diff --git a/hooks/site-wysiwyd-check.sh b/hooks/site-wysiwyd-check.sh
new file mode 100755
index 0000000..f715823
--- /dev/null
+++ b/hooks/site-wysiwyd-check.sh
@@ -0,0 +1,75 @@
+#!/bin/sh
+# site-wysiwyd-check.sh — PostToolUse(Edit|Write) advisory hook.
+#
+# Detects frontend source edits on a project that has a live dev server
+# (.keisei/dev-server.pid exists) and reports visual drift against the
+# most recent approved screenshot (.keisei/target.png).
+#
+# Non-blocking: every exit path is `exit 0`. If any dependency is missing
+# (jq, mock-render, visual-diff, live server, target.png) the hook silently
+# no-ops. Drift is printed to stderr; Claude Code surfaces it as advisory.
+#
+# Stdin: JSON with tool_input.file_path.
+
+command -v jq >/dev/null 2>&1 || exit 0
+
+set -eu
+
+FILE=$(jq -r '.tool_input.file_path // empty' 2>/dev/null || true)
+[ -n "$FILE" ] || exit 0
+
+# Extension whitelist — only frontend source files trigger a drift check.
+EXT=$(printf '%s' "${FILE##*.}" | tr '[:upper:]' '[:lower:]')
+case "$EXT" in
+    tsx|vue|svelte|astro|css|html|jsx|ts) ;;
+    *) exit 0 ;;
+esac
+
+# Walk up from the edited file looking for .keisei/dev-server.pid. Stop at /
+# or at $HOME to avoid unbounded traversal.
+dir=$(dirname "$FILE")
+pid_file=""
+while [ "$dir" != "/" ] && [ "$dir" != "$HOME" ] && [ -n "$dir" ]; do
+    if [ -f "$dir/.keisei/dev-server.pid" ]; then
+        pid_file="$dir/.keisei/dev-server.pid"
+        break
+    fi
+    parent=$(dirname "$dir")
+    [ "$parent" = "$dir" ] && break
+    dir="$parent"
+done
+[ -n "$pid_file" ] || exit 0
+
+PROJECT_DIR=$(dirname "$(dirname "$pid_file")")
+TARGET_PNG="$PROJECT_DIR/.keisei/target.png"
+[ -f "$TARGET_PNG" ] || exit 0
+
+# Locate mock-render + visual-diff in the Rust primitive workspace.
+RUST_BIN="$HOME/.claude/agents/_primitives/_rust/target/release"
+MOCK="$RUST_BIN/mock-render"
+DIFF="$RUST_BIN/visual-diff"
+[ -x "$MOCK" ] || exit 0
+[ -x "$DIFF" ] || exit 0
+
+# Read dev-server URL (default http://localhost:3000 if unrecorded).
+URL_FILE="$PROJECT_DIR/.keisei/dev-server.url"
+if [ -f "$URL_FILE" ]; then
+    URL=$(head -n1 "$URL_FILE")
+else
+    URL="http://localhost:3000"
+fi
+[ -n "$URL" ] || exit 0
+
+# Let HMR settle before screenshotting.
+sleep 0.5
+
+CURRENT_PNG="$PROJECT_DIR/.keisei/current.png"
+"$MOCK" screenshot "$URL" --out "$CURRENT_PNG" >/dev/null 2>&1 || exit 0
+[ -f "$CURRENT_PNG" ] || exit 0
+
+drift=$("$DIFF" "$TARGET_PNG" "$CURRENT_PNG" 2>/dev/null || true)
+if [ -n "$drift" ]; then
+    echo "[site-wysiwyd] drift vs $TARGET_PNG: $drift" >&2
+fi
+
+exit 0