diff --git a/CMakeLists.txt b/CMakeLists.txt
index a7272ee7..37255ce9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -721,6 +721,7 @@ if(CORTEXT_EMBED_OBJSTORE)
   if(WIN32)
     list(APPEND CORTEXT_OBJSTORE_SRCS
       ${CMAKE_CURRENT_SOURCE_DIR}/third_party/sqlite-objstore/src/win_dirent.c
+      ${CMAKE_CURRENT_SOURCE_DIR}/third_party/sqlite-objstore/src/win_posix.c
     )
   endif()
   if(CORTEXT_TARGET_AARCH64)
diff --git a/third_party/sqlite-objstore/src/CMakeLists.txt b/third_party/sqlite-objstore/src/CMakeLists.txt
index fb72fb55..40dec625 100644
--- a/third_party/sqlite-objstore/src/CMakeLists.txt
+++ b/third_party/sqlite-objstore/src/CMakeLists.txt
@@ -80,7 +80,7 @@ if(OBJSTORE_BACKEND_VFS)
 endif()
 
 if(WIN32)
-    list(APPEND OBJSTORE_SOURCES win_dirent.c)
+    list(APPEND OBJSTORE_SOURCES win_dirent.c win_posix.c)
 endif()
 
 function(_objstore_configure_library target)
diff --git a/third_party/sqlite-objstore/src/backend_file.c b/third_party/sqlite-objstore/src/backend_file.c
index f1fefd05..09f269e8 100644
--- a/third_party/sqlite-objstore/src/backend_file.c
+++ b/third_party/sqlite-objstore/src/backend_file.c
@@ -28,8 +28,10 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#if defined(_WIN32)
+#include "win_posix.h"
+#else
 #include <unistd.h>
-#if !defined(_WIN32)
 #include <sys/file.h>
 #endif
 #if defined(__linux__)
diff --git a/third_party/sqlite-objstore/src/backend_portable.c b/third_party/sqlite-objstore/src/backend_portable.c
index 754bb13f..8f3ccfc2 100644
--- a/third_party/sqlite-objstore/src/backend_portable.c
+++ b/third_party/sqlite-objstore/src/backend_portable.c
@@ -14,7 +14,11 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#if defined(_WIN32)
+#include "win_posix.h"
+#else
 #include <unistd.h>
+#endif
 
 #if defined(__EMSCRIPTEN__)
 #include <emscripten.h>
diff --git a/third_party/sqlite-objstore/src/win_posix.c b/third_party/sqlite-objstore/src/win_posix.c
new file mode 100644
index 00000000..81eeb271
--- /dev/null
+++ b/third_party/sqlite-objstore/src/win_posix.c
@@ -0,0 +1,63 @@
+#include "win_posix.h"
+
+#define WIN32_LEAN_AND_MEAN 1
+#include <windows.h>
+
+int
+flock (int fd, int how)
+{
+  HANDLE handle;
+  BOOL exclusive = (how & LOCK_EX) != 0;
+
+  handle = (HANDLE) _get_osfhandle (fd);
+  if (handle == INVALID_HANDLE_VALUE)
+    {
+      errno = EBADF;
+      return -1;
+    }
+
+  if (how & LOCK_UN)
+    {
+      if (UnlockFileEx (handle, 0, 1, 0, (LPOVERLAPPED) 0) != 0)
+        {
+          return 0;
+        }
+      errno = (GetLastError () == ERROR_NOT_LOCKED) ? EINVAL : EIO;
+      return -1;
+    }
+
+  /*
+   * LockFileEx is always non-blocking; the POSIX callers of this shim use
+   * both modes (LOCK_NB for the primary probe, blocking waits while the
+   * primary finishes), so emulate the blocking form with a short retry
+   * loop on the dedicated lock file.
+   */
+  for (;;)
+    {
+      if (LockFileEx (handle, exclusive ? LOCKFILE_EXCLUSIVE_LOCK : 0,
+                      0, 1, 0, (LPOVERLAPPED) 0) != 0)
+        {
+          return 0;
+        }
+      if (how & LOCK_NB)
+        {
+          errno = (GetLastError () == ERROR_LOCK_VIOLATION) ? EACCES : EIO;
+          return -1;
+        }
+      Sleep (10);
+    }
+}
+
+int
+ftruncate (int fd, off_t length)
+{
+  errno_t err;
+
+  err = _chsize_s (fd, length);
+  if (err != 0)
+    {
+      errno = (int) err;
+      return -1;
+    }
+  return 0;
+}
diff --git a/third_party/sqlite-objstore/src/win_posix.h b/third_party/sqlite-objstore/src/win_posix.h
new file mode 100644
index 00000000..85c0e89d
--- /dev/null
+++ b/third_party/sqlite-objstore/src/win_posix.h
@@ -0,0 +1,66 @@
+#ifndef OBJSTORE_WIN_POSIX_H
+#define OBJSTORE_WIN_POSIX_H
+
+/*
+ * Minimal POSIX unistd/fcntl/sys-file subset for Windows (MSVC and clang-cl).
+ *
+ * The objstore file backends use open/read/write/close, fsync, ftruncate,
+ * flock, the O_* open flags, and a few unistd functions. UCRT provides the
+ * underscore-prefixed primitives (_open, _read, _write, _close, _commit,
+ * _chsize_s, _get_osfhandle) and the prefixed _O_* flags, but not the bare
+ * POSIX names or the un-prefixed O_* aliases in plain C mode. On other
+ * platforms the real <unistd.h> and <sys/file.h> are used instead.
+ *
+ * Every definition is guarded so this header also compiles under MinGW,
+ * where the bare names already exist.
+ */
+
+#define WIN32_LEAN_AND_MEAN 1
+#include <direct.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <io.h>
+#include <stdio.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#ifndef O_RDONLY
+#define O_RDONLY _O_RDONLY
+#define O_WRONLY _O_WRONLY
+#define O_RDWR _O_RDWR
+#define O_CREAT _O_CREAT
+#define O_TRUNC _O_TRUNC
+#define O_EXCL _O_EXCL
+#define O_APPEND _O_APPEND
+#endif
+#ifndef O_CLOEXEC
+#define O_CLOEXEC _O_NOINHERIT
+#endif
+#ifndef open
+#define open _open
+#define read _read
+#define write _write
+#define close _close
+#endif
+#ifndef fsync
+#define fsync(fd) _commit(fd)
+#endif
+#ifndef LOCK_SH
+#define LOCK_SH 1
+#define LOCK_EX 2
+#define LOCK_NB 4
+#define LOCK_UN 8
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int flock (int fd, int how);
+int ftruncate (int fd, off_t length);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* OBJSTORE_WIN_POSIX_H */