diff --git a/DESIGN.md b/DESIGN.md
index 22bccb8..cbef937 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -68,9 +68,11 @@ set -e
 
 # Detect OS
 OS=$(uname -s | tr '[:upper:]' '[:lower:]')
+EXT=""
 case "$OS" in
   linux*) OS="linux" ;;
   darwin*) OS="macos" ;;
+  mingw*|msys*|cygwin*) OS="windows"; EXT=".exe" ;;
   *) echo "Error: Unsupported OS: $OS" >&2; exit 1 ;;
 esac
 
@@ -82,7 +84,7 @@ case "$ARCH" in
   *) echo "Error: Unsupported architecture: $ARCH" >&2; exit 1 ;;
 esac
 
-BINARY="$(dirname "$0")/.rnr/bin/rnr-${OS}-${ARCH}"
+BINARY="$(dirname "$0")/.rnr/bin/rnr-${OS}-${ARCH}${EXT}"
 
 if [ ! -f "$BINARY" ]; then
   echo "Error: rnr is not configured for ${OS}-${ARCH}." >&2
diff --git a/src/commands/init.rs b/src/commands/init.rs
index 4949885..422074e 100644
--- a/src/commands/init.rs
+++ b/src/commands/init.rs
@@ -264,9 +264,11 @@ set -e
 
 # Detect OS
 OS=$(uname -s | tr '[:upper:]' '[:lower:]')
+EXT=""
 case "$OS" in
   linux*) OS="linux" ;;
   darwin*) OS="macos" ;;
+  mingw*|msys*|cygwin*) OS="windows"; EXT=".exe" ;;
   *) echo "Error: Unsupported OS: $OS" >&2; exit 1 ;;
 esac
 
@@ -278,7 +280,7 @@ case "$ARCH" in
   *) echo "Error: Unsupported architecture: $ARCH" >&2; exit 1 ;;
 esac
 
-BINARY="$(dirname "$0")/.rnr/bin/rnr-${OS}-${ARCH}"
+BINARY="$(dirname "$0")/.rnr/bin/rnr-${OS}-${ARCH}${EXT}"
 
 if [ ! -f "$BINARY" ]; then
   echo "Error: rnr is not configured for ${OS}-${ARCH}." >&2