diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index 911274b..f7709a7 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -271,6 +271,24 @@ CNC_SIM_RS274_FILE_MODE=1 CNC_SIM_RS274_VAR="$var_file" \
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_tool_number.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_tool_number.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_convert.cc
+# convert_tool_select() and convert_m() M61 both route through
+# src/emc/rs274ngc/interp_find.cc find_tool_index() for tooldata lookup.
+tool_number_error_cases=(
+    "linuxcnc_t_missing_tool_error|Requested tool 99 not found in the tool table"
+    "linuxcnc_m61_missing_tool_error|Requested tool 99 not found in the tool table"
+)
+for case in "${tool_number_error_cases[@]}"; do
+    fixture=${case%%|*}
+    expected=${case#*|}
+    if CNC_SIM_RS274_VAR="$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
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_program_stops.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_program_stops.json"
diff --git a/test-native.sh b/test-native.sh
index 506187a..f63bb8f 100755
--- a/test-native.sh
+++ b/test-native.sh
@@ -147,6 +147,7 @@ grep -F 'tests/gcode/linuxcnc_lathe_diameter_g76.ngc' docs/linuxcnc-porting.md >
 grep -F 'missing source-linked G7 diameter-mode G76 final threading pass' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'convert_tool_length_offset() rejects invalid G43.2 H/axis combinations' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'linuxcnc_g43_2_missing_h_axis_error|G43.2: No axes specified and H word missing' test-linuxcnc-source-link.sh >/dev/null
+grep -F 'convert_tool_select() and convert_m() M61 both route through' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'read_atsign()' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'tests/gcode/linuxcnc_polar_coordinates.ngc' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'tests/gcode/linuxcnc_polar_coordinate_variants.ngc' test-linuxcnc-source-link.sh >/dev/null
diff --git a/tests/gcode/linuxcnc_m61_missing_tool_error.ngc b/tests/gcode/linuxcnc_m61_missing_tool_error.ngc
new file mode 100644
index 0000000..761f708
--- /dev/null
+++ b/tests/gcode/linuxcnc_m61_missing_tool_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+M61 Q99
+M30
diff --git a/tests/gcode/linuxcnc_t_missing_tool_error.ngc b/tests/gcode/linuxcnc_t_missing_tool_error.ngc
new file mode 100644
index 0000000..357aaab
--- /dev/null
+++ b/tests/gcode/linuxcnc_t_missing_tool_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+T99
+M30