diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index b8994f8c8..1b5a0162f 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -564,8 +564,15 @@ class NinjaBackend(backends.Backend):
         # and locale dependent. Any attempt at converting it to
         # Python strings leads to failure. We _must_ do this detection
         # in raw byte mode and write the result in raw bytes.
+        #
+        # Pass the compiler's external args (e.g. -I flags from c_args /
+        # cpp_args in a native file) so that cl.exe can locate system headers
+        # even when the INCLUDE environment variable is not set — for example,
+        # when using a bundled MSVC toolchain outside a VS Developer Shell.
+        extra_args = self.environment.coredata.get_external_args(
+            MachineChoice.HOST, compiler.language)
         pc = subprocess.Popen(compiler.get_exelist() +
-                              ['/showIncludes', '/c', filebase],
+                              ['/showIncludes', '/c', filebase] + extra_args,
                               cwd=self.environment.get_scratch_dir(),
                               stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         (stdout, stderr) = pc.communicate()