diff --git a/Sources/NIOPosix/WindowsCompat.swift b/Sources/NIOPosix/WindowsCompat.swift
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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftNIO open source project
+//
+// Copyright (c) 2025 Apple Inc. and the SwiftNIO project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+#if os(Windows)
+import WinSDK
+import ucrt
+
+/// SwiftNIO leans on a number of small POSIX conveniences that are absent from
+/// the Windows CRT surface area.  The declarations in this file bridge a subset
+/// of those APIs to unblock the Windows build without forcing every call site to
+/// grow additional platform checks.
+
+/// Standard POSIX-style file descriptor numbers used by example code and tests.
+public let STDIN_FILENO: CInt = 0
+public let STDOUT_FILENO: CInt = 1
+public let STDERR_FILENO: CInt = 2
+
+/// Windows-compatible implementation of `setenv(3)` that mirrors the common
+/// POSIX semantics used within SwiftNIO.  Internally this delegates to
+/// `_putenv_s`, applying the documented overwrite behaviour and translating the
+/// CRT error codes back into `errno` so the return value matches the POSIX
+/// contract (0 on success, -1 on failure).
+@discardableResult
+public func setenv(_ name: String, _ value: String, _ overwrite: Int32) -> Int32 {
+    if overwrite == 0 {
+        var duplicated: UnsafeMutablePointer<CChar>?
+        var size: size_t = 0
+        let dupResult = _dupenv_s(&duplicated, &size, name)
+        defer { free(duplicated) }
+
+        if dupResult != 0 {
+            _set_errno(dupResult)
+            return -1
+        }
+
+        if duplicated != nil {
+            // Variable already exists and we were asked not to overwrite it.
+            return 0
+        }
+    }
+
+    let putResult = _putenv_s(name, value)
+    if putResult == 0 {
+        return 0
+    }
+
+    _set_errno(putResult)
+    return -1
+}
+
+/// Windows-compatible implementation of `unsetenv(3)` expressed in terms of the
+/// CRT's `_putenv_s`.  Setting the variable to an empty string removes it from
+/// the environment block.  We surface errors via `errno` to preserve the POSIX
+/// contract.
+@discardableResult
+public func unsetenv(_ name: String) -> Int32 {
+    let result = _putenv_s(name, "")
+    if result == 0 {
+        return 0
+    }
+
+    _set_errno(result)
+    return -1
+}
+
+#endif