diff --git a/docs/linuxcnc-porting.md b/docs/linuxcnc-porting.md
index ab03b01..bcb24ec 100644
--- a/docs/linuxcnc-porting.md
+++ b/docs/linuxcnc-porting.md
@@ -200,7 +200,7 @@ This matrix tracks LinuxCNC feature coverage for the web/WASM simulator. A featu
 | Spindle, coolant, program stops, and current tool number `M3/M4/M5`, `M7/M8/M9`, `M0/M1/M60/M30`, `M61 Q...` | covered through LinuxCNC native/source backends, including source-linked `convert_m()` rejection of out-of-range `M3/M4/M5 $` spindle selectors and negative `M61 Q` | `tests/gcode/linuxcnc_spindle_direction.ngc`, `tests/gcode/linuxcnc_coolant.ngc`, `tests/gcode/linuxcnc_program_stops.ngc`, `tests/gcode/linuxcnc_tool_number.ngc`, `tests/gcode/linuxcnc_m3_spindle_dollar_out_of_range_error.ngc`, `tests/gcode/linuxcnc_m4_spindle_dollar_out_of_range_error.ngc`, `tests/gcode/linuxcnc_m5_spindle_dollar_out_of_range_error.ngc`, `tests/gcode/linuxcnc_m61_negative_q_error.ngc` |
 | Spindle speed mode `G96/G97` | covered with Canon `SET_SPINDLE_MODE` preserved as mode state events, including `G96 D...` max-RPM value and source-linked `check_other_codes()` rejection of missing `S` with `G96` | `tests/gcode/linuxcnc_spindle_modes.ngc`, `tests/gcode/linuxcnc_g96_missing_s_word_error.ngc` |
 | Spindle orientation `M19 R... P... Q...` | covered with Canon `ORIENT_SPINDLE` and `WAIT_SPINDLE_ORIENT_COMPLETE` preserved as temporary mode state events, including source-linked `check_other_codes()` validation for `P/R` words and `convert_m()` rejection of out-of-range `$` selectors and nonpositive `Q` waits | `tests/gcode/linuxcnc_spindle_orient.ngc`, `tests/gcode/linuxcnc_m19_spindle_dollar_out_of_range_error.ngc`, `tests/gcode/linuxcnc_m19_p_not_integer_error.ngc`, `tests/gcode/linuxcnc_m19_p_out_of_range_error.ngc`, `tests/gcode/linuxcnc_m19_r_out_of_range_error.ngc`, `tests/gcode/linuxcnc_m19_zero_q_error.ngc` |
-| Spindle-synchronized feed and threading `G33/G33.1/G76` | covered through smoke, LinuxCNC native, and source-linked paths with Canon `START_SPEED_FEED_SYNCH`/`STOP_SPEED_FEED_SYNCH` preserved, rigid-tap line numbers normalized, G76 multi-pass threading smoke-covered, and source-linked `check_other_codes()` validation for required/disallowed `K/F/P/I/J` words | `tests/gcode/linuxcnc_threading_sync.ngc`, `tests/gcode/linuxcnc_threading_cycle.ngc`, `tests/gcode/linuxcnc_g33_missing_k_word_error.ngc`, `tests/gcode/linuxcnc_g33_f_word_error.ngc`, `tests/gcode/linuxcnc_g33_1_missing_k_word_error.ngc`, `tests/gcode/linuxcnc_g33_1_f_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_p_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_i_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_j_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_k_word_error.ngc`, smoke API regression |
+| Spindle-synchronized feed and threading `G33/G33.1/G76` | covered through smoke, LinuxCNC native, and source-linked paths with Canon `START_SPEED_FEED_SYNCH`/`STOP_SPEED_FEED_SYNCH` preserved, rigid-tap line numbers normalized, G76 multi-pass threading smoke-covered, source-linked `check_other_codes()` validation for required/disallowed `K/F/P/I/J` words, and source-linked `convert_straight()`/`convert_threading_cycle()` rejection of invalid spindle state, spindle selector, rotary-axis motion, and G76 `I/J/K` geometry | `tests/gcode/linuxcnc_threading_sync.ngc`, `tests/gcode/linuxcnc_threading_cycle.ngc`, `tests/gcode/linuxcnc_g33_missing_k_word_error.ngc`, `tests/gcode/linuxcnc_g33_f_word_error.ngc`, `tests/gcode/linuxcnc_g33_1_missing_k_word_error.ngc`, `tests/gcode/linuxcnc_g33_1_f_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_p_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_i_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_j_word_error.ngc`, `tests/gcode/linuxcnc_g76_missing_k_word_error.ngc`, `tests/gcode/linuxcnc_g33_stopped_spindle_error.ngc`, `tests/gcode/linuxcnc_g33_1_stopped_spindle_error.ngc`, `tests/gcode/linuxcnc_g33_spindle_dollar_out_of_range_error.ngc`, `tests/gcode/linuxcnc_g33_1_spindle_dollar_out_of_range_error.ngc`, `tests/gcode/linuxcnc_g76_spindle_dollar_out_of_range_error.ngc`, `tests/gcode/linuxcnc_g76_stopped_spindle_error.ngc`, `tests/gcode/linuxcnc_g76_rotary_axis_error.ngc`, `tests/gcode/linuxcnc_g76_i_zero_error.ngc`, `tests/gcode/linuxcnc_g76_j_zero_error.ngc`, `tests/gcode/linuxcnc_g76_k_less_equal_j_error.ngc`, smoke API regression |
 | Lathe diameter/radius mode `G7/G8` | covered through LinuxCNC native/source backends for same-block X scaling, `G76` I/J/K threading geometry interaction, and `#<_lathe_diameter_mode>`/`#<_lathe_radius_mode>` readonly named-parameter state | `tests/gcode/linuxcnc_lathe_diameter_mode.ngc`, `tests/gcode/linuxcnc_lathe_diameter_g76.ngc` |
 | Digital/analog I/O `M62`-`M68` | covered with Canon digital output, analog output, and input wait callbacks preserved as temporary state events, including source-linked `convert_m()` validation for required `P`/`E`, M66 input selection, M66 wait timeout, and analog wait restrictions | `tests/gcode/linuxcnc_io_controls.ngc`, `tests/gcode/linuxcnc_m62_missing_p_word_error.ngc`, `tests/gcode/linuxcnc_m63_missing_p_word_error.ngc`, `tests/gcode/linuxcnc_m64_missing_p_word_error.ngc`, `tests/gcode/linuxcnc_m65_missing_p_word_error.ngc`, `tests/gcode/linuxcnc_m66_missing_input_error.ngc`, `tests/gcode/linuxcnc_m66_both_inputs_error.ngc`, `tests/gcode/linuxcnc_m66_zero_timeout_error.ngc`, `tests/gcode/linuxcnc_m66_analog_wait_error.ngc`, `tests/gcode/linuxcnc_m67_missing_e_word_error.ngc`, `tests/gcode/linuxcnc_m68_missing_e_word_error.ngc` |
 | Override controls `M48`-`M53` | covered with Canon feed override, spindle speed override, adaptive feed, and feed hold callbacks preserved as temporary state events | `tests/gcode/linuxcnc_override_controls.ngc` |
diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index 2635916..ec8945e 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -857,6 +857,33 @@ for case in "${threading_check_error_cases[@]}"; do
     fi
     grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
 done
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_convert.cc
+# convert_straight() and convert_threading_cycle() reject invalid
+# spindle state/selector, rotary-axis motion, and G76 I/J/K geometry
+# before issuing synchronized threading motion.
+threading_convert_error_cases=(
+    "linuxcnc_g33_stopped_spindle_error|Spindle not turning in G33"
+    "linuxcnc_g33_1_stopped_spindle_error|Spindle not turning in G33.1"
+    "linuxcnc_g33_spindle_dollar_out_of_range_error|Invalid spindle (\$) number in G33 move"
+    "linuxcnc_g33_1_spindle_dollar_out_of_range_error|Invalid spindle (\$) number in G33.1 move"
+    "linuxcnc_g76_spindle_dollar_out_of_range_error|Invalid D-number in G76 cycle"
+    "linuxcnc_g76_stopped_spindle_error|Chosen spindle (0) not turning in G76"
+    "linuxcnc_g76_rotary_axis_error|Cannot move rotary axes with G76"
+    "linuxcnc_g76_i_zero_error|In G76, I must not be 0"
+    "linuxcnc_g76_j_zero_error|In G76, J must be greater than 0"
+    "linuxcnc_g76_k_less_equal_j_error|In G76, K must be greater than J"
+)
+for case in "${threading_convert_error_cases[@]}"; do
+    fixture=${case%%|*}
+    expected=${case#*|}
+    if CNC_SIM_RS274_FILE_MODE=1 CNC_SIM_RS274_VAR="$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_read.cc
 # read_items() only consumes a leading N word before item dispatch;
 # read_one_item() then uses the default reader table from interp_array.cc
diff --git a/tests/gcode/linuxcnc_g33_1_spindle_dollar_out_of_range_error.ngc b/tests/gcode/linuxcnc_g33_1_spindle_dollar_out_of_range_error.ngc
new file mode 100644
index 0000000..d021e8b
--- /dev/null
+++ b/tests/gcode/linuxcnc_g33_1_spindle_dollar_out_of_range_error.ngc
@@ -0,0 +1,4 @@
+G21 G90 G17
+S500 M3
+G33.1 Z0 K1 $1
+M30
diff --git a/tests/gcode/linuxcnc_g33_1_stopped_spindle_error.ngc b/tests/gcode/linuxcnc_g33_1_stopped_spindle_error.ngc
new file mode 100644
index 0000000..675622e
--- /dev/null
+++ b/tests/gcode/linuxcnc_g33_1_stopped_spindle_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G33.1 Z0 K1
+M30
diff --git a/tests/gcode/linuxcnc_g33_spindle_dollar_out_of_range_error.ngc b/tests/gcode/linuxcnc_g33_spindle_dollar_out_of_range_error.ngc
new file mode 100644
index 0000000..1b95bf6
--- /dev/null
+++ b/tests/gcode/linuxcnc_g33_spindle_dollar_out_of_range_error.ngc
@@ -0,0 +1,4 @@
+G21 G90 G17
+S500 M3
+G33 Z0 K1 $1
+M30
diff --git a/tests/gcode/linuxcnc_g33_stopped_spindle_error.ngc b/tests/gcode/linuxcnc_g33_stopped_spindle_error.ngc
new file mode 100644
index 0000000..0609e0a
--- /dev/null
+++ b/tests/gcode/linuxcnc_g33_stopped_spindle_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G33 Z0 K1
+M30
diff --git a/tests/gcode/linuxcnc_g76_i_zero_error.ngc b/tests/gcode/linuxcnc_g76_i_zero_error.ngc
new file mode 100644
index 0000000..8d48521
--- /dev/null
+++ b/tests/gcode/linuxcnc_g76_i_zero_error.ngc
@@ -0,0 +1,4 @@
+G20 G90 G18
+S500 M3
+G76 P0.05 Z-1 I0 J0.008 K0.045 Q29.5 L2 E0.045
+M30
diff --git a/tests/gcode/linuxcnc_g76_j_zero_error.ngc b/tests/gcode/linuxcnc_g76_j_zero_error.ngc
new file mode 100644
index 0000000..e9fc2c2
--- /dev/null
+++ b/tests/gcode/linuxcnc_g76_j_zero_error.ngc
@@ -0,0 +1,4 @@
+G20 G90 G18
+S500 M3
+G76 P0.05 Z-1 I-.075 J0 K0.045 Q29.5 L2 E0.045
+M30
diff --git a/tests/gcode/linuxcnc_g76_k_less_equal_j_error.ngc b/tests/gcode/linuxcnc_g76_k_less_equal_j_error.ngc
new file mode 100644
index 0000000..d237297
--- /dev/null
+++ b/tests/gcode/linuxcnc_g76_k_less_equal_j_error.ngc
@@ -0,0 +1,4 @@
+G20 G90 G18
+S500 M3
+G76 P0.05 Z-1 I-.075 J0.045 K0.045 Q29.5 L2 E0.045
+M30
diff --git a/tests/gcode/linuxcnc_g76_rotary_axis_error.ngc b/tests/gcode/linuxcnc_g76_rotary_axis_error.ngc
new file mode 100644
index 0000000..ebac43c
--- /dev/null
+++ b/tests/gcode/linuxcnc_g76_rotary_axis_error.ngc
@@ -0,0 +1,4 @@
+G20 G90 G18
+S500 M3
+G76 P0.05 Z-1 A1 I-.075 J0.008 K0.045 Q29.5 L2 E0.045
+M30
diff --git a/tests/gcode/linuxcnc_g76_spindle_dollar_out_of_range_error.ngc b/tests/gcode/linuxcnc_g76_spindle_dollar_out_of_range_error.ngc
new file mode 100644
index 0000000..70406dd
--- /dev/null
+++ b/tests/gcode/linuxcnc_g76_spindle_dollar_out_of_range_error.ngc
@@ -0,0 +1,4 @@
+G20 G90 G18
+S500 M3
+G76 P0.05 Z-1 I-.075 J0.008 K0.045 Q29.5 L2 E0.045 $1
+M30
diff --git a/tests/gcode/linuxcnc_g76_stopped_spindle_error.ngc b/tests/gcode/linuxcnc_g76_stopped_spindle_error.ngc
new file mode 100644
index 0000000..50157ea
--- /dev/null
+++ b/tests/gcode/linuxcnc_g76_stopped_spindle_error.ngc
@@ -0,0 +1,3 @@
+G20 G90 G18
+G76 P0.05 Z-1 I-.075 J0.008 K0.045 Q29.5 L2 E0.045
+M30