diff --git a/src/config.rs b/src/config.rs
index 7121415..2355b43 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -701,6 +701,11 @@ impl Config {
             }
         }
 
+        // Validate notification thresholds
+        if self.notifications.cache_threshold_pct > 100 {
+            anyhow::bail!("notifications.cache_threshold_pct must be 0–100, got {}", self.notifications.cache_threshold_pct);
+        }
+
         // Validate SMB auth
         if self.smb_auth.enabled {
             if self.smb_auth.username.is_none() {
diff --git a/src/daemon.rs b/src/daemon.rs
index 88cbf49..fa5c36b 100644
--- a/src/daemon.rs
+++ b/src/daemon.rs
@@ -70,6 +70,8 @@ pub struct DaemonStatus {
     pub nas_offline_notified: bool,
     /// Cache warning level already notified (0=none, 3=writeback depth).
     pub cache_notified_level: u8,
+    /// Whether we've already sent the cache-threshold notification (reset when usage drops).
+    pub cache_threshold_notified: bool,
 }
 
 impl DaemonStatus {
@@ -108,6 +110,7 @@ impl DaemonStatus {
             nas_offline_since: None,
             nas_offline_notified: false,
             cache_notified_level: 0,
+            cache_threshold_notified: false,
         }
     }
 
diff --git a/src/supervisor.rs b/src/supervisor.rs
index 201ce49..0ebb7ec 100644
--- a/src/supervisor.rs
+++ b/src/supervisor.rs
@@ -910,6 +910,23 @@ fn supervise(
                     status.nas_offline_notified = false;
                 }
 
+                // Cache usage % notification
+                if notif.cache_threshold_pct > 0 {
+                    let total_cache: u64 = status.shares.iter().map(|s| s.cache_bytes).sum();
+                    if let Some(max_bytes) = parse_size_bytes(&config.cache.max_size) {
+                        let pct = (total_cache as f64 / max_bytes as f64 * 100.0) as u8;
+                        if pct >= notif.cache_threshold_pct && !status.cache_threshold_notified {
+                            send_webhook_notification(&url, &format!(
+                                "\u{26a0}\u{fe0f} Warpgate: cache usage {}% — consider cleaning", pct
+                            ));
+                            status.cache_threshold_notified = true;
+                        } else if pct < notif.cache_threshold_pct.saturating_sub(5) {
+                            // Hysteresis: reset when usage drops 5% below threshold
+                            status.cache_threshold_notified = false;
+                        }
+                    }
+                }
+
                 // Write-back depth notification
                 if total_dirty >= notif.writeback_depth {
                     if status.cache_notified_level < 3 {