diff --git a/Sources/System/Internals/WindowsSyscallAdapters.swift b/Sources/System/Internals/WindowsSyscallAdapters.swift
index 3c6055ec..e583e41d 100644
--- a/Sources/System/Internals/WindowsSyscallAdapters.swift
+++ b/Sources/System/Internals/WindowsSyscallAdapters.swift
@@ -111,14 +111,26 @@ internal func lseek(
 internal func read(
   _ fd: Int32, _ buf: UnsafeMutableRawPointer!, _ nbyte: Int
 ) -> Int {
-  Int(_read(fd, buf, numericCast(nbyte)))
+  // _read takes an unsigned int count; reject sizes that would overflow it so
+  // numericCast cannot trap (pread/pwrite apply the same guard).
+  if nbyte > Int(DWORD.max) {
+    ucrt._set_errno(EINVAL)
+    return -1
+  }
+  return Int(_read(fd, buf, numericCast(nbyte)))
 }
 
 @inline(__always)
 internal func write(
   _ fd: Int32, _ buf: UnsafeRawPointer!, _ nbyte: Int
 ) -> Int {
-  Int(_write(fd, buf, numericCast(nbyte)))
+  // _write takes an unsigned int count; reject sizes that would overflow it so
+  // numericCast cannot trap (pread/pwrite apply the same guard).
+  if nbyte > Int(DWORD.max) {
+    ucrt._set_errno(EINVAL)
+    return -1
+  }
+  return Int(_write(fd, buf, numericCast(nbyte)))
 }
 
 @inline(__always)
@@ -158,6 +170,18 @@ internal func pread(
   // NOTE: this is a non-owning handle, do *not* call CloseHandle on it
   let hFile: HANDLE = HANDLE(bitPattern: handle)!
 
+  // POSIX pread/pwrite leave the file offset unchanged, but issuing an
+  // OVERLAPPED read/write against a synchronous handle updates it. Save the
+  // current position and restore it afterwards (as ftruncate does).
+  var liCurrentOffset = LARGE_INTEGER(QuadPart: 0)
+  if !SetFilePointerEx(hFile, liCurrentOffset, &liCurrentOffset, FILE_CURRENT) {
+    ucrt._set_errno(_mapWindowsErrorToErrno(GetLastError()))
+    return -1
+  }
+  defer {
+    _ = SetFilePointerEx(hFile, liCurrentOffset, nil, FILE_BEGIN)
+  }
+
   var ovlOverlapped: OVERLAPPED = OVERLAPPED()
   ovlOverlapped.OffsetHigh = DWORD(UInt32(offset >> 32) & 0xffffffff)
   ovlOverlapped.Offset = DWORD(UInt32(offset >> 0) & 0xffffffff)
@@ -186,6 +210,18 @@ internal func pwrite(
   // NOTE: this is a non-owning handle, do *not* call CloseHandle on it
   let hFile: HANDLE = HANDLE(bitPattern: handle)!
 
+  // POSIX pread/pwrite leave the file offset unchanged, but issuing an
+  // OVERLAPPED read/write against a synchronous handle updates it. Save the
+  // current position and restore it afterwards (as ftruncate does).
+  var liCurrentOffset = LARGE_INTEGER(QuadPart: 0)
+  if !SetFilePointerEx(hFile, liCurrentOffset, &liCurrentOffset, FILE_CURRENT) {
+    ucrt._set_errno(_mapWindowsErrorToErrno(GetLastError()))
+    return -1
+  }
+  defer {
+    _ = SetFilePointerEx(hFile, liCurrentOffset, nil, FILE_BEGIN)
+  }
+
   var ovlOverlapped: OVERLAPPED = OVERLAPPED()
   ovlOverlapped.OffsetHigh = DWORD(UInt32(offset >> 32) & 0xffffffff)
   ovlOverlapped.Offset = DWORD(UInt32(offset >> 0) & 0xffffffff)
diff --git a/Tests/SystemTests/FileOperationsTest.swift b/Tests/SystemTests/FileOperationsTest.swift
index abc4ecdb..be8f48cc 100644
--- a/Tests/SystemTests/FileOperationsTest.swift
+++ b/Tests/SystemTests/FileOperationsTest.swift
@@ -153,6 +153,36 @@ final class FileOperationsTest: XCTestCase {
     }
   }
 
+  func testPositionedIODoesNotMoveFileOffset() throws {
+    try withTemporaryFilePath(basename: "testPositionedIO") { path in
+      let fd = try FileDescriptor.open(
+        path.appending("f.txt"), .readWrite,
+        options: [.create, .truncate], permissions: .ownerReadWrite)
+      defer { try? fd.close() }
+
+      try fd.writeAll("0123456789".utf8)
+
+      // Park the file offset at a known position.
+      XCTAssertEqual(try fd.seek(offset: 3, from: .start), 3)
+
+      // A positioned read must not move the file offset (POSIX pread).
+      var readBuf = [UInt8](repeating: 0, count: 2)
+      let n = try readBuf.withUnsafeMutableBytes {
+        try fd.read(fromAbsoluteOffset: 6, into: $0)
+      }
+      XCTAssertEqual(n, 2)
+      XCTAssertEqual(Array(readBuf), Array("67".utf8))
+      XCTAssertEqual(try fd.seek(offset: 0, from: .current), 3)
+
+      // A positioned write must not move the file offset either (POSIX pwrite).
+      let m = try Array("ab".utf8).withUnsafeBytes {
+        try fd.write(toAbsoluteOffset: 8, $0)
+      }
+      XCTAssertEqual(m, 2)
+      XCTAssertEqual(try fd.seek(offset: 0, from: .current), 3)
+    }
+  }
+
   func testHelpers() {
     // TODO: Test writeAll, writeAll(toAbsoluteOffset), closeAfter
   }
diff --git a/Tests/SystemTests/FileOperationsTestWindows.swift b/Tests/SystemTests/FileOperationsTestWindows.swift
index 856d74db..cc6b8d3c 100644
--- a/Tests/SystemTests/FileOperationsTestWindows.swift
+++ b/Tests/SystemTests/FileOperationsTestWindows.swift
@@ -297,6 +297,48 @@ final class FileOperationsTestWindows: XCTestCase {
     }
   }
 
+  /// The sequential read/write adapters must reject a byte count exceeding
+  /// DWORD.max with EINVAL, matching the positioned pread/pwrite guard.
+  /// Without the guard, `numericCast(count)` into the CRT's unsigned int would
+  /// trap. We pass an oversized *count* over a small allocation; the guard
+  /// rejects it before any bytes are touched.
+  func testSequentialBufferSizeLimit() throws {
+    try withTemporaryFilePath(basename: "testSequentialBufferSizeLimit") { path in
+      let fd = try FileDescriptor.open(
+        path.appending("test.txt"),
+        .readWrite,
+        options: [.create, .truncate],
+        permissions: .ownerReadWrite
+      )
+      defer { try? fd.close() }
+
+      try fd.writeAll("test data".utf8)
+
+      let buffer = UnsafeMutableRawBufferPointer.allocate(byteCount: 1024, alignment: 1)
+      defer { buffer.deallocate() }
+
+      let oversizedCount = Int(DWORD.max) + 1
+      let oversizedBuffer = UnsafeMutableRawBufferPointer(
+        start: buffer.baseAddress,
+        count: oversizedCount
+      )
+
+      do {
+        _ = try fd.read(into: oversizedBuffer)
+        XCTFail("Expected EINVAL for buffer size exceeding DWORD.max")
+      } catch let err as Errno {
+        XCTAssertEqual(err, .invalidArgument, "Expected EINVAL, got \(err)")
+      }
+
+      do {
+        _ = try fd.write(UnsafeRawBufferPointer(oversizedBuffer))
+        XCTFail("Expected EINVAL for buffer size exceeding DWORD.max")
+      } catch let err as Errno {
+        XCTAssertEqual(err, .invalidArgument, "Expected EINVAL, got \(err)")
+      }
+    }
+  }
+
   func testCloseOnExecOpenOption() throws {
     try withTemporaryFilePath(basename: "testCloseOnExec") { path in