diff --git a/app/celer-g4/GlobalSetup.cc b/app/celer-g4/GlobalSetup.cc
index 8cd2e2c77e..ceb65b87c7 100644
--- a/app/celer-g4/GlobalSetup.cc
+++ b/app/celer-g4/GlobalSetup.cc
@@ -133,6 +133,16 @@ void GlobalSetup::ReadInput(std::string const& filename)
         CELER_ASSERT(instream);
         nlohmann::json::parse(*instream).get_to(input_);
 
+        if (input_.cuda_stack_size != RunInput::unspecified)
+        {
+            options_->cuda_stack_size = input_.cuda_stack_size;
+        }
+        if (input_.cuda_heap_size != RunInput::unspecified)
+        {
+            options_->cuda_heap_size = input_.cuda_heap_size;
+        }
+        celeritas::environment().merge(input_.environ);
+
         if constexpr (CELERITAS_CORE_GEO == CELERITAS_CORE_GEO_ORANGE)
         {
             static char const fi_hack_envname[] = "ORANGE_FORCE_INPUT";
@@ -179,8 +189,6 @@ void GlobalSetup::ReadInput(std::string const& filename)
         options_->sd.enabled = input_.sd_type != SensitiveDetectorType::none;
         options_->slot_diagnostic_prefix = input_.slot_diagnostic_prefix;
 
-        options_->cuda_stack_size = input_.cuda_stack_size;
-        options_->cuda_heap_size = input_.cuda_heap_size;
         options_->action_times = input_.action_times;
         options_->default_stream = input_.default_stream;
         options_->track_order = input_.track_order;
diff --git a/app/celer-g4/RunInput.hh b/app/celer-g4/RunInput.hh
index 98bf858872..18378d4ed1 100644
--- a/app/celer-g4/RunInput.hh
+++ b/app/celer-g4/RunInput.hh
@@ -12,10 +12,18 @@
 #include "corecel/Types.hh"
 #include "corecel/cont/Array.hh"
 #include "corecel/sys/Device.hh"
+#include "corecel/sys/Environment.hh"
 #include "celeritas/ext/GeantPhysicsOptions.hh"
 #include "celeritas/field/FieldDriverOptions.hh"
 #include "celeritas/phys/PrimaryGeneratorOptions.hh"
 
+#ifdef _WIN32
+#    include <cstdlib>
+#    ifdef environ
+#        undef environ
+#    endif
+#endif
+
 namespace celeritas
 {
 namespace app
@@ -54,8 +62,9 @@ struct RunInput
     static constexpr size_type unspecified{static_cast<size_type>(-1)};
 
     // Global environment
-    size_type cuda_stack_size{};
-    size_type cuda_heap_size{};
+    size_type cuda_stack_size{unspecified};
+    size_type cuda_heap_size{unspecified};
+    Environment environ;  //!< Supplement existing env variables
 
     // Problem definition
     std::string geometry_file;  //!< Path to GDML file
diff --git a/app/celer-g4/RunInputIO.json.cc b/app/celer-g4/RunInputIO.json.cc
index 3e05ce117a..00054f116d 100644
--- a/app/celer-g4/RunInputIO.json.cc
+++ b/app/celer-g4/RunInputIO.json.cc
@@ -12,6 +12,7 @@
 #include "corecel/io/Logger.hh"
 #include "corecel/io/StringEnumMapper.hh"
 #include "corecel/sys/Environment.hh"
+#include "corecel/sys/EnvironmentIO.json.hh"
 #include "celeritas/Types.hh"
 #include "celeritas/TypesIO.hh"
 #include "celeritas/ext/GeantPhysicsOptionsIO.json.hh"
@@ -73,6 +74,10 @@ void from_json(nlohmann::json const& j, RunInput& v)
     // Check version (if available)
     check_format(j, "celer-g4");
 
+    RI_LOAD_OPTION(cuda_heap_size);
+    RI_LOAD_OPTION(cuda_stack_size);
+    RI_LOAD_REQUIRED(environ);
+
     RI_LOAD_REQUIRED(geometry_file);
     RI_LOAD_OPTION(event_file);
 
@@ -172,6 +177,10 @@ void to_json(nlohmann::json& j, RunInput const& v)
     // Save version and format type
     save_format(j, "celer-g4");
 
+    RI_SAVE_OPTION(cuda_stack_size);
+    RI_SAVE_OPTION(cuda_heap_size);
+    RI_SAVE(environ);
+
     RI_SAVE(geometry_file);
     RI_SAVE_OPTION(event_file);
 
@@ -184,8 +193,6 @@ void to_json(nlohmann::json& j, RunInput const& v)
     RI_SAVE_OPTION(max_steps);
     RI_SAVE(initializer_capacity);
     RI_SAVE(secondary_stack_factor);
-    RI_SAVE_OPTION(cuda_stack_size);
-    RI_SAVE_OPTION(cuda_heap_size);
     RI_SAVE(action_times);
     RI_SAVE(default_stream);
     RI_SAVE(auto_flush);