diff --git a/internal/api/diagnostics.go b/internal/api/diagnostics.go
index 17700ca..336da97 100644
--- a/internal/api/diagnostics.go
+++ b/internal/api/diagnostics.go
@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"sort"
 	"strings"
 	"time"
 
@@ -32,6 +33,26 @@ type DiagnosticsInfo struct {
 	NodeSnapshots []monitoring.NodeMemorySnapshot `json:"nodeSnapshots,omitempty"`
 	// GuestSnapshots captures recent per-guest memory breakdowns (VM/LXC) with the raw Proxmox fields.
 	GuestSnapshots []monitoring.GuestMemorySnapshot `json:"guestSnapshots,omitempty"`
+	// MemorySources summarizes how many nodes currently rely on each memory source per instance.
+	MemorySources []MemorySourceStat `json:"memorySources,omitempty"`
+}
+
+// MemorySourceStat aggregates memory-source usage per instance.
+type MemorySourceStat struct {
+	Instance    string `json:"instance"`
+	Source      string `json:"source"`
+	NodeCount   int    `json:"nodeCount"`
+	LastUpdated string `json:"lastUpdated"`
+	Fallback    bool   `json:"fallback"`
+}
+
+func isFallbackMemorySource(source string) bool {
+	switch strings.ToLower(source) {
+	case "", "unknown", "nodes-endpoint", "node-status-used", "previous-snapshot":
+		return true
+	default:
+		return false
+	}
 }
 
 // NodeDiagnostic contains diagnostic info for a Proxmox node
@@ -338,6 +359,54 @@ func (r *Router) handleDiagnostics(w http.ResponseWriter, req *http.Request) {
 		snapshots := r.monitor.GetDiagnosticSnapshots()
 		if len(snapshots.Nodes) > 0 {
 			diag.NodeSnapshots = snapshots.Nodes
+
+			type memorySourceAgg struct {
+				stat   MemorySourceStat
+				latest time.Time
+			}
+
+			sourceAverages := make(map[string]*memorySourceAgg)
+			for _, snap := range snapshots.Nodes {
+				source := snap.MemorySource
+				if source == "" {
+					source = "unknown"
+				}
+
+				key := fmt.Sprintf("%s|%s", snap.Instance, source)
+				entry, ok := sourceAverages[key]
+				if !ok {
+					entry = &memorySourceAgg{
+						stat: MemorySourceStat{
+							Instance: snap.Instance,
+							Source:   source,
+							Fallback: isFallbackMemorySource(source),
+						},
+					}
+					sourceAverages[key] = entry
+				}
+
+				entry.stat.NodeCount++
+				if snap.RetrievedAt.After(entry.latest) {
+					entry.latest = snap.RetrievedAt
+				}
+			}
+
+			if len(sourceAverages) > 0 {
+				diag.MemorySources = make([]MemorySourceStat, 0, len(sourceAverages))
+				for _, entry := range sourceAverages {
+					if !entry.latest.IsZero() {
+						entry.stat.LastUpdated = entry.latest.UTC().Format(time.RFC3339)
+					}
+					diag.MemorySources = append(diag.MemorySources, entry.stat)
+				}
+
+				sort.Slice(diag.MemorySources, func(i, j int) bool {
+					if diag.MemorySources[i].Instance == diag.MemorySources[j].Instance {
+						return diag.MemorySources[i].Source < diag.MemorySources[j].Source
+					}
+					return diag.MemorySources[i].Instance < diag.MemorySources[j].Instance
+				})
+			}
 		}
 		if len(snapshots.Guests) > 0 {
 			diag.GuestSnapshots = snapshots.Guests
diff --git a/internal/monitoring/diagnostic_snapshots.go b/internal/monitoring/diagnostic_snapshots.go
index 184a6e1..9f02aa2 100644
--- a/internal/monitoring/diagnostic_snapshots.go
+++ b/internal/monitoring/diagnostic_snapshots.go
@@ -6,6 +6,8 @@ import (
 	"time"
 
 	"github.com/rcourtman/pulse-go-rewrite/internal/models"
+	"github.com/rs/zerolog"
+	"github.com/rs/zerolog/log"
 )
 
 // NodeMemoryRaw captures the raw memory fields returned by Proxmox for a node.
@@ -88,6 +90,70 @@ func makeGuestSnapshotKey(instance, guestType, node string, vmid int) string {
 	return fmt.Sprintf("%s|%s|%s|%d", instance, guestType, node, vmid)
 }
 
+func (m *Monitor) logNodeMemorySource(instance, node string, snapshot NodeMemorySnapshot) {
+	if m == nil {
+		return
+	}
+
+	key := makeNodeSnapshotKey(instance, node)
+
+	source := snapshot.MemorySource
+	var prevSource string
+	m.diagMu.RLock()
+	if existing, ok := m.nodeSnapshots[key]; ok {
+		prevSource = existing.MemorySource
+	}
+	m.diagMu.RUnlock()
+
+	if prevSource == snapshot.MemorySource {
+		return
+	}
+
+	var evt *zerolog.Event
+	switch source {
+	case "", "nodes-endpoint", "node-status-used", "previous-snapshot":
+		evt = log.Warn()
+	default:
+		evt = log.Debug()
+	}
+
+	evt = evt.
+		Str("instance", instance).
+		Str("node", node).
+		Str("memorySource", source).
+		Str("proxmoxSource", snapshot.Raw.ProxmoxMemorySource)
+
+	if snapshot.FallbackReason != "" {
+		evt = evt.Str("fallbackReason", snapshot.FallbackReason)
+	}
+	if snapshot.Raw.Available > 0 {
+		evt = evt.Uint64("rawAvailable", snapshot.Raw.Available)
+	}
+	if snapshot.Raw.Buffers > 0 {
+		evt = evt.Uint64("rawBuffers", snapshot.Raw.Buffers)
+	}
+	if snapshot.Raw.Cached > 0 {
+		evt = evt.Uint64("rawCached", snapshot.Raw.Cached)
+	}
+	if snapshot.Raw.TotalMinusUsed > 0 {
+		evt = evt.Uint64("rawTotalMinusUsed", snapshot.Raw.TotalMinusUsed)
+	}
+	if snapshot.Memory.Total > 0 {
+		evt = evt.Int64("total", snapshot.Memory.Total)
+	}
+	if snapshot.Memory.Used > 0 {
+		evt = evt.Int64("used", snapshot.Memory.Used)
+	}
+	if snapshot.Memory.Free > 0 {
+		evt = evt.Int64("free", snapshot.Memory.Free)
+	}
+	if snapshot.Memory.Usage > 0 {
+		evt = evt.Float64("usage", snapshot.Memory.Usage)
+	}
+
+	evt.Msg("Node memory source updated")
+}
+
 func (m *Monitor) recordNodeSnapshot(instance, node string, snapshot NodeMemorySnapshot) {
 	if m == nil {
 		return
@@ -99,6 +165,8 @@ func (m *Monitor) recordNodeSnapshot(instance, node string, snapshot NodeMemoryS
 		snapshot.RetrievedAt = time.Now()
 	}
 
+	m.logNodeMemorySource(instance, node, snapshot)
+
 	m.diagMu.Lock()
 	defer m.diagMu.Unlock()
 	if m.nodeSnapshots == nil {
diff --git a/pkg/proxmox/client_test.go b/pkg/proxmox/client_test.go
index fe106fe..01a36ed 100644
--- a/pkg/proxmox/client_test.go
+++ b/pkg/proxmox/client_test.go
@@ -101,6 +101,17 @@ func TestMemoryStatusEffectiveAvailable(t *testing.T) {
 			status: MemoryStatus{Total: 32 * 1024, Free: 4 * 1024, Buffers: 2 * 1024, Cached: 6 * 1024},
 			want:   12 * 1024,
 		},
+		{
+			name: "available zero but buffers and cache present",
+			status: MemoryStatus{
+				Total:   64 * 1024,
+				Used:    40 * 1024,
+				Free:    6 * 1024,
+				Buffers: 8 * 1024,
+				Cached:  10 * 1024,
+			},
+			want: 24 * 1024,
+		},
 		{
 			name:   "caps derived value at total",
 			status: MemoryStatus{Total: 8 * 1024, Free: 4 * 1024, Buffers: 4 * 1024, Cached: 4 * 1024},
@@ -196,6 +207,19 @@ func TestMemoryStatusEffectiveAvailable_RegressionIssue435(t *testing.T) {
 			wantUsedPct:   42.4, // Should be ~42%, not 86%!
 			description:   "Real user report: 86% shown when actual usage is 42%",
 		},
+		{
+			name: "missing available but buffers/cached still present (issue 553 guard)",
+			status: MemoryStatus{
+				Total:   34359738368, // 32GB
+				Used:    27917287424, // ~26GB reported used (includes cache)
+				Free:    2147483648,  // 2GB
+				Buffers: 3221225472,  // 3GB
+				Cached:  8053063680,  // 7.5GB
+			},
+			wantAvailable: 13421772800, // Free + Buffers + Cached
+			wantUsedPct:   60.94,
+			description:   "When available=0 but buffers/cached exist, derive reclaimable memory instead of alerting",
+		},
 		{
 			name: "missing all cache-aware fields - fallback to zero",
 			status: MemoryStatus{