diff --git a/wasm-port/docs/porting-steps-standalone.md b/wasm-port/docs/porting-steps-standalone.md
index a981175..1d15488 100644
--- a/wasm-port/docs/porting-steps-standalone.md
+++ b/wasm-port/docs/porting-steps-standalone.md
@@ -488,6 +488,11 @@ Current verified progress:
   script compares that map against every `.c` and `.cc` entry in
   `tools/source-manifest.txt`, so future LinuxCNC source additions fail
   validation unless they also get an explicit standalone source compile probe.
+- `tools/verify_no_standalone_cnc_semantics.sh` now guards against
+  reintroducing a standalone `Interp::convert_g()` definition outside
+  `vendor/linuxcnc/`. This keeps G-code group conversion on the vendored
+  LinuxCNC `interp_convert.cc` path instead of allowing the wrapper layer to
+  grow another project-authored conversion implementation.
 
 ## Phase 4: Port INI Parsing Without Editing Upstream
 
@@ -786,8 +791,8 @@ Keep these documents under `wasm-port/docs/`:
 Continue expanding the standalone interpreter core from the verified minimal
 traverse path:
 
-1. replace the temporary minimal `convert_g()` implementation with calls into
-   vendored LinuxCNC interpreter conversion code.
+1. keep `convert_g()` execution on vendored LinuxCNC interpreter conversion
+   code and reject any new standalone `Interp::convert_g()` implementation.
 2. identify and shim the native runtime symbols blocking direct compilation of
    `interp_convert.cc`, `interp_execute.cc`, and related interpreter files.
 3. keep fixture coverage as regression protection while deleting temporary
diff --git a/wasm-port/tests/native/verify_native_probes.sh b/wasm-port/tests/native/verify_native_probes.sh
index 5976218..74cfbfb 100755
--- a/wasm-port/tests/native/verify_native_probes.sh
+++ b/wasm-port/tests/native/verify_native_probes.sh
@@ -10,6 +10,7 @@ CANON_ERROR_FIXTURE_DIR="$ROOT_DIR/tests/fixtures/canon_errors"
 NAMEDPARAM_INI_FIXTURE="$ROOT_DIR/tests/fixtures/ini/namedparams.ini"
 
 "$ROOT_DIR/tools/verify_vendor_sync.sh"
+"$ROOT_DIR/tools/verify_no_standalone_cnc_semantics.sh"
 "$ROOT_DIR/tools/build_native_probes.sh"
 
 check_source_probe_coverage() {
diff --git a/wasm-port/tools/verify_no_standalone_cnc_semantics.sh b/wasm-port/tools/verify_no_standalone_cnc_semantics.sh
new file mode 100755
index 0000000..41a63a9
--- /dev/null
+++ b/wasm-port/tools/verify_no_standalone_cnc_semantics.sh
@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "$0")/.." && pwd)"
+
+TMP_FILE="$(mktemp)"
+trap 'rm -f "$TMP_FILE"' EXIT
+
+find \
+  "$ROOT_DIR/runtime" \
+  "$ROOT_DIR/tests" \
+  "$ROOT_DIR/tools" \
+  -type f \
+  \( -name '*.c' -o -name '*.cc' -o -name '*.cpp' -o -name '*.h' -o -name '*.hh' -o -name '*.hpp' -o -name '*.js' -o -name '*.ts' \) \
+  ! -path "$ROOT_DIR/vendor/*" \
+  ! -path "$ROOT_DIR/runtime/ui/ini-panel/linuxcnc_ini.js" \
+  -print0 |
+  xargs -0 grep -nE '^[[:space:]]*int[[:space:]]+Interp::convert_g[[:space:]]*\(' > "$TMP_FILE" || true
+
+if [[ -s "$TMP_FILE" ]]; then
+  echo "standalone Interp::convert_g definitions are not allowed:" >&2
+  cat "$TMP_FILE" >&2
+  exit 1
+fi
+
+echo "standalone CNC semantics guard complete"