diff --git a/c/doctor.py b/c/doctor.py
index 0fb74af..da64704 100644
--- a/c/doctor.py
+++ b/c/doctor.py
@@ -2,6 +2,7 @@
 """Read-only installation diagnostics for colibri."""
 
 import os
+import sys
 import json
 import subprocess
 from pathlib import Path
@@ -63,7 +64,16 @@ def run_doctor(model, ram_gb=0, context=4096, gpu_indices=None, vram_gb=0, *,
         checks.append(_check("storage.persistence", "skip", "persistence requires a model directory"))
 
     engine = Path(engine_path)
-    if engine.is_file() and os.access(engine, os.X_OK):
+    # On Windows, os.access(X_OK) always returns True for any existing file
+    # (NTFS has no execute bit; executability is governed by file extension).
+    # So a chmod(0o644) "non-executable" scenario can't be detected via X_OK
+    # on Windows. Use a platform-aware check: on POSIX, honor the mode bits;
+    # on Windows, any existing file is treated as executable. (#141)
+    if sys.platform == "win32":
+        engine_ok = engine.is_file()
+    else:
+        engine_ok = engine.is_file() and os.access(engine, os.X_OK)
+    if engine_ok:
         checks.append(_check("engine.binary", "pass", "engine executable is ready", path=str(engine)))
     elif engine.is_file():
         checks.append(_check("engine.binary", "fail", "engine exists but is not executable", path=str(engine)))
diff --git a/c/tests/test_doctor.py b/c/tests/test_doctor.py
index 1647a0d..8950147 100644
--- a/c/tests/test_doctor.py
+++ b/c/tests/test_doctor.py
@@ -104,7 +104,14 @@ class DoctorTest(unittest.TestCase):
         report = self.report(ram_gb=40)
         checks = self.checks_by_id(report)
 
-        self.assertEqual(checks["engine.binary"]["status"], "fail")
+        # On Windows chmod(0o644) does not remove executability (NTFS has no
+        # execute bit; os.access(X_OK) is always True for existing files), so
+        # the engine.binary check stays "pass" there. The excessive-RAM check
+        # (memory.ram) is platform-independent and must still fail. (#141)
+        if sys.platform == "win32":
+            self.assertEqual(checks["engine.binary"]["status"], "pass")
+        else:
+            self.assertEqual(checks["engine.binary"]["status"], "fail")
         self.assertEqual(checks["memory.ram"]["status"], "fail")
         self.assertEqual(report["status"], "error")