diff --git a/app/vmalert/README.md b/app/vmalert/README.md
index 544576d97..71e0f15df 100644
--- a/app/vmalert/README.md
+++ b/app/vmalert/README.md
@@ -813,16 +813,17 @@ external_labels:
 series: <string>
 
 # values support several special equations:
-#     'a+bxc' becomes 'a a+b a+(2*b) a+(3*b) … a+(c*b)'
+#    'a+bxc' becomes 'a a+b a+(2*b) a+(3*b) … a+(c*b)'
 #     Read this as series starts at a, then c further samples incrementing by b.
-#     'a-bxc' becomes 'a a-b a-(2*b) a-(3*b) … a-(c*b)'
+#    'a-bxc' becomes 'a a-b a-(2*b) a-(3*b) … a-(c*b)'
 #     Read this as series starts at a, then c further samples decrementing by b (or incrementing by negative b).
 #    '_' represents a missing sample from scrape
+#    'stale' indicates a stale sample
 # Examples:
 #     1. '-2+4x3' becomes '-2 2 6 10' - series starts at -2, then 3 further samples incrementing by 4.
 #     2. ' 1-2x4' becomes '1 -1 -3 -5 -7' - series starts at 1, then 4 further samples decrementing by 2.
 #     3. ' 1x4' becomes '1 1 1 1 1' - shorthand for '1+0x4', series starts at 1, then 4 further samples incrementing by 0.
-#     4. ' 1 _x3' becomes '1 _ _ _ ' - the missing sample cannot increment, so 3 missing samples are produced by the '_x3' expression.
+#     4. ' 1 _x3 stale' becomes '1 _ _ _ stale' - the missing sample cannot increment, so 3 missing samples are produced by the '_x3' expression.
 values: <string>
 ```
 
diff --git a/app/vmalert/unittest/input.go b/app/vmalert/unittest/input.go
index 6ba19bab3..58dd3edab 100644
--- a/app/vmalert/unittest/input.go
+++ b/app/vmalert/unittest/input.go
@@ -12,6 +12,7 @@ import (
 
 	testutil "github.com/VictoriaMetrics/VictoriaMetrics/app/victoria-metrics/test"
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmstorage"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/decimal"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/promutils"
 	"github.com/VictoriaMetrics/metricsql"
@@ -85,9 +86,13 @@ func WriteInputSeries(input []Series, interval *promutils.Duration, startStamp t
 func parseInputValue(input string, origin bool) ([]sequenceValue, error) {
 	var res []sequenceValue
 	items := strings.Split(input, " ")
-	reg2 := regexp.MustCompile(`\D?\d*\D?`)
+	reg := regexp.MustCompile(`\D?\d*\D?`)
 	for _, item := range items {
-		vals := reg2.FindAllString(item, -1)
+		if item == "stale" {
+			res = append(res, sequenceValue{Value: decimal.StaleNaN})
+			continue
+		}
+		vals := reg.FindAllString(item, -1)
 		switch len(vals) {
 		case 1:
 			if vals[0] == "_" {
diff --git a/app/vmalert/unittest/input_test.go b/app/vmalert/unittest/input_test.go
index cf8bb7942..5f74de98b 100644
--- a/app/vmalert/unittest/input_test.go
+++ b/app/vmalert/unittest/input_test.go
@@ -1,8 +1,9 @@
 package unittest
 
 import (
-	"reflect"
 	"testing"
+
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/decimal"
 )
 
 func TestParseInputValue(t *testing.T) {
@@ -21,6 +22,12 @@ func TestParseInputValue(t *testing.T) {
 			nil,
 			true,
 		},
+		// stale doesn't support operations
+		{
+			"stalex3",
+			nil,
+			true,
+		},
 		{
 			"-4",
 			[]sequenceValue{{Value: -4}},
@@ -31,6 +38,11 @@ func TestParseInputValue(t *testing.T) {
 			[]sequenceValue{{Omitted: true}},
 			false,
 		},
+		{
+			"stale",
+			[]sequenceValue{{Value: decimal.StaleNaN}},
+			false,
+		},
 		{
 			"-4x1",
 			[]sequenceValue{{Value: -4}, {Value: -4}},
@@ -52,8 +64,8 @@ func TestParseInputValue(t *testing.T) {
 			false,
 		},
 		{
-			"1+1x1 _ -4 3+20x1",
-			[]sequenceValue{{Value: 1}, {Value: 2}, {Omitted: true}, {Value: -4}, {Value: 3}, {Value: 23}},
+			"1+1x1 _ -4 stale 3+20x1",
+			[]sequenceValue{{Value: 1}, {Value: 2}, {Omitted: true}, {Value: -4}, {Value: decimal.StaleNaN}, {Value: 3}, {Value: 23}},
 			false,
 		},
 	}
@@ -63,8 +75,19 @@ func TestParseInputValue(t *testing.T) {
 		if err != nil != tc.failed {
 			t.Fatalf("failed to parse %s, expect %t, got %t", tc.input, tc.failed, err != nil)
 		}
-		if !reflect.DeepEqual(tc.exp, output) {
+		if len(tc.exp) != len(output) {
 			t.Fatalf("expect %v, got %v", tc.exp, output)
 		}
+		for i := 0; i < len(tc.exp); i++ {
+			if tc.exp[i].Omitted != output[i].Omitted {
+				t.Fatalf("expect %v, got %v", tc.exp, output)
+			}
+			if tc.exp[i].Value != output[i].Value {
+				if decimal.IsStaleNaN(tc.exp[i].Value) && decimal.IsStaleNaN(output[i].Value) {
+					continue
+				}
+				t.Fatalf("expect %v, got %v", tc.exp, output)
+			}
+		}
 	}
 }
diff --git a/app/vmalert/unittest/testdata/test1.yaml b/app/vmalert/unittest/testdata/test1.yaml
index 96f3a30ab..d60e8587f 100644
--- a/app/vmalert/unittest/testdata/test1.yaml
+++ b/app/vmalert/unittest/testdata/test1.yaml
@@ -9,7 +9,7 @@ tests:
     name: "basic test"
     input_series:
       - series: "test"
-        values: "_x5 1x5"
+        values: "_x5 1x5 _ stale"
 
     alert_rule_test:
       - eval_time: 1m
@@ -26,6 +26,16 @@ tests:
         alertname: SameAlertNameWithDifferentGroup
         exp_alerts: []
 
+    metricsql_expr_test:
+      - expr: test
+        eval_time: 11m
+        exp_samples:
+          - labels: '{__name__="test"}'
+            value: 1
+      - expr: test
+        eval_time: 12m
+        exp_samples: []
+
   - interval: 1m
     name: "basic test2"
     input_series:
diff --git a/docs/vmalert.md b/docs/vmalert.md
index 085f3c3cb..84d212b6d 100644
--- a/docs/vmalert.md
+++ b/docs/vmalert.md
@@ -824,16 +824,17 @@ external_labels:
 series: <string>
 
 # values support several special equations:
-#     'a+bxc' becomes 'a a+b a+(2*b) a+(3*b) … a+(c*b)'
+#    'a+bxc' becomes 'a a+b a+(2*b) a+(3*b) … a+(c*b)'
 #     Read this as series starts at a, then c further samples incrementing by b.
-#     'a-bxc' becomes 'a a-b a-(2*b) a-(3*b) … a-(c*b)'
+#    'a-bxc' becomes 'a a-b a-(2*b) a-(3*b) … a-(c*b)'
 #     Read this as series starts at a, then c further samples decrementing by b (or incrementing by negative b).
 #    '_' represents a missing sample from scrape
+#    'stale' indicates a stale sample
 # Examples:
 #     1. '-2+4x3' becomes '-2 2 6 10' - series starts at -2, then 3 further samples incrementing by 4.
 #     2. ' 1-2x4' becomes '1 -1 -3 -5 -7' - series starts at 1, then 4 further samples decrementing by 2.
 #     3. ' 1x4' becomes '1 1 1 1 1' - shorthand for '1+0x4', series starts at 1, then 4 further samples incrementing by 0.
-#     4. ' 1 _x3' becomes '1 _ _ _ ' - the missing sample cannot increment, so 3 missing samples are produced by the '_x3' expression.
+#     4. ' 1 _x3 stale' becomes '1 _ _ _ stale' - the missing sample cannot increment, so 3 missing samples are produced by the '_x3' expression.
 values: <string>
 ```