diff --git a/core/src/linuxcnc_rs274_backend.cpp b/core/src/linuxcnc_rs274_backend.cpp
index 54c911a..47fd35c 100644
--- a/core/src/linuxcnc_rs274_backend.cpp
+++ b/core/src/linuxcnc_rs274_backend.cpp
@@ -183,8 +183,14 @@ int parse_linuxcnc_rs274_backend(CanonEventSink &sink,
         return -1;
     }
 
+    // LinuxCNC source basis: rs274ngc_pre.cc uses
+    // RS274NGC_PARAMETER_FILE_NAME_DEFAULT when the external parameter file
+    // name is empty. Reset the bridge every parse so a prior native env
+    // override does not leak into later default parses.
     if (const char *parameter_file = std::getenv("CNC_SIM_RS274_VAR")) {
         SET_PARAMETER_FILE_NAME(parameter_file);
+    } else {
+        SET_PARAMETER_FILE_NAME("rs274ngc.var");
     }
     cnc_sim_init_minimal_linuxcnc_tooldata();
     sink.clear_callback_status();
diff --git a/core/tests/cnc_sim_api_smoke.cpp b/core/tests/cnc_sim_api_smoke.cpp
index cf4b770..53c3d86 100644
--- a/core/tests/cnc_sim_api_smoke.cpp
+++ b/core/tests/cnc_sim_api_smoke.cpp
@@ -6163,6 +6163,48 @@ int main() {
                      "expected LinuxCNC rotary/UVW persistent numbered parameters to reload from parameter file");
     }
 
+    const std::string parameter_env_leak_guard_var_path = temp_dir.file("parameter_env_leak_guard.var");
+    {
+        std::ofstream var_file(parameter_env_leak_guard_var_path);
+        var_file << "5001\t17.000000\n";
+    }
+    {
+        ScopedEnv var_env("CNC_SIM_RS274_VAR", parameter_env_leak_guard_var_path.c_str());
+        const char parameter_env_leak_guard_set_program[] =
+            "#5001 = 91\n"
+            "M30\n";
+        events.clear();
+        ok &= expect(cnc_sim_parse_program(sim,
+                                           parameter_env_leak_guard_set_program,
+                                           sizeof(parameter_env_leak_guard_set_program) - 1) == 0,
+                     cnc_sim_last_error(sim));
+    }
+    {
+        const char parameter_env_default_check_program[] =
+            "F100\n"
+            "O10 if [#5001 EQ 91]\n"
+            "G1 X1\n"
+            "O10 endif\n";
+        events.clear();
+        ok &= expect(cnc_sim_parse_program(sim,
+                                           parameter_env_default_check_program,
+                                           sizeof(parameter_env_default_check_program) - 1) == 0,
+                     cnc_sim_last_error(sim));
+        bool saw_stale_env_parameter = false;
+        for (const auto &event : events) {
+            saw_stale_env_parameter = saw_stale_env_parameter ||
+                                      (event.type == CNC_SIM_EVENT_LINEAR_FEED &&
+                                       event.line == 3 &&
+                                       near(event.end.x, 1.0));
+        }
+        // LinuxCNC source basis: rs274ngc_pre.cc rs274_ngc_init() falls back
+        // to RS274NGC_PARAMETER_FILE_NAME_DEFAULT when the external parameter
+        // file name is empty. The project env override must not persist after
+        // CNC_SIM_RS274_VAR is unset.
+        ok &= expect(!saw_stale_env_parameter,
+                     "expected LinuxCNC parameter env override not to leak into default parses");
+    }
+
     const char volatile_global_set_program[] =
         "#31 = 7\n"
         "M30\n";
diff --git a/test-native.sh b/test-native.sh
index f0668e1..8f2892d 100755
--- a/test-native.sh
+++ b/test-native.sh
@@ -79,6 +79,8 @@ grep -F 'metadata:src/emc/task/taskclass.cc:source basis for wasm-safe builtin P
 grep -F 'metadata:src/emc/task/emccanon.cc:source basis for wasm-safe tooldata API declaration shim' linuxcnc-rs274-wasm-source-files.txt >/dev/null
 grep -F 'metadata:src/emc/sai/dummyemcstat.cc:source basis for wasm-safe emcStatus singleton shim' linuxcnc-rs274-wasm-source-files.txt >/dev/null
 grep -F 'blocked:src/emc/rs274ngc/gcodemodule.cc:Python gcode module binding, tracked for full rs274ngc source coverage' linuxcnc-rs274-wasm-source-files.txt >/dev/null
+grep -F 'expected LinuxCNC parameter env override not to leak into default parses' core/tests/cnc_sim_api_smoke.cpp >/dev/null
+grep -F 'SET_PARAMETER_FILE_NAME("rs274ngc.var")' core/src/linuxcnc_rs274_backend.cpp >/dev/null
 grep -F 'metadata:src/emc/nml_intf/emcops.cc:source basis for native source-link EMC status constructor support object' linuxcnc-rs274-source-files.txt >/dev/null
 grep -F 'metadata:src/emc/sai/dummyemcstat.cc:source basis for native source-link emcStatus singleton support object' linuxcnc-rs274-source-files.txt >/dev/null
 grep -F 'metadata:src/emc/sai/Submakefile:LinuxCNC SAI build metadata defining dummyemcstat support object' linuxcnc-rs274-source-files.txt >/dev/null