diff --git a/TESTING.md b/TESTING.md
index 8259500..2fb2248 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -38,6 +38,23 @@ LAN test host:  <lan-host-ip>
 
 Use the same room code everywhere, for example `ROOM1`.
 
+## Build Prerequisites
+
+On Windows, use the Rust MSVC toolchain and install Visual Studio Build Tools
+with the C++ build tools. The dependency stack includes native code, so tools
+such as `cl.exe` and `lib.exe` must be available in the build environment.
+TAP-Windows6 must be installed before running the client.
+
+Quick Windows checks:
+
+```powershell
+rustc -vV
+where.exe cl
+where.exe lib
+```
+
+`rustc -vV` should report a `host` containing `x86_64-pc-windows-msvc`.
+
 ## Build
 
 On the relay or Linux build host: