diff --git a/plugins/core/hooks/hooks.json b/plugins/core/hooks/hooks.json
index c7f2d7c..d900d8a 100644
--- a/plugins/core/hooks/hooks.json
+++ b/plugins/core/hooks/hooks.json
@@ -1,5 +1,16 @@
 {
   "hooks": {
+    "SessionStart": [
+      {
+        "matcher": "startup|resume|clear|compact",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/session-multiplexing.sh"
+          }
+        ]
+      }
+    ],
     "PostToolUse": [
       {
         "matcher": "TodoWrite",
diff --git a/plugins/core/hooks/session-multiplexing.sh b/plugins/core/hooks/session-multiplexing.sh
new file mode 100755
index 0000000..f4a9ba1
--- /dev/null
+++ b/plugins/core/hooks/session-multiplexing.sh
@@ -0,0 +1,36 @@
+#!/bin/bash
+# SessionStart hook: Detect multiple active Claude Code sessions
+# Touches a session file and counts active sessions (modified in last 2 hours)
+# When 3+ active, outputs a reminder to re-ground context in responses
+
+# Read JSON input from stdin
+INPUT=$(cat)
+
+# Extract session_id
+SESSION_ID=$(echo "$INPUT" | jq -r '.session_id // empty')
+
+if [ -z "$SESSION_ID" ]; then
+    exit 0
+fi
+
+# Session tracking directory
+SESSION_DIR="$HOME/.claude/sessions"
+mkdir -p "$SESSION_DIR"
+
+# Sanitize session_id to prevent path traversal — use only the basename
+SESSION_FILE="$SESSION_DIR/$(basename "$SESSION_ID")"
+touch "$SESSION_FILE"
+
+# Count sessions modified in last 2 hours (find -mmin works on both macOS and Linux)
+ACTIVE_COUNT=$(find "$SESSION_DIR" -type f -mmin -120 2>/dev/null | wc -l | tr -d ' ')
+
+# Clean up sessions older than 24 hours
+find "$SESSION_DIR" -type f -mmin +1440 -delete 2>/dev/null
+
+# Threshold: 3+ sessions is where context-juggling becomes error-prone.
+# Below 3, a user can reasonably track what each window is doing.
+if [ "$ACTIVE_COUNT" -ge 3 ]; then
+    echo "Multi-session mode: $ACTIVE_COUNT active Claude Code sessions detected. The user is juggling multiple windows — always include explicit context (repo name, branch, what we're working on) when reporting status or completing work."
+fi
+
+exit 0