diff --git a/check-linuxcnc-wasm-shims-source-map.sh b/check-linuxcnc-wasm-shims-source-map.sh
index 50aff26..a22ad23 100755
--- a/check-linuxcnc-wasm-shims-source-map.sh
+++ b/check-linuxcnc-wasm-shims-source-map.sh
@@ -57,7 +57,18 @@ grep -F 'int set_print_rcs_error_info(const char *file, int line)' \
 grep -F 'print_rcs_error_new' "$linuxcnc_root/src/libnml/rcs/rcs_print.cc" >/dev/null
 grep -F 'retval = rcs_vprint(_fmt, args, 1);' \
     "$linuxcnc_root/src/libnml/rcs/rcs_print.cc" >/dev/null
-grep -F 'rtapi_snprintf' "$linuxcnc_root/src/rtapi/uspace_common.h" >/dev/null
+grep -F 'extern int rtapi_snprintf(char *buf, unsigned long int size,' \
+    "$linuxcnc_root/src/rtapi/rtapi.h" >/dev/null
+grep -F 'extern int rtapi_vsnprintf(char *buf, unsigned long size,' \
+    "$linuxcnc_root/src/rtapi/rtapi.h" >/dev/null
+grep -F 'int rtapi_snprintf(char *buffer, unsigned long int size, const char *msg, ...) {' \
+    "$linuxcnc_root/src/rtapi/uspace_common.h" >/dev/null
+grep -F 'result = vsnprintf(buffer, size, msg, args);' \
+    "$linuxcnc_root/src/rtapi/uspace_common.h" >/dev/null
+grep -F 'int rtapi_vsnprintf(char *buffer, unsigned long int size, const char *fmt,' \
+    "$linuxcnc_root/src/rtapi/uspace_common.h" >/dev/null
+grep -F 'return vsnprintf(buffer, size, fmt, args);' \
+    "$linuxcnc_root/src/rtapi/uspace_common.h" >/dev/null
 grep -F 'class PythonPlugin' "$linuxcnc_root/src/emc/pythonplugin/python_plugin.hh" >/dev/null
 grep -F 'PLUGIN_PYTHON_NOT_INITIALIZED = -12,' "$linuxcnc_root/src/emc/pythonplugin/python_plugin.hh" >/dev/null
 grep -F 'PLUGIN_NO_CALLABLE = 1,' "$linuxcnc_root/src/emc/pythonplugin/python_plugin.hh" >/dev/null
@@ -223,6 +234,14 @@ for needle in [
     if needle not in rcs_print_shim:
         raise SystemExit(f"RCS print shim missing split print-error boundary: {needle}")
 
+rtapi_compat_shim = Path("core/wasm_shims/rtapi_compat.cc").read_text(encoding="utf-8")
+for needle in [
+    "rtapi_snprintf",
+    "std::vsnprintf(buf, static_cast<std::size_t>(size), fmt, ap)",
+]:
+    if needle not in rtapi_compat_shim:
+        raise SystemExit(f"RTAPI compatibility shim missing userspace snprintf boundary: {needle}")
+
 for phrase in [
     "must not add CNC behavior",
     "must not expand the temporary smoke parser",
@@ -241,6 +260,8 @@ for phrase in [
     "negative-status short-circuit behavior",
     "LinuxCNC `rcs_print_error` split entry points",
     "without native print routing",
+    "LinuxCNC userspace `rtapi_snprintf()` formatting",
+    "`uspace_common.h` `vsnprintf()` implementation path",
     "./check-linuxcnc-wasm-shims-source-map.sh",
     "does not interpret G-code",
 ]:
diff --git a/docs/linuxcnc-wasm-shims-source-map.md b/docs/linuxcnc-wasm-shims-source-map.md
index 3126901..f4095a5 100644
--- a/docs/linuxcnc-wasm-shims-source-map.md
+++ b/docs/linuxcnc-wasm-shims-source-map.md
@@ -21,7 +21,7 @@ only; it must not add CNC behavior and must not expand the temporary smoke parse
 | `core/wasm_shims/nml_status_shim.cc` | `src/libnml/nml/nmlmsg.cc`, `src/libnml/nml/stat_msg.cc` | Provides NML status constructors needed by source-linked EMC status objects. |
 | `core/wasm_shims/python_c_api_shim.cc` | `src/emc/rs274ngc/interp_python.cc`, `src/emc/rs274ngc/interp_namedparams.cc`, `src/emc/rs274ngc/gcodemodule.cc`, `src/emc/pythonplugin/python_plugin.cc` | Satisfies Python C API symbols while Python/Boost.Python remap paths remain tracked blockers. |
 | `core/wasm_shims/rcs_print_shim.cc` | `src/libnml/rcs/rcs_print.cc`, `src/libnml/rcs/rcs_print.hh` | Provides the LinuxCNC `rcs_print_error` split entry points used by NML validation without native print routing. |
-| `core/wasm_shims/rtapi_compat.cc` | `src/rtapi/uspace_common.h`, `src/rtapi/rtapi.h` | Provides userspace RTAPI formatting declarations used by RS274/NML code. |
+| `core/wasm_shims/rtapi_compat.cc` | `src/rtapi/uspace_common.h`, `src/rtapi/rtapi.h` | Provides LinuxCNC userspace `rtapi_snprintf()` formatting by following the `rtapi.h` declaration and `uspace_common.h` `vsnprintf()` implementation path. |
 | `core/wasm_shims/tooldata/tooldata_mmap_backend.cc` | `src/emc/tooldata/tooldata_mmap.cc` | Replaces native mmap storage with an in-memory WASM-safe backend for probes. |
 | `core/wasm_shims/tooldata/tooldata_runtime_stubs.cc` | `src/emc/tooldata/tooldata_nml.cc`, `src/emc/tooldata/tooldata_db.cc` | Satisfies NML and external tool database entry points without native services, preserving inactive-DB return values. |
 | `core/wasm_shims/pythonplugin/python_plugin.cc` | `src/emc/pythonplugin/python_plugin.cc`, `src/emc/pythonplugin/python_plugin.hh` | Keeps LinuxCNC PythonPlugin API and negative-status short-circuit behavior available while Python execution is disabled for browser-safe probes. |