diff --git a/run_pbwo4.cc b/run_pbwo4.cc
index 1754eb1..22e3895 100644
--- a/run_pbwo4.cc
+++ b/run_pbwo4.cc
@@ -1,6 +1,7 @@
 #include "GeometryDescriptor.hh"
 #include "G4System.hh"
 
+#include <cstdlib>
 #include <iostream>
 #include <string>
 #include <vector>
@@ -8,6 +9,18 @@
 int main(int argc, char** argv) {
     int nEvents = 10;
 
+    int seed = -1;
+    if (const char* seedEnv = std::getenv("MINICALOSIM_SEED")) {
+        try {
+            std::size_t pos = 0;
+            std::string seedStr(seedEnv);
+            int parsed = std::stoi(seedStr, &pos);
+            if (pos == seedStr.size()) seed = parsed;
+        } catch (const std::exception&) {
+            // Not a valid integer; fall back to time-based seed.
+        }
+    }
+
     if (argc > 2) {
         std::cerr << "Usage: run_pbwo4 [nEvents]" << std::endl;
         return 1;
@@ -28,7 +41,7 @@ int main(int argc, char** argv) {
     gd.addLayer(20.0, "G4_PbWO4", true, 10, 10);
 
     G4System g4;
-    g4.init(gd, -1);
+    g4.init(gd, seed);
     g4.run_batch(nEvents, {"e-"}, 1.0, 1.0, outfile);
 
     std::cout << "Saved " << nEvents << " events to " << outfile << std::endl;
diff --git a/run_sampling.cc b/run_sampling.cc
index 1f329c6..b2da2b1 100644
--- a/run_sampling.cc
+++ b/run_sampling.cc
@@ -1,6 +1,7 @@
 #include "GeometryDescriptor.hh"
 #include "G4System.hh"
 
+#include <cstdlib>
 #include <iostream>
 #include <stdexcept>
 #include <string>
@@ -111,12 +112,24 @@ int main(int argc, char** argv) {
         return 1;
     }
 
+    int seed = -1;
+    if (const char* seedEnv = std::getenv("MINICALOSIM_SEED")) {
+        try {
+            std::size_t pos = 0;
+            std::string seedStr(seedEnv);
+            int parsed = std::stoi(seedStr, &pos);
+            if (pos == seedStr.size()) seed = parsed;
+        } catch (const std::exception&) {
+            // Not a valid integer; fall back to time-based seed.
+        }
+    }
+
     GeometryDescriptor gd = builder();
 
     std::string outfile = "sampling_" + configName + "_" + std::to_string(nEvents) + "events_hits.root";
 
     G4System g4;
-    g4.init(gd, -1);
+    g4.init(gd, seed);
     g4.run_batch(nEvents, {"e-"}, 1.0, 1.0, outfile);
 
     std::cout << "Saved " << nEvents << " events to " << outfile << std::endl;