diff --git a/cp-algo/algebra/modint.hpp b/cp-algo/algebra/modint.hpp
index f9928bb..ce9195f 100644
--- a/cp-algo/algebra/modint.hpp
+++ b/cp-algo/algebra/modint.hpp
@@ -5,7 +5,7 @@
 namespace cp_algo::algebra {
     template<typename modint>
     struct modint_base {
-        static int mod() {
+        static int64_t mod() {
             return modint::mod();
         }
         modint_base(): r(0) {}
@@ -20,7 +20,11 @@ namespace cp_algo::algebra {
             return to_modint() *= t.inv();
         }
         modint& operator *= (const modint &t) {
-            r *= t.r; if(mod()) {r %= mod();}
+            if(mod() <= uint32_t(-1)) {
+                r = r * t.r % mod();
+            } else {
+                r = __int128(r) * t.r % mod();
+            }
             return to_modint();
         }
         modint& operator += (const modint &t) {
@@ -36,11 +40,10 @@ namespace cp_algo::algebra {
         modint operator * (const modint &t) const {return modint(to_modint()) *= t;}
         modint operator / (const modint &t) const {return modint(to_modint()) /= t;}
         auto operator <=> (const modint_base &t) const = default;
-        explicit operator int() const {return r;}
         int64_t rem() const {return 2 * r > (uint64_t)mod() ? r - mod() : r;}
 
         // Only use if you really know what you're doing!
-        uint64_t modmod() const {return 8LL * mod() * mod();};
+        uint64_t modmod() const {return 8ULL * mod() * mod();};
         void add_unsafe(uint64_t t) {r += t;}
         void pseudonormalize() {r = std::min(r, r - modmod());}
         modint const& normalize() {
@@ -65,21 +68,30 @@ namespace cp_algo::algebra {
         return out << x.getr();
     }
 
-    template<int m>
+    template<int64_t m>
     struct modint: modint_base<modint<m>> {
-        static constexpr int mod() {return m;}
+        static constexpr int64_t mod() {return m;}
         using Base = modint_base<modint<m>>;
         using Base::Base;
     };
 
     struct dynamic_modint: modint_base<dynamic_modint> {
-        static int mod() {return m;}
-        static void switch_mod(int nm) {m = nm;}
+        static int64_t mod() {return m;}
+        static void switch_mod(int64_t nm) {m = nm;}
         using Base = modint_base<dynamic_modint>;
         using Base::Base;
+
+        // Wrapper for temp switching
+        auto static with_switched_mod(int64_t tmp, auto callback) {
+            auto prev = mod();
+            switch_mod(tmp);
+            auto res = callback();
+            switch_mod(prev);
+            return res;
+        }
     private:
-        static int m;
+        static int64_t m;
     };
-    int dynamic_modint::m = 0;
+    int64_t dynamic_modint::m = 0;
 }
 #endif // CP_ALGO_ALGEBRA_MODINT_HPP
diff --git a/cp-algo/algebra/number_theory.hpp b/cp-algo/algebra/number_theory.hpp
index e5f27b3..3e5ce0f 100644
--- a/cp-algo/algebra/number_theory.hpp
+++ b/cp-algo/algebra/number_theory.hpp
@@ -27,5 +27,44 @@ namespace cp_algo::algebra {
             }
         }
     }
+
+    template<typename base>
+    requires(std::is_base_of_v<modint_base<base>, base>)
+    bool is_prime_mod() {
+        auto m = base::mod();
+        if(m == 1 || m % 2 == 0) {
+            return m == 2;
+        }
+        auto m1 = m - 1;
+        int d = 0;
+        while(m1 % 2 == 0) {
+            m1 /= 2;
+            d++;
+        }
+        auto test = [&](auto x) {
+            x = bpow(x, m1);
+            if(x == 0 || x == 1 || x == -1) {
+                return true;
+            }
+            for(int i = 0; i <= d; i++) {
+                if(x == -1) {
+                    return true;
+                }
+                x *= x;
+            }
+            return false;
+        };
+        for(base b: {2, 325, 9375, 28178, 450775, 9780504, 1795265022}) {
+            if(!test(b)) {
+                return false;
+            }
+        }
+        return true;
+    }
+    bool is_prime(int64_t m) {
+        return dynamic_modint::with_switched_mod(m, [](){
+            return is_prime_mod<dynamic_modint>();
+        });
+    }
 }
 #endif // CP_ALGO_ALGEBRA_NUMBER_THEORY_HPP
diff --git a/verify/number_theory/primality.test.cpp b/verify/number_theory/primality.test.cpp
new file mode 100644
index 0000000..3a76db6
--- /dev/null
+++ b/verify/number_theory/primality.test.cpp
@@ -0,0 +1,26 @@
+// @brief Primality Test
+#define PROBLEM "https://judge.yosupo.jp/problem/primality_test"
+#pragma GCC optimize("Ofast,unroll-loops")
+#pragma GCC target("avx2,tune=native")
+#include "cp-algo/algebra/number_theory.hpp"
+#include <bits/stdc++.h>
+
+using namespace std;
+using namespace cp_algo::algebra;
+
+void solve() {
+    int64_t m;
+    cin >> m;
+    cout << (is_prime(m) ? "Yes" : "No") << "\n";
+}
+
+signed main() {
+    //freopen("input.txt", "r", stdin);
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+    int t = 1;
+    cin >> t;
+    while(t--) {
+        solve();
+    }
+}