diff --git a/test-linuxcnc-rs274-native.sh b/test-linuxcnc-rs274-native.sh
index d5122a0..af2ce64 100755
--- a/test-linuxcnc-rs274-native.sh
+++ b/test-linuxcnc-rs274-native.sh
@@ -43,6 +43,7 @@ var_file="$build_dir/rs274ngc.var"
 base_var_file="$build_dir/rs274ngc-base.var"
 ini_named_parameter_file="$(pwd)/tests/gcode/linuxcnc_ini_named_parameter.ini"
 disable_g92_persistence_ini_file="$(pwd)/tests/gcode/linuxcnc_disable_g92_persistence.ini"
+disable_fanuc_style_sub_ini_file="$(pwd)/tests/gcode/linuxcnc_disable_fanuc_style_sub.ini"
 output_dir="$build_dir/outputs"
 mkdir -p "$output_dir"
 cp "$linuxcnc_root/tests/halui/jogging/sim.var" "$base_var_file"
@@ -823,6 +824,25 @@ for case in "${oword_reader_error_cases[@]}"; do
     fi
     grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_$fixture.err"
 done
+# Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads
+# [RS274NGC] DISABLE_FANUC_STYLE_SUB, and interp_read.cc rejects
+# Fanuc-style M98/M99 and bare O-word subroutine forms when enabled.
+disable_fanuc_style_error_cases=(
+    "linuxcnc_disable_fanuc_style_m98_error|DISABLE_FANUC_STYLE_SUB set in INI file, but found m98"
+    "linuxcnc_disable_fanuc_style_m99_error|DISABLE_FANUC_STYLE_SUB set in INI file, but found m99"
+    "linuxcnc_disable_fanuc_style_bare_oword_error|DISABLE_FANUC_STYLE_SUB disabled in INI file, but found bare O-word"
+)
+for case in "${disable_fanuc_style_error_cases[@]}"; do
+    fixture=${case%%|*}
+    expected=${case#*|}
+    if INI_FILE_NAME="$disable_fanuc_style_sub_ini_file" CNC_SIM_RS274_FILE_MODE=1 CNC_SIM_RS274_VAR="$var_file" \
+        "$build_dir/linuxcnc_rs274_dump" "tests/gcode/$fixture.ngc" \
+        >"$output_dir/cnc_sim_linuxcnc_$fixture.json" 2>"$output_dir/cnc_sim_linuxcnc_$fixture.err"; then
+        echo "expected $fixture.ngc to fail" >&2
+        exit 1
+    fi
+    grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_$fixture.err"
+done
 # Source basis: LinuxCNC src/emc/rs274ngc/interp_o_word.cc
 # convert_control_functions() rejects unmatched, duplicate, and
 # out-of-context O-word control labels during execution. control_save_offset()
diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index 182e3a9..9657eb9 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -176,6 +176,7 @@ var_file="$build_dir/rs274ngc-source.var"
 base_var_file="$build_dir/rs274ngc-source-base.var"
 ini_named_parameter_file="$(pwd)/tests/gcode/linuxcnc_ini_named_parameter.ini"
 disable_g92_persistence_ini_file="$(pwd)/tests/gcode/linuxcnc_disable_g92_persistence.ini"
+disable_fanuc_style_sub_ini_file="$(pwd)/tests/gcode/linuxcnc_disable_fanuc_style_sub.ini"
 multi_spindle_modes_file="$(pwd)/tests/gcode/linuxcnc_multi_spindle_modes.ini"
 multi_spindle_override_file="$(pwd)/tests/gcode/linuxcnc_multi_spindle_override.ini"
 multi_spindle_threading_file="$(pwd)/tests/gcode/linuxcnc_multi_spindle_threading.ini"
@@ -1355,6 +1356,25 @@ for case in "${oword_reader_error_cases[@]}"; do
     fi
     grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
 done
+# Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads
+# [RS274NGC] DISABLE_FANUC_STYLE_SUB, and interp_read.cc rejects
+# Fanuc-style M98/M99 and bare O-word subroutine forms when enabled.
+disable_fanuc_style_error_cases=(
+    "linuxcnc_disable_fanuc_style_m98_error|DISABLE_FANUC_STYLE_SUB set in INI file, but found m98"
+    "linuxcnc_disable_fanuc_style_m99_error|DISABLE_FANUC_STYLE_SUB set in INI file, but found m99"
+    "linuxcnc_disable_fanuc_style_bare_oword_error|DISABLE_FANUC_STYLE_SUB disabled in INI file, but found bare O-word"
+)
+for case in "${disable_fanuc_style_error_cases[@]}"; do
+    fixture=${case%%|*}
+    expected=${case#*|}
+    if INI_FILE_NAME="$disable_fanuc_style_sub_ini_file" CNC_SIM_RS274_FILE_MODE=1 CNC_SIM_RS274_VAR="$var_file" \
+        "$build_dir/linuxcnc_rs274_source_dump" "tests/gcode/$fixture.ngc" \
+        >"$output_dir/cnc_sim_linuxcnc_source_$fixture.json" 2>"$output_dir/cnc_sim_linuxcnc_source_$fixture.err"; then
+        echo "expected $fixture.ngc to fail" >&2
+        exit 1
+    fi
+    grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
+done
 # Source basis: LinuxCNC src/emc/rs274ngc/interp_o_word.cc
 # convert_control_functions() rejects unmatched, duplicate, and
 # out-of-context O-word control labels during execution.
diff --git a/tests/gcode/linuxcnc_disable_fanuc_style_bare_oword_error.ngc b/tests/gcode/linuxcnc_disable_fanuc_style_bare_oword_error.ngc
new file mode 100644
index 0000000..c850bb5
--- /dev/null
+++ b/tests/gcode/linuxcnc_disable_fanuc_style_bare_oword_error.ngc
@@ -0,0 +1,5 @@
+G21 G90
+(Source basis: LinuxCNC src/emc/rs274ngc/interp_read.cc rejects bare O-word definitions when DISABLE_FANUC_STYLE_SUB is enabled.)
+O100
+M99
+M30
diff --git a/tests/gcode/linuxcnc_disable_fanuc_style_m98_error.ngc b/tests/gcode/linuxcnc_disable_fanuc_style_m98_error.ngc
new file mode 100644
index 0000000..3daeaf7
--- /dev/null
+++ b/tests/gcode/linuxcnc_disable_fanuc_style_m98_error.ngc
@@ -0,0 +1,6 @@
+G21 G90
+(Source basis: LinuxCNC src/emc/rs274ngc/interp_read.cc rejects M98 when DISABLE_FANUC_STYLE_SUB is enabled.)
+M98 P100
+M30
+O100
+M99
diff --git a/tests/gcode/linuxcnc_disable_fanuc_style_m99_error.ngc b/tests/gcode/linuxcnc_disable_fanuc_style_m99_error.ngc
new file mode 100644
index 0000000..a5524bb
--- /dev/null
+++ b/tests/gcode/linuxcnc_disable_fanuc_style_m99_error.ngc
@@ -0,0 +1,7 @@
+G21 G90
+(Source basis: LinuxCNC src/emc/rs274ngc/interp_read.cc rejects Fanuc-style M99 when DISABLE_FANUC_STYLE_SUB is enabled.)
+O100 sub
+M99
+O100 endsub
+O100 call
+M30
diff --git a/tests/gcode/linuxcnc_disable_fanuc_style_sub.ini b/tests/gcode/linuxcnc_disable_fanuc_style_sub.ini
new file mode 100644
index 0000000..052f26f
--- /dev/null
+++ b/tests/gcode/linuxcnc_disable_fanuc_style_sub.ini
@@ -0,0 +1,2 @@
+[RS274NGC]
+DISABLE_FANUC_STYLE_SUB = 1