diff --git a/internal/monitoring/diagnostic_snapshots_test.go b/internal/monitoring/diagnostic_snapshots_test.go
index 6082c2e..e4d259b 100644
--- a/internal/monitoring/diagnostic_snapshots_test.go
+++ b/internal/monitoring/diagnostic_snapshots_test.go
@@ -3,6 +3,8 @@ package monitoring
 import (
 	"testing"
 	"time"
+
+	"github.com/rcourtman/pulse-go-rewrite/internal/models"
 )
 
 func TestMakeNodeSnapshotKey(t *testing.T) {
@@ -524,6 +526,238 @@ func TestRecordGuestSnapshot(t *testing.T) {
 	})
 }
 
+func TestLogNodeMemorySource(t *testing.T) {
+	t.Run("nil Monitor is no-op", func(t *testing.T) {
+		var m *Monitor
+		// Should not panic
+		m.logNodeMemorySource("instance", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+		})
+	})
+
+	t.Run("same source as existing snapshot - no log emitted", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Pre-populate with existing snapshot
+		key := makeNodeSnapshotKey("pve1", "node1")
+		m.nodeSnapshots[key] = NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+		}
+
+		// Should not panic and should return early (same source)
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+		})
+	})
+
+	t.Run("empty source triggers warn level", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - empty source triggers Warn
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "",
+		})
+	})
+
+	t.Run("nodes-endpoint source triggers warn level", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - nodes-endpoint triggers Warn
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "nodes-endpoint",
+		})
+	})
+
+	t.Run("node-status-used source triggers warn level", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - node-status-used triggers Warn
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "node-status-used",
+		})
+	})
+
+	t.Run("previous-snapshot source triggers warn level", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - previous-snapshot triggers Warn
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "previous-snapshot",
+		})
+	})
+
+	t.Run("rrd-data source triggers debug level", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - rrd-data triggers Debug
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+		})
+	})
+
+	t.Run("rrd-available source triggers debug level", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - rrd-available triggers Debug
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-available",
+		})
+	})
+
+	t.Run("source change from existing triggers log", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Pre-populate with existing snapshot
+		key := makeNodeSnapshotKey("pve1", "node1")
+		m.nodeSnapshots[key] = NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+		}
+
+		// Different source should trigger logging
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-available",
+		})
+	})
+
+	t.Run("FallbackReason is logged when present", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - FallbackReason present
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource:   "previous-snapshot",
+			FallbackReason: "no rrd data available",
+		})
+	})
+
+	t.Run("Raw fields are logged when > 0", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - various Raw fields > 0
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+			Raw: NodeMemoryRaw{
+				Available:           1000000000,
+				Buffers:             200000000,
+				Cached:              500000000,
+				TotalMinusUsed:      800000000,
+				RRDAvailable:        900000000,
+				RRDUsed:             100000000,
+				RRDTotal:            1000000000,
+				ProxmoxMemorySource: "rrd",
+			},
+		})
+	})
+
+	t.Run("Memory fields are logged when > 0", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// Should not panic - Memory fields > 0
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+			Memory: models.Memory{
+				Total: 16000000000,
+				Used:  8000000000,
+				Free:  8000000000,
+				Usage: 0.5,
+			},
+		})
+	})
+
+	t.Run("all warn sources", func(t *testing.T) {
+		warnSources := []string{"", "nodes-endpoint", "node-status-used", "previous-snapshot"}
+
+		for _, source := range warnSources {
+			t.Run("source_"+source, func(t *testing.T) {
+				m := &Monitor{
+					nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+					guestSnapshots: make(map[string]GuestMemorySnapshot),
+				}
+
+				// Should not panic
+				m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+					MemorySource: source,
+				})
+			})
+		}
+	})
+
+	t.Run("debug sources", func(t *testing.T) {
+		debugSources := []string{"rrd-data", "rrd-available", "node-status-available", "calculated"}
+
+		for _, source := range debugSources {
+			t.Run("source_"+source, func(t *testing.T) {
+				m := &Monitor{
+					nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+					guestSnapshots: make(map[string]GuestMemorySnapshot),
+				}
+
+				// Should not panic
+				m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+					MemorySource: source,
+				})
+			})
+		}
+	})
+
+	t.Run("new node with no prior snapshot logs", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// No existing snapshot, empty prevSource should not match non-empty source
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "rrd-data",
+		})
+	})
+
+	t.Run("new node with empty source matches no prior snapshot", func(t *testing.T) {
+		m := &Monitor{
+			nodeSnapshots:  make(map[string]NodeMemorySnapshot),
+			guestSnapshots: make(map[string]GuestMemorySnapshot),
+		}
+
+		// No existing snapshot means prevSource is "", matching empty MemorySource
+		// This should skip logging due to prevSource == snapshot.MemorySource
+		m.logNodeMemorySource("pve1", "node1", NodeMemorySnapshot{
+			MemorySource: "",
+		})
+	})
+}
+
 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/docker_commands_test.go b/internal/monitoring/docker_commands_test.go
index 533bb0b..e0752e2 100644
--- a/internal/monitoring/docker_commands_test.go
+++ b/internal/monitoring/docker_commands_test.go
@@ -287,6 +287,84 @@ func TestDockerCommandHasExpired(t *testing.T) {
 	}
 }
 
+func TestMarkFailed(t *testing.T) {
+	t.Parallel()
+
+	t.Run("sets status to failed", func(t *testing.T) {
+		t.Parallel()
+
+		cmd := newDockerHostCommand(DockerCommandTypeStop, "test", dockerCommandDefaultTTL, nil)
+		cmd.markFailed("connection lost")
+
+		if cmd.status.Status != DockerCommandStatusFailed {
+			t.Errorf("expected status %q, got %q", DockerCommandStatusFailed, cmd.status.Status)
+		}
+	})
+
+	t.Run("sets FailedAt to current time", func(t *testing.T) {
+		t.Parallel()
+
+		cmd := newDockerHostCommand(DockerCommandTypeStop, "test", dockerCommandDefaultTTL, nil)
+		before := time.Now().UTC()
+		cmd.markFailed("timeout")
+		after := time.Now().UTC()
+
+		if cmd.status.FailedAt == nil {
+			t.Fatal("expected FailedAt to be set")
+		}
+		if cmd.status.FailedAt.Before(before) || cmd.status.FailedAt.After(after) {
+			t.Errorf("FailedAt %v not in expected range [%v, %v]", *cmd.status.FailedAt, before, after)
+		}
+	})
+
+	t.Run("sets UpdatedAt to current time", func(t *testing.T) {
+		t.Parallel()
+
+		cmd := newDockerHostCommand(DockerCommandTypeStop, "test", dockerCommandDefaultTTL, nil)
+		originalUpdatedAt := cmd.status.UpdatedAt
+		time.Sleep(time.Millisecond) // Ensure time advances
+		before := time.Now().UTC()
+		cmd.markFailed("error")
+		after := time.Now().UTC()
+
+		if !cmd.status.UpdatedAt.After(originalUpdatedAt) {
+			t.Errorf("UpdatedAt should be updated; original=%v, new=%v", originalUpdatedAt, cmd.status.UpdatedAt)
+		}
+		if cmd.status.UpdatedAt.Before(before) || cmd.status.UpdatedAt.After(after) {
+			t.Errorf("UpdatedAt %v not in expected range [%v, %v]", cmd.status.UpdatedAt, before, after)
+		}
+	})
+
+	t.Run("sets FailureReason to provided reason", func(t *testing.T) {
+		t.Parallel()
+
+		cmd := newDockerHostCommand(DockerCommandTypeStop, "test", dockerCommandDefaultTTL, nil)
+		reason := "agent unreachable: connection refused"
+		cmd.markFailed(reason)
+
+		if cmd.status.FailureReason != reason {
+			t.Errorf("expected FailureReason %q, got %q", reason, cmd.status.FailureReason)
+		}
+	})
+
+	t.Run("accepts empty reason string", func(t *testing.T) {
+		t.Parallel()
+
+		cmd := newDockerHostCommand(DockerCommandTypeStop, "test", dockerCommandDefaultTTL, nil)
+		cmd.markFailed("")
+
+		if cmd.status.Status != DockerCommandStatusFailed {
+			t.Errorf("expected status %q, got %q", DockerCommandStatusFailed, cmd.status.Status)
+		}
+		if cmd.status.FailureReason != "" {
+			t.Errorf("expected empty FailureReason, got %q", cmd.status.FailureReason)
+		}
+		if cmd.status.FailedAt == nil {
+			t.Error("expected FailedAt to be set even with empty reason")
+		}
+	})
+}
+
 func TestAcknowledgeDockerCommandErrorPaths(t *testing.T) {
 	t.Parallel()
 
diff --git a/internal/monitoring/monitor_test.go b/internal/monitoring/monitor_test.go
new file mode 100644
index 0000000..327b63f
--- /dev/null
+++ b/internal/monitoring/monitor_test.go
@@ -0,0 +1,164 @@
+package monitoring
+
+import (
+	"testing"
+	"time"
+)
+
+func TestParseDurationEnv(t *testing.T) {
+	const testKey = "TEST_DURATION_ENV"
+	defaultVal := 30 * time.Second
+
+	t.Run("empty env var returns default", func(t *testing.T) {
+		t.Setenv(testKey, "")
+		result := parseDurationEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected %v, got %v", defaultVal, result)
+		}
+	})
+
+	t.Run("unset env var returns default", func(t *testing.T) {
+		// t.Setenv automatically cleans up, so not setting means unset
+		result := parseDurationEnv("UNSET_DURATION_KEY_12345", defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected %v, got %v", defaultVal, result)
+		}
+	})
+
+	t.Run("valid duration seconds", func(t *testing.T) {
+		t.Setenv(testKey, "1s")
+		result := parseDurationEnv(testKey, defaultVal)
+		expected := 1 * time.Second
+		if result != expected {
+			t.Errorf("expected %v, got %v", expected, result)
+		}
+	})
+
+	t.Run("valid duration minutes", func(t *testing.T) {
+		t.Setenv(testKey, "5m")
+		result := parseDurationEnv(testKey, defaultVal)
+		expected := 5 * time.Minute
+		if result != expected {
+			t.Errorf("expected %v, got %v", expected, result)
+		}
+	})
+
+	t.Run("valid duration composite", func(t *testing.T) {
+		t.Setenv(testKey, "2h30m")
+		result := parseDurationEnv(testKey, defaultVal)
+		expected := 2*time.Hour + 30*time.Minute
+		if result != expected {
+			t.Errorf("expected %v, got %v", expected, result)
+		}
+	})
+
+	t.Run("valid duration milliseconds", func(t *testing.T) {
+		t.Setenv(testKey, "500ms")
+		result := parseDurationEnv(testKey, defaultVal)
+		expected := 500 * time.Millisecond
+		if result != expected {
+			t.Errorf("expected %v, got %v", expected, result)
+		}
+	})
+
+	t.Run("invalid duration returns default", func(t *testing.T) {
+		t.Setenv(testKey, "invalid")
+		result := parseDurationEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected default %v, got %v", defaultVal, result)
+		}
+	})
+
+	t.Run("numeric without unit returns default", func(t *testing.T) {
+		t.Setenv(testKey, "100")
+		result := parseDurationEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected default %v, got %v", defaultVal, result)
+		}
+	})
+
+	t.Run("negative duration parses correctly", func(t *testing.T) {
+		t.Setenv(testKey, "-5s")
+		result := parseDurationEnv(testKey, defaultVal)
+		expected := -5 * time.Second
+		if result != expected {
+			t.Errorf("expected %v, got %v", expected, result)
+		}
+	})
+}
+
+func TestParseIntEnv(t *testing.T) {
+	const testKey = "TEST_INT_ENV"
+	defaultVal := 42
+
+	t.Run("empty env var returns default", func(t *testing.T) {
+		t.Setenv(testKey, "")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected %d, got %d", defaultVal, result)
+		}
+	})
+
+	t.Run("unset env var returns default", func(t *testing.T) {
+		result := parseIntEnv("UNSET_INT_KEY_12345", defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected %d, got %d", defaultVal, result)
+		}
+	})
+
+	t.Run("valid positive integer", func(t *testing.T) {
+		t.Setenv(testKey, "100")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != 100 {
+			t.Errorf("expected 100, got %d", result)
+		}
+	})
+
+	t.Run("valid zero", func(t *testing.T) {
+		t.Setenv(testKey, "0")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != 0 {
+			t.Errorf("expected 0, got %d", result)
+		}
+	})
+
+	t.Run("valid negative integer", func(t *testing.T) {
+		t.Setenv(testKey, "-50")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != -50 {
+			t.Errorf("expected -50, got %d", result)
+		}
+	})
+
+	t.Run("invalid string returns default", func(t *testing.T) {
+		t.Setenv(testKey, "not-a-number")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected default %d, got %d", defaultVal, result)
+		}
+	})
+
+	t.Run("float returns default", func(t *testing.T) {
+		t.Setenv(testKey, "3.14")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected default %d, got %d", defaultVal, result)
+		}
+	})
+
+	t.Run("whitespace returns default", func(t *testing.T) {
+		t.Setenv(testKey, "  ")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected default %d, got %d", defaultVal, result)
+		}
+	})
+
+	t.Run("number with trailing text returns default", func(t *testing.T) {
+		t.Setenv(testKey, "100abc")
+		result := parseIntEnv(testKey, defaultVal)
+		if result != defaultVal {
+			t.Errorf("expected default %d, got %d", defaultVal, result)
+		}
+	})
+}