diff --git a/ground_suite/windows/runner/test_utf8_from_utf16.py b/ground_suite/windows/runner/test_utf8_from_utf16.py
new file mode 100644
index 0000000..9cd9879
--- /dev/null
+++ b/ground_suite/windows/runner/test_utf8_from_utf16.py
@@ -0,0 +1,121 @@
+#!/usr/bin/env python3
+"""Run the production conversion function with a controlled Windows API stub.
+
+This host check compiles only Utf8FromUtf16 from utils.cpp. It checks API arguments,
+counts, and failure handling using 16-bit wchar_t and known UTF-16/UTF-8 data. It
+does not build the Windows runner or validate Windows' Unicode implementation.
+"""
+
+import argparse
+from pathlib import Path
+import subprocess
+
+
+PREFIX = r"""
+#include <cassert>
+#include <cstring>
+#include <string>
+
+static_assert(sizeof(wchar_t) == 2, "test requires UTF-16 code units");
+constexpr unsigned CP_UTF8 = 65001;
+constexpr unsigned WC_ERR_INVALID_CHARS = 0x80;
+static const wchar_t* expected_source;
+static std::string expected_utf8;
+static int query_result;
+static int conversion_result;
+static int calls;
+
+int WideCharToMultiByte(unsigned code_page, unsigned flags, const wchar_t* source,
+                        int input_length, char* target, int capacity,
+                        const char* default_char, int* used_default_char) {
+  ++calls;
+  assert(code_page == CP_UTF8 && flags == WC_ERR_INVALID_CHARS);
+  assert(source == expected_source && input_length == -1);
+  assert(default_char == nullptr && used_default_char == nullptr);
+  if (target == nullptr) {
+    assert(calls == 1 && capacity == 0);
+    return query_result;
+  }
+  assert(calls == 2 && capacity == query_result);
+  assert(capacity == static_cast<int>(expected_utf8.size()) + 1);
+  std::memcpy(target, expected_utf8.c_str(), expected_utf8.size() + 1);
+  return conversion_result;
+}
+"""
+
+SUFFIX = r"""
+void configure(const wchar_t* source, const std::string& expected, int query,
+               int converted) {
+  expected_source = source;
+  expected_utf8 = expected;
+  query_result = query;
+  conversion_result = converted;
+  calls = 0;
+}
+
+int main() {
+  assert(Utf8FromUtf16(nullptr).empty());
+  assert(calls == 0);
+
+  const wchar_t empty[] = {0};
+  configure(empty, "", 1, 1);
+  assert(Utf8FromUtf16(empty).empty());
+  assert(calls == 2);
+
+  // Independent known encoding: A, U+00E9, U+1F642 (a surrogate pair).
+  const wchar_t text[] = {0x0041, 0x00e9, 0xd83d, 0xde42, 0};
+  const std::string utf8 = "A\xc3\xa9\xf0\x9f\x99\x82";
+  configure(text, utf8, 8, 8);
+  assert(Utf8FromUtf16(text) == utf8);
+  assert(calls == 2);
+
+  // WC_ERR_INVALID_CHARS reports an isolated surrogate as a zero query result.
+  const wchar_t invalid[] = {0xd800, 0};
+  for (int failed : {0, -1}) {
+    configure(invalid, "", failed, 0);
+    assert(Utf8FromUtf16(invalid).empty());
+    assert(calls == 1);
+  }
+  for (int failed_or_mismatched : {0, -1, 7, 9}) {
+    configure(text, utf8, 8, failed_or_mismatched);
+    assert(Utf8FromUtf16(text).empty());
+    assert(calls == 2);
+  }
+}
+"""
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--work-dir", type=Path, required=True)
+    parser.add_argument("--cxx", default="c++")
+    args = parser.parse_args()
+    work = args.work_dir.resolve()
+    work.mkdir(parents=True, exist_ok=True)
+    source = Path(__file__).with_name("utils.cpp").read_text()
+    start = source.index("std::string Utf8FromUtf16(")
+    end = source.index("\n}", start) + 2
+    fixture = work / "utf8_from_utf16.cpp"
+    fixture.write_text(PREFIX + source[start:end] + SUFFIX)
+    binary = work / "utf8_from_utf16_test"
+    subprocess.run(
+        [
+            args.cxx,
+            "-std=c++17",
+            "-fshort-wchar",
+            "-Wall",
+            "-Wextra",
+            "-Werror",
+            "-pedantic",
+            str(fixture),
+            "-o",
+            str(binary),
+        ],
+        check=True,
+    )
+    subprocess.run([str(binary)], check=True)
+    print("Utf8FromUtf16 host stub checks passed (9 cases, 16-bit wchar_t)")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/ground_suite/windows/runner/utils.cpp b/ground_suite/windows/runner/utils.cpp
index 3a0b465..cb5a55a 100644
--- a/ground_suite/windows/runner/utils.cpp
+++ b/ground_suite/windows/runner/utils.cpp
@@ -45,21 +45,24 @@ std::string Utf8FromUtf16(const wchar_t* utf16_string) {
   if (utf16_string == nullptr) {
     return std::string();
   }
-  unsigned int target_length = ::WideCharToMultiByte(
+  const int target_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,
-      -1, nullptr, 0, nullptr, nullptr)
-    -1; // remove the trailing null character
-  int input_length = (int)wcslen(utf16_string);
+      -1, nullptr, 0, nullptr, nullptr);
+  if (target_length <= 0) {
+    return std::string();
+  }
   std::string utf8_string;
-  if (target_length == 0 || target_length > utf8_string.max_size()) {
+  if (static_cast<size_t>(target_length) > utf8_string.max_size()) {
     return utf8_string;
   }
+  // With a terminated input, both calls include the trailing null character.
   utf8_string.resize(target_length);
   int converted_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,
-      input_length, utf8_string.data(), target_length, nullptr, nullptr);
-  if (converted_length == 0) {
+      -1, utf8_string.data(), target_length, nullptr, nullptr);
+  if (converted_length != target_length) {
     return std::string();
   }
+  utf8_string.resize(static_cast<size_t>(converted_length) - 1);
   return utf8_string;
 }