicate
 \ range from 1 to n
 | return 1 for those divisible by n, and 0 for the others
 Σ take the sum
 =2 if the sum is 2, then it's a prime