diff --git a/.github/workflows/portability.yml b/.github/workflows/portability.yml
index a5ee4068..ebf511c2 100644
--- a/.github/workflows/portability.yml
+++ b/.github/workflows/portability.yml
@@ -57,7 +57,8 @@ jobs:
           cmake -S . -B build -G Ninja
           cmake --build build --parallel 4
           ctest --test-dir build -L 'unit|vendor' --output-on-failure --no-tests=error
-          python tests/interop.py --self --bin build/bin
+          objdump -p build/bin/ss-server.exe | sed -n '/DLL Name/p'
+          python tests/interop.py --self --bin build/bin --isolate-windows-runtime
       - name: Installed consumers
         run: |
           cmake --install build --prefix "$PWD/install"
diff --git a/docs/measurements/bundled-binaries-macos-arm64.json b/docs/measurements/bundled-binaries-macos-arm64.json
index 33079f16..d7e2a17b 100644
--- a/docs/measurements/bundled-binaries-macos-arm64.json
+++ b/docs/measurements/bundled-binaries-macos-arm64.json
@@ -1,5 +1,5 @@
 {
-  "commit": "feature/self-contained-portable-c (working tree)",
+  "commit": "46d3acd9d6b72e088b688c2f84aae5c0466885d8",
   "platform": "Darwin arm64",
   "binaries": {
     "ss-local": {
diff --git a/docs/measurements/minimal-binaries-macos-arm64.json b/docs/measurements/minimal-binaries-macos-arm64.json
index 6aa37b27..1a57434c 100644
--- a/docs/measurements/minimal-binaries-macos-arm64.json
+++ b/docs/measurements/minimal-binaries-macos-arm64.json
@@ -1,5 +1,5 @@
 {
-  "commit": "feature/self-contained-portable-c (working tree)",
+  "commit": "46d3acd9d6b72e088b688c2f84aae5c0466885d8",
   "platform": "Darwin arm64",
   "binaries": {
     "ss-local": {
diff --git a/tests/interop.py b/tests/interop.py
index a708a1d8..39a413b3 100644
--- a/tests/interop.py
+++ b/tests/interop.py
@@ -137,7 +137,7 @@ def wait_ready(process, port):
 
 
 @contextlib.contextmanager
-def peers(commands, ports):
+def peers(commands, ports, environment=None):
     processes = []
     with tempfile.TemporaryDirectory(prefix="ss-interop-") as temp, contextlib.ExitStack() as stack:
         logs = []
@@ -146,7 +146,7 @@ def peers(commands, ports):
                 path = Path(temp) / f"peer-{index}.log"
                 log = stack.enter_context(path.open("wb"))
                 logs.append(path)
-                process = subprocess.Popen(command, stdout=log, stderr=subprocess.STDOUT)
+                process = subprocess.Popen(command, stdout=log, stderr=subprocess.STDOUT, env=environment)
                 processes.append(process)
                 wait_ready(process, port)
             yield
@@ -172,9 +172,18 @@ def main():
     parser.add_argument("--method", help="run one cipher")
     parser.add_argument("--bin", default=os.environ.get("SS_BIN_DIR", "build/bin"), help="program directory")
     parser.add_argument("--plugin", help="SIP003 fixture executable (requires --self)")
+    parser.add_argument("--isolate-windows-runtime", action="store_true",
+                        help="remove MSYS2/toolchain DLL directories from child PATH")
     args = parser.parse_args()
     if args.plugin and not args.self:
         parser.error("--plugin requires --self")
+    environment = None
+    if args.isolate_windows_runtime:
+        if os.name != "nt":
+            parser.error("--isolate-windows-runtime requires Windows")
+        environment = os.environ.copy()
+        system_root = environment["SystemRoot"]
+        environment["PATH"] = os.pathsep.join((system_root, os.path.join(system_root, "System32")))
     binary_dir = Path(args.bin).resolve()
     suffix = ".exe" if os.name == "nt" else ""
     local, server = [str(binary_dir / (name + suffix)) for name in ("ss-local", "ss-server")]
@@ -219,7 +228,7 @@ def main():
                     elif direction.startswith("Rust client"):
                         commands[1] = [rust_local, "-b", f"127.0.0.1:{local_port}", "-s", f"127.0.0.1:{server_port}", "-k", psk, "-m", method, "-U"]
                     try:
-                        with peers(commands, [server_port, local_port]):
+                        with peers(commands, [server_port, local_port], environment):
                             for marker in markers:
                                 deadline = time.monotonic() + 10
                                 while not marker.exists() and time.monotonic() < deadline:
diff --git a/tests/test_jconf.c b/tests/test_jconf.c
index 00ba8b00..03f2a5f9 100644
--- a/tests/test_jconf.c
+++ b/tests/test_jconf.c
@@ -9,8 +9,6 @@
 #include "test_helpers.h"
 #ifndef _WIN32
 #include <sys/wait.h>
-#else
-#include <process.h>
 #endif
 
 int verbose = 0;
@@ -151,9 +149,21 @@ test_read_jconf_rejects_out_of_range_int_options(void)
     fclose(f);
 
 #ifdef _WIN32
-    const char *args[] = {test_executable, "--read-config", path, NULL};
-    intptr_t status = _spawnv(_P_WAIT, test_executable, args);
-    assert(status > 0);
+    char command[3 * 4096];
+    assert(snprintf(command, sizeof(command), "\"%s\" --read-config \"%s\"",
+                    test_executable, path) < (int)sizeof(command));
+    STARTUPINFOA startup = {0};
+    PROCESS_INFORMATION child = {0};
+    startup.cb = sizeof(startup);
+    assert(CreateProcessA(test_executable, command, NULL, NULL, FALSE, 0,
+                          NULL, NULL, &startup, &child));
+    CloseHandle(child.hThread);
+    assert(WaitForSingleObject(child.hProcess, 10000) == WAIT_OBJECT_0);
+    DWORD status;
+    assert(GetExitCodeProcess(child.hProcess, &status));
+    CloseHandle(child.hProcess);
+    /* FATAL exits with -1, which is a valid nonzero DWORD exit status. */
+    assert(status != 0);
 #else
     pid_t pid = fork();
     assert(pid >= 0);