diff --git a/internal/monitoring/diagnostic_snapshots_test.go b/internal/monitoring/diagnostic_snapshots_test.go
index e4d259bc5..6d83eab7c 100644
--- a/internal/monitoring/diagnostic_snapshots_test.go
+++ b/internal/monitoring/diagnostic_snapshots_test.go
@@ -758,6 +758,213 @@ func TestLogNodeMemorySource(t *testing.T) {
 	})
 }
 
+func TestRecordNodeSnapshot(t *testing.T) {
+	tests := []struct {
+		name             string
+		monitor          *Monitor
+		instance         string
+		node             string
+		snapshot         NodeMemorySnapshot
+		wantPanic        bool
+		wantMapLen       int
+		wantInstance     string
+		wantNode         string
+		checkRetrievedAt string // "set" (auto-set), "preserved" (kept from input), "skip" (don't check)
+		inputRetrievedAt time.Time
+	}{
+		{
+			name:             "nil Monitor returns early without panic",
+			monitor:          nil,
+			instance:         "pve1",
+			node:             "node1",
+			snapshot:         NodeMemorySnapshot{},
+			wantPanic:        false,
+			checkRetrievedAt: "skip",
+		},
+		{
+			name: "nil nodeSnapshots map gets initialized",
+			monitor: &Monitor{
+				nodeSnapshots:  nil,
+				guestSnapshots: make(map[string]GuestMemorySnapshot),
+			},
+			instance:         "pve1",
+			node:             "node1",
+			snapshot:         NodeMemorySnapshot{MemorySource: "test"},
+			wantMapLen:       1,
+			wantInstance:     "pve1",
+			wantNode:         "node1",
+			checkRetrievedAt: "set",
+		},
+		{
+			name: "Instance and Node are set from parameters",
+			monitor: &Monitor{
+				nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+				guestSnapshots: make(map[string]GuestMemorySnapshot),
+			},
+			instance: "my-instance",
+			node:     "my-node",
+			snapshot: NodeMemorySnapshot{
+				Instance: "wrong-instance",
+				Node:     "wrong-node",
+			},
+			wantMapLen:       1,
+			wantInstance:     "my-instance",
+			wantNode:         "my-node",
+			checkRetrievedAt: "set",
+		},
+		{
+			name: "zero RetrievedAt gets set to current time",
+			monitor: &Monitor{
+				nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+				guestSnapshots: make(map[string]GuestMemorySnapshot),
+			},
+			instance:         "pve1",
+			node:             "node1",
+			snapshot:         NodeMemorySnapshot{},
+			wantMapLen:       1,
+			wantInstance:     "pve1",
+			wantNode:         "node1",
+			checkRetrievedAt: "set",
+		},
+		{
+			name: "non-zero RetrievedAt is preserved",
+			monitor: &Monitor{
+				nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+				guestSnapshots: make(map[string]GuestMemorySnapshot),
+			},
+			instance:         "pve1",
+			node:             "node1",
+			snapshot:         NodeMemorySnapshot{RetrievedAt: time.Date(2024, 6, 15, 12, 0, 0, 0, time.UTC)},
+			wantMapLen:       1,
+			wantInstance:     "pve1",
+			wantNode:         "node1",
+			checkRetrievedAt: "preserved",
+			inputRetrievedAt: time.Date(2024, 6, 15, 12, 0, 0, 0, time.UTC),
+		},
+		{
+			name: "snapshot is stored with correct key",
+			monitor: &Monitor{
+				nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+				guestSnapshots: make(map[string]GuestMemorySnapshot),
+			},
+			instance:         "pve1",
+			node:             "node1",
+			snapshot:         NodeMemorySnapshot{MemorySource: "rrd-available"},
+			wantMapLen:       1,
+			wantInstance:     "pve1",
+			wantNode:         "node1",
+			checkRetrievedAt: "set",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			beforeRecord := time.Now()
+
+			// Call recordNodeSnapshot - should not panic
+			tc.monitor.recordNodeSnapshot(tc.instance, tc.node, tc.snapshot)
+
+			afterRecord := time.Now()
+
+			// For nil monitor, we just verify no panic occurred
+			if tc.monitor == nil {
+				return
+			}
+
+			// Verify map was initialized and has expected length
+			if tc.monitor.nodeSnapshots == nil {
+				t.Fatal("nodeSnapshots should have been initialized")
+			}
+			if len(tc.monitor.nodeSnapshots) != tc.wantMapLen {
+				t.Errorf("nodeSnapshots length = %d, want %d", len(tc.monitor.nodeSnapshots), tc.wantMapLen)
+			}
+
+			// Verify snapshot was stored with correct key
+			key := makeNodeSnapshotKey(tc.instance, tc.node)
+			stored, ok := tc.monitor.nodeSnapshots[key]
+			if !ok {
+				t.Fatalf("snapshot not found with key %q", key)
+			}
+
+			// Verify Instance and Node were set from parameters
+			if stored.Instance != tc.wantInstance {
+				t.Errorf("Instance = %q, want %q", stored.Instance, tc.wantInstance)
+			}
+			if stored.Node != tc.wantNode {
+				t.Errorf("Node = %q, want %q", stored.Node, tc.wantNode)
+			}
+
+			// Verify RetrievedAt handling
+			switch tc.checkRetrievedAt {
+			case "set":
+				if stored.RetrievedAt.Before(beforeRecord) || stored.RetrievedAt.After(afterRecord) {
+					t.Errorf("RetrievedAt = %v, want between %v and %v", stored.RetrievedAt, beforeRecord, afterRecord)
+				}
+			case "preserved":
+				if !stored.RetrievedAt.Equal(tc.inputRetrievedAt) {
+					t.Errorf("RetrievedAt = %v, want %v", stored.RetrievedAt, tc.inputRetrievedAt)
+				}
+			}
+		})
+	}
+}
+
+func TestRecordNodeSnapshot_MultipleSnapshots(t *testing.T) {
+	t.Run("records multiple nodes with different keys", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		m.recordNodeSnapshot("pve1", "node1", NodeMemorySnapshot{MemorySource: "src1"})
+		m.recordNodeSnapshot("pve1", "node2", NodeMemorySnapshot{MemorySource: "src2"})
+		m.recordNodeSnapshot("pve2", "node1", NodeMemorySnapshot{MemorySource: "src3"})
+
+		if len(m.nodeSnapshots) != 3 {
+			t.Fatalf("Expected 3 node snapshots, got %d", len(m.nodeSnapshots))
+		}
+
+		// Verify each one exists with correct data
+		key1 := makeNodeSnapshotKey("pve1", "node1")
+		key2 := makeNodeSnapshotKey("pve1", "node2")
+		key3 := makeNodeSnapshotKey("pve2", "node1")
+
+		if m.nodeSnapshots[key1].MemorySource != "src1" {
+			t.Errorf("Snapshot 1 MemorySource = %q, want %q", m.nodeSnapshots[key1].MemorySource, "src1")
+		}
+		if m.nodeSnapshots[key2].MemorySource != "src2" {
+			t.Errorf("Snapshot 2 MemorySource = %q, want %q", m.nodeSnapshots[key2].MemorySource, "src2")
+		}
+		if m.nodeSnapshots[key3].MemorySource != "src3" {
+			t.Errorf("Snapshot 3 MemorySource = %q, want %q", m.nodeSnapshots[key3].MemorySource, "src3")
+		}
+	})
+
+	t.Run("overwrites existing snapshot with same key", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		m.recordNodeSnapshot("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "old-source",
+		})
+
+		m.recordNodeSnapshot("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "new-source",
+		})
+
+		if len(m.nodeSnapshots) != 1 {
+			t.Fatalf("Expected 1 node snapshot after overwrite, got %d", len(m.nodeSnapshots))
+		}
+
+		key := makeNodeSnapshotKey("pve1", "node1")
+		if m.nodeSnapshots[key].MemorySource != "new-source" {
+			t.Errorf("MemorySource = %q, want %q", m.nodeSnapshots[key].MemorySource, "new-source")
+		}
+	})
+}
+
 func TestGetDiagnosticSnapshots(t *testing.T) {
 	t.Run("nil Monitor returns empty set with non-nil slices", func(t *testing.T) {
 		var m *Monitor
diff --git a/internal/monitoring/monitor_host_agents_test.go b/internal/monitoring/monitor_host_agents_test.go
index c38e43c84..adf10d9b4 100644
--- a/internal/monitoring/monitor_host_agents_test.go
+++ b/internal/monitoring/monitor_host_agents_test.go
@@ -197,3 +197,321 @@ func TestRemoveHostAgentUnbindsToken(t *testing.T) {
 		t.Fatalf("expected token binding to be cleared after host removal")
 	}
 }
+
+func TestEvaluateHostAgentsEmptyHostsList(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	// No hosts in state - should complete without error or state changes
+	monitor.evaluateHostAgents(time.Now())
+
+	snapshot := monitor.state.GetSnapshot()
+	if len(snapshot.Hosts) != 0 {
+		t.Errorf("expected 0 hosts, got %d", len(snapshot.Hosts))
+	}
+	if len(snapshot.ConnectionHealth) != 0 {
+		t.Errorf("expected 0 connection health entries, got %d", len(snapshot.ConnectionHealth))
+	}
+}
+
+func TestEvaluateHostAgentsZeroIntervalUsesDefault(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	hostID := "host-zero-interval"
+	// IntervalSeconds = 0, LastSeen = now, should use default interval (30s)
+	// Default window = 30s * 4 = 120s, but minimum is 30s, so window = 30s
+	// With LastSeen = now, the host should be healthy
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "zero-interval.local",
+		Status:          "unknown",
+		IntervalSeconds: 0, // Zero interval - should use default
+		LastSeen:        time.Now(),
+	})
+
+	monitor.evaluateHostAgents(time.Now())
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || !healthy {
+		t.Fatalf("expected connection health true for zero-interval host with recent LastSeen, got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "online" {
+			t.Errorf("expected host status online, got %q", host.Status)
+		}
+	}
+}
+
+func TestEvaluateHostAgentsNegativeIntervalUsesDefault(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	hostID := "host-negative-interval"
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "negative-interval.local",
+		Status:          "unknown",
+		IntervalSeconds: -10, // Negative interval - should use default
+		LastSeen:        time.Now(),
+	})
+
+	monitor.evaluateHostAgents(time.Now())
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || !healthy {
+		t.Fatalf("expected connection health true for negative-interval host with recent LastSeen, got %v (exists=%v)", healthy, ok)
+	}
+}
+
+func TestEvaluateHostAgentsWindowClampedToMinimum(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	hostID := "host-min-window"
+	// IntervalSeconds = 1, so window = 1s * 4 = 4s, but minimum is 30s
+	// Host last seen 25s ago should still be healthy (within 30s minimum window)
+	now := time.Now()
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "min-window.local",
+		Status:          "unknown",
+		IntervalSeconds: 1, // Very small interval
+		LastSeen:        now.Add(-25 * time.Second),
+	})
+
+	monitor.evaluateHostAgents(now)
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || !healthy {
+		t.Fatalf("expected connection health true (window clamped to minimum 30s), got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "online" {
+			t.Errorf("expected host status online, got %q", host.Status)
+		}
+	}
+}
+
+func TestEvaluateHostAgentsWindowClampedToMaximum(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	hostID := "host-max-window"
+	// IntervalSeconds = 300 (5 min), so window = 300s * 4 = 1200s (20 min)
+	// But maximum is 10 min = 600s
+	// Host last seen 11 minutes ago should be unhealthy (outside 10 min max window)
+	now := time.Now()
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "max-window.local",
+		Status:          "online",
+		IntervalSeconds: 300, // 5 minute interval
+		LastSeen:        now.Add(-11 * time.Minute),
+	})
+
+	monitor.evaluateHostAgents(now)
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || healthy {
+		t.Fatalf("expected connection health false (window clamped to maximum 10m), got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "offline" {
+			t.Errorf("expected host status offline, got %q", host.Status)
+		}
+	}
+}
+
+func TestEvaluateHostAgentsRecentLastSeenIsHealthy(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	hostID := "host-recent"
+	now := time.Now()
+	// IntervalSeconds = 30, window = 30s * 4 = 120s (clamped to min 30s is not needed)
+	// LastSeen = 10s ago, should be healthy
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "recent.local",
+		Status:          "unknown",
+		IntervalSeconds: 30,
+		LastSeen:        now.Add(-10 * time.Second),
+	})
+
+	monitor.evaluateHostAgents(now)
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || !healthy {
+		t.Fatalf("expected connection health true for recent LastSeen, got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "online" {
+			t.Errorf("expected host status online, got %q", host.Status)
+		}
+	}
+}
+
+func TestEvaluateHostAgentsZeroLastSeenIsUnhealthy(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	hostID := "host-zero-lastseen"
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "zero-lastseen.local",
+		Status:          "online",
+		IntervalSeconds: 30,
+		LastSeen:        time.Time{}, // Zero time
+	})
+
+	monitor.evaluateHostAgents(time.Now())
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || healthy {
+		t.Fatalf("expected connection health false for zero LastSeen, got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "offline" {
+			t.Errorf("expected host status offline for zero LastSeen, got %q", host.Status)
+		}
+	}
+}
+
+func TestEvaluateHostAgentsOldLastSeenIsUnhealthy(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: alerts.NewManager(),
+		config:       &config.Config{},
+	}
+	t.Cleanup(func() { monitor.alertManager.Stop() })
+
+	hostID := "host-old-lastseen"
+	now := time.Now()
+	// IntervalSeconds = 30, window = 30s * 4 = 120s
+	// LastSeen = 5 minutes ago, should be unhealthy
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "old-lastseen.local",
+		Status:          "online",
+		IntervalSeconds: 30,
+		LastSeen:        now.Add(-5 * time.Minute),
+	})
+
+	monitor.evaluateHostAgents(now)
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || healthy {
+		t.Fatalf("expected connection health false for old LastSeen, got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "offline" {
+			t.Errorf("expected host status offline for old LastSeen, got %q", host.Status)
+		}
+	}
+}
+
+func TestEvaluateHostAgentsNilAlertManagerOnline(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: nil, // No alert manager
+		config:       &config.Config{},
+	}
+
+	hostID := "host-nil-am-online"
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "nil-am-online.local",
+		Status:          "unknown",
+		IntervalSeconds: 30,
+		LastSeen:        time.Now(),
+	})
+
+	// Should not panic with nil alertManager
+	monitor.evaluateHostAgents(time.Now())
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || !healthy {
+		t.Fatalf("expected connection health true, got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "online" {
+			t.Errorf("expected host status online, got %q", host.Status)
+		}
+	}
+}
+
+func TestEvaluateHostAgentsNilAlertManagerOffline(t *testing.T) {
+	monitor := &Monitor{
+		state:        models.NewState(),
+		alertManager: nil, // No alert manager
+		config:       &config.Config{},
+	}
+
+	hostID := "host-nil-am-offline"
+	monitor.state.UpsertHost(models.Host{
+		ID:              hostID,
+		Hostname:        "nil-am-offline.local",
+		Status:          "online",
+		IntervalSeconds: 30,
+		LastSeen:        time.Time{}, // Zero time - unhealthy
+	})
+
+	// Should not panic with nil alertManager
+	monitor.evaluateHostAgents(time.Now())
+
+	snapshot := monitor.state.GetSnapshot()
+	connKey := hostConnectionPrefix + hostID
+	if healthy, ok := snapshot.ConnectionHealth[connKey]; !ok || healthy {
+		t.Fatalf("expected connection health false, got %v (exists=%v)", healthy, ok)
+	}
+
+	for _, host := range snapshot.Hosts {
+		if host.ID == hostID && host.Status != "offline" {
+			t.Errorf("expected host status offline, got %q", host.Status)
+		}
+	}
+}
diff --git a/internal/monitoring/temperature_test.go b/internal/monitoring/temperature_test.go
index 9b9bcabb9..1065808ca 100644
--- a/internal/monitoring/temperature_test.go
+++ b/internal/monitoring/temperature_test.go
@@ -2359,6 +2359,246 @@ func TestHandleProxyFailure_PlainError_TriggersDisablePath(t *testing.T) {
 	}
 }
 
+// =============================================================================
+// Tests for handleProxyHostFailure
+// =============================================================================
+
+func TestHandleProxyHostFailure_EmptyHost(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: make(map[string]*proxyHostState),
+	}
+
+	tc.handleProxyHostFailure("", fmt.Errorf("some error"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	if len(tc.proxyHostStates) != 0 {
+		t.Errorf("expected no state change for empty host, got %d entries", len(tc.proxyHostStates))
+	}
+}
+
+func TestHandleProxyHostFailure_WhitespaceOnlyHost(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: make(map[string]*proxyHostState),
+	}
+
+	tc.handleProxyHostFailure("   ", fmt.Errorf("some error"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	if len(tc.proxyHostStates) != 0 {
+		t.Errorf("expected no state change for whitespace-only host, got %d entries", len(tc.proxyHostStates))
+	}
+}
+
+func TestHandleProxyHostFailure_FirstFailureCreatesState(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: make(map[string]*proxyHostState),
+	}
+
+	tc.handleProxyHostFailure("192.168.1.100", fmt.Errorf("connection refused"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	state, exists := tc.proxyHostStates["192.168.1.100"]
+	if !exists {
+		t.Fatal("expected host state to be created")
+	}
+	if state.failures != 1 {
+		t.Errorf("expected failures to be 1, got %d", state.failures)
+	}
+	if state.lastError != "connection refused" {
+		t.Errorf("expected lastError to be 'connection refused', got %q", state.lastError)
+	}
+	if !state.cooldownUntil.IsZero() {
+		t.Errorf("expected cooldownUntil to be zero (threshold not reached), got %s", state.cooldownUntil)
+	}
+}
+
+func TestHandleProxyHostFailure_SubsequentFailuresIncrement(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: map[string]*proxyHostState{
+			"192.168.1.100": {
+				failures:  1,
+				lastError: "first error",
+			},
+		},
+	}
+
+	tc.handleProxyHostFailure("192.168.1.100", fmt.Errorf("second error"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	state := tc.proxyHostStates["192.168.1.100"]
+	if state.failures != 2 {
+		t.Errorf("expected failures to be 2, got %d", state.failures)
+	}
+	if state.lastError != "second error" {
+		t.Errorf("expected lastError to be 'second error', got %q", state.lastError)
+	}
+	if !state.cooldownUntil.IsZero() {
+		t.Errorf("expected cooldownUntil to remain zero (threshold not reached), got %s", state.cooldownUntil)
+	}
+}
+
+func TestHandleProxyHostFailure_ReachesThresholdSetsCooldown(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: map[string]*proxyHostState{
+			"192.168.1.100": {
+				failures:  proxyFailureThreshold - 1, // one failure away
+				lastError: "previous error",
+			},
+		},
+	}
+
+	before := time.Now()
+	tc.handleProxyHostFailure("192.168.1.100", fmt.Errorf("final error"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	state := tc.proxyHostStates["192.168.1.100"]
+	// failures should be reset to 0 after reaching threshold
+	if state.failures != 0 {
+		t.Errorf("expected failures to be reset to 0 after reaching threshold, got %d", state.failures)
+	}
+	if state.lastError != "final error" {
+		t.Errorf("expected lastError to be 'final error', got %q", state.lastError)
+	}
+	// cooldownUntil should be set in the future
+	if !state.cooldownUntil.After(before) {
+		t.Errorf("expected cooldownUntil to be set in the future, got %s", state.cooldownUntil)
+	}
+	// cooldownUntil should be approximately proxyRetryInterval from now
+	expectedMin := before.Add(proxyRetryInterval - time.Second)
+	if state.cooldownUntil.Before(expectedMin) {
+		t.Errorf("expected cooldownUntil to be at least %s, got %s", expectedMin, state.cooldownUntil)
+	}
+}
+
+func TestHandleProxyHostFailure_LastErrorStored(t *testing.T) {
+	tests := []struct {
+		name        string
+		err         error
+		wantLastErr string
+	}{
+		{
+			name:        "simple error message",
+			err:         fmt.Errorf("connection refused"),
+			wantLastErr: "connection refused",
+		},
+		{
+			name:        "error with whitespace is trimmed",
+			err:         fmt.Errorf("  timeout waiting for response  "),
+			wantLastErr: "timeout waiting for response",
+		},
+		{
+			name:        "empty error message",
+			err:         fmt.Errorf(""),
+			wantLastErr: "",
+		},
+		{
+			name:        "multiline error gets first part after trim",
+			err:         fmt.Errorf("network error\ndetails here"),
+			wantLastErr: "network error\ndetails here",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tc := &TemperatureCollector{
+				proxyHostStates: make(map[string]*proxyHostState),
+			}
+
+			tc.handleProxyHostFailure("192.168.1.100", tt.err)
+
+			tc.proxyMu.Lock()
+			state := tc.proxyHostStates["192.168.1.100"]
+			tc.proxyMu.Unlock()
+
+			if state.lastError != tt.wantLastErr {
+				t.Errorf("expected lastError to be %q, got %q", tt.wantLastErr, state.lastError)
+			}
+		})
+	}
+}
+
+func TestHandleProxyHostFailure_NilStateInMap(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: map[string]*proxyHostState{
+			"192.168.1.100": nil, // nil state in map
+		},
+	}
+
+	tc.handleProxyHostFailure("192.168.1.100", fmt.Errorf("some error"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	state := tc.proxyHostStates["192.168.1.100"]
+	if state == nil {
+		t.Fatal("expected new state to be created for nil entry")
+	}
+	if state.failures != 1 {
+		t.Errorf("expected failures to be 1, got %d", state.failures)
+	}
+}
+
+func TestHandleProxyHostFailure_TrimsHostWhitespace(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: make(map[string]*proxyHostState),
+	}
+
+	tc.handleProxyHostFailure("  192.168.1.100  ", fmt.Errorf("some error"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	// State should be stored under trimmed key
+	if _, exists := tc.proxyHostStates["192.168.1.100"]; !exists {
+		t.Error("expected state to be stored under trimmed host key")
+	}
+	if _, exists := tc.proxyHostStates["  192.168.1.100  "]; exists {
+		t.Error("state should not be stored under untrimmed host key")
+	}
+}
+
+func TestHandleProxyHostFailure_MultipleHostsIndependent(t *testing.T) {
+	tc := &TemperatureCollector{
+		proxyHostStates: make(map[string]*proxyHostState),
+	}
+
+	// First host gets multiple failures
+	tc.handleProxyHostFailure("192.168.1.100", fmt.Errorf("error 1"))
+	tc.handleProxyHostFailure("192.168.1.100", fmt.Errorf("error 2"))
+
+	// Second host gets one failure
+	tc.handleProxyHostFailure("192.168.1.101", fmt.Errorf("different error"))
+
+	tc.proxyMu.Lock()
+	defer tc.proxyMu.Unlock()
+
+	state1 := tc.proxyHostStates["192.168.1.100"]
+	state2 := tc.proxyHostStates["192.168.1.101"]
+
+	if state1.failures != 2 {
+		t.Errorf("expected host 1 failures to be 2, got %d", state1.failures)
+	}
+	if state2.failures != 1 {
+		t.Errorf("expected host 2 failures to be 1, got %d", state2.failures)
+	}
+	if state1.lastError != "error 2" {
+		t.Errorf("expected host 1 lastError to be 'error 2', got %q", state1.lastError)
+	}
+	if state2.lastError != "different error" {
+		t.Errorf("expected host 2 lastError to be 'different error', got %q", state2.lastError)
+	}
+}
+
 // Helper functions for test setup
 
 func intPtr(i int) *int {