diff --git a/src/client/components/FilterPanel.tsx b/src/client/components/FilterPanel.tsx
index e3877cb..d793299 100644
--- a/src/client/components/FilterPanel.tsx
+++ b/src/client/components/FilterPanel.tsx
@@ -2,15 +2,17 @@ import React, { useState } from "react";
 import type { MessageCategory } from "../lib/types";
 import { ALL_CATEGORIES, CATEGORY_LABELS } from "../lib/types";
 import { CATEGORY_COLORS } from "../lib/constants";
+import { Tooltip } from "./Tooltip";
 
 interface Props {
   enabledCategories: Set<MessageCategory>;
   onToggle: (cat: MessageCategory) => void;
   autoRedactEnabled: boolean;
   onAutoRedactToggle: (enabled: boolean) => void;
+  sensitiveCount: number;
 }
 
-export function FilterPanel({ enabledCategories, onToggle, autoRedactEnabled, onAutoRedactToggle }: Props) {
+export function FilterPanel({ enabledCategories, onToggle, autoRedactEnabled, onAutoRedactToggle, sensitiveCount }: Props) {
   const [collapsed, setCollapsed] = useState(false);
 
   const enabledCount = enabledCategories.size;
@@ -69,18 +71,29 @@ export function FilterPanel({ enabledCategories, onToggle, autoRedactEnabled, on
           </div>
 
           <div className="mt-3 pt-3 border-t border-border-muted">
-            <label className="flex items-center gap-2.5 py-1.5 px-2 rounded-md cursor-pointer hover:bg-surface-inset transition-colors">
-              <input
-                type="checkbox"
-                checked={autoRedactEnabled}
-                onChange={(e) => onAutoRedactToggle(e.target.checked)}
-                className="custom-checkbox checkbox-danger"
-              />
-              <svg className="w-3.5 h-3.5 text-red-500 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-                <path strokeLinecap="round" strokeLinejoin="round" d="M9 12.75L11.25 15 15 9.75m-3-7.036A11.959 11.959 0 013.598 6 11.99 11.99 0 003 9.749c0 5.592 3.824 10.29 9 11.623 5.176-1.332 9-6.03 9-11.622 0-1.31-.21-2.571-.598-3.751h-.152c-3.196 0-6.1-1.248-8.25-3.285z" />
-              </svg>
-              <span className="text-body text-foreground-secondary">Auto-redact sensitive</span>
-            </label>
+            <Tooltip
+              content="Automatically detect and replace sensitive content (API keys, tokens, passwords, emails, IPs, etc.) with placeholder labels"
+              side="top"
+              delayMs={150}
+            >
+              <label className="flex items-center gap-2.5 py-1.5 px-2 rounded-md cursor-pointer hover:bg-surface-inset transition-colors">
+                <input
+                  type="checkbox"
+                  checked={autoRedactEnabled}
+                  onChange={(e) => onAutoRedactToggle(e.target.checked)}
+                  className="custom-checkbox checkbox-danger"
+                />
+                <svg className="w-3.5 h-3.5 text-red-500 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+                  <path strokeLinecap="round" strokeLinejoin="round" d="M9 12.75L11.25 15 15 9.75m-3-7.036A11.959 11.959 0 013.598 6 11.99 11.99 0 003 9.749c0 5.592 3.824 10.29 9 11.623 5.176-1.332 9-6.03 9-11.622 0-1.31-.21-2.571-.598-3.751h-.152c-3.196 0-6.1-1.248-8.25-3.285z" />
+                </svg>
+                <span className="text-body text-foreground-secondary flex-1">Auto-redact sensitive</span>
+                {sensitiveCount > 0 && (
+                  <span className="text-caption tabular-nums px-1.5 py-0.5 rounded-full bg-red-500/10 text-red-400 border border-red-500/20 ml-auto">
+                    {sensitiveCount}
+                  </span>
+                )}
+              </label>
+            </Tooltip>
           </div>
         </div>
       )}
diff --git a/src/client/components/Tooltip.tsx b/src/client/components/Tooltip.tsx
new file mode 100644
index 0000000..10cc380
--- /dev/null
+++ b/src/client/components/Tooltip.tsx
@@ -0,0 +1,81 @@
+import React, { useState, useRef, useCallback, useEffect } from "react";
+
+interface Props {
+  content: React.ReactNode;
+  children: React.ReactElement;
+  delayMs?: number;
+  side?: "top" | "bottom";
+}
+
+export function Tooltip({ content, children, delayMs = 150, side = "top" }: Props) {
+  const [visible, setVisible] = useState(false);
+  const [position, setPosition] = useState<{ x: number; y: number } | null>(null);
+  const triggerRef = useRef<HTMLElement>(null);
+  const tooltipRef = useRef<HTMLDivElement>(null);
+  const timerRef = useRef<ReturnType<typeof setTimeout>>();
+
+  const show = useCallback(() => {
+    timerRef.current = setTimeout(() => setVisible(true), delayMs);
+  }, [delayMs]);
+
+  const hide = useCallback(() => {
+    clearTimeout(timerRef.current);
+    setVisible(false);
+  }, []);
+
+  // Clean up timer on unmount
+  useEffect(() => () => clearTimeout(timerRef.current), []);
+
+  // Recompute position when visible
+  useEffect(() => {
+    if (!visible || !triggerRef.current) return;
+    const rect = triggerRef.current.getBoundingClientRect();
+    setPosition({
+      x: rect.left + rect.width / 2,
+      y: side === "top" ? rect.top : rect.bottom,
+    });
+  }, [visible, side]);
+
+  // Nudge tooltip horizontally if it overflows the viewport
+  useEffect(() => {
+    if (!visible || !tooltipRef.current || !position) return;
+    const el = tooltipRef.current;
+    const tooltipRect = el.getBoundingClientRect();
+    const pad = 8;
+    if (tooltipRect.left < pad) {
+      el.style.transform = `translateX(${pad - tooltipRect.left}px)`;
+    } else if (tooltipRect.right > window.innerWidth - pad) {
+      el.style.transform = `translateX(${window.innerWidth - pad - tooltipRect.right}px)`;
+    } else {
+      el.style.transform = "";
+    }
+  }, [visible, position]);
+
+  return (
+    <>
+      {React.cloneElement(children, {
+        ref: triggerRef,
+        onMouseEnter: show,
+        onMouseLeave: hide,
+        onFocus: show,
+        onBlur: hide,
+      })}
+      {visible && position && (
+        <div
+          ref={tooltipRef}
+          role="tooltip"
+          className="tooltip-popup"
+          style={{
+            position: "fixed",
+            left: position.x,
+            top: side === "top" ? position.y - 8 : position.y + 8,
+            zIndex: 9999,
+          }}
+          data-side={side}
+        >
+          {content}
+        </div>
+      )}
+    </>
+  );
+}
diff --git a/src/client/styles/main.css b/src/client/styles/main.css
index 7a04c9f..3b4cf4c 100644
--- a/src/client/styles/main.css
+++ b/src/client/styles/main.css
@@ -552,3 +552,43 @@ mark.search-highlight {
     @apply focus-visible:ring-red-500;
   }
 }
+
+/* ═══════════════════════════════════════════════
+   Tooltip
+   ═══════════════════════════════════════════════ */
+
+.tooltip-popup {
+  pointer-events: none;
+  max-width: 280px;
+  padding: 0.375rem 0.625rem;
+  font-size: 0.75rem;
+  line-height: 1.45;
+  color: var(--color-foreground);
+  background: var(--color-surface-overlay);
+  border: 1px solid var(--color-border);
+  border-radius: 0.375rem;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.35), 0 0 0 1px rgba(0, 0, 0, 0.1);
+  animation: tooltip-in 120ms cubic-bezier(0.16, 1, 0.3, 1);
+  white-space: normal;
+}
+
+.tooltip-popup[data-side="top"] {
+  transform-origin: bottom center;
+  translate: -50% -100%;
+}
+
+.tooltip-popup[data-side="bottom"] {
+  transform-origin: top center;
+  translate: -50% 0;
+}
+
+@keyframes tooltip-in {
+  from {
+    opacity: 0;
+    scale: 0.96;
+  }
+  to {
+    opacity: 1;
+    scale: 1;
+  }
+}