diff --git a/sparkplug_plugin/unit_test.go b/sparkplug_plugin/unit_test.go
index 93fea42..81fc8cd 100644
--- a/sparkplug_plugin/unit_test.go
+++ b/sparkplug_plugin/unit_test.go
@@ -110,6 +110,37 @@ var _ = Describe("AliasCache Unit Tests", func() {
 			Expect(count).To(Equal(2))
 			Expect(*dataMetrics[0].Name).To(Equal("Temperature"))
 			Expect(*dataMetrics[1].Name).To(Equal("Pressure"))
+			//TODO: test on type
+		})
+
+		// ENG-5126: the BIRTH-cached datatype must be restored on DATA metrics
+		// even when the publisher already sent the metric name alongside the
+		// alias. Without the datatype, extractMetricValue cannot reinterpret
+		// signed integer wire values (two's-complement packed into the unsigned
+		// int_value/long_value fields) and Int32(-12) surfaces as 4294967284.
+		It("restores the datatype when DATA carries the name alongside the alias", func() {
+			birthMetrics := []*sparkplugb.Payload_Metric{
+				{
+					Name:     stringPtr("Temperature"),
+					Alias:    uint64Ptr(100),
+					Datatype: uint32Ptr(sparkplugplugin.SparkplugDataTypeInt32),
+				},
+			}
+			Expect(cache.CacheAliases("TestFactory/Line1", birthMetrics)).To(Equal(1))
+
+			dataMetrics := []*sparkplugb.Payload_Metric{
+				{
+					Name:  stringPtr("Temperature"),
+					Alias: uint64Ptr(100),
+					Value: &sparkplugb.Payload_Metric_IntValue{IntValue: 4294967284}, // -12 as Int32
+				},
+			}
+
+			cache.ResolveAliases("TestFactory/Line1", dataMetrics)
+
+			Expect(dataMetrics[0].Datatype).NotTo(BeNil(),
+				"datatype from the BIRTH certificate must be restored even when DATA already carries the name")
+			Expect(*dataMetrics[0].Datatype).To(Equal(sparkplugplugin.SparkplugDataTypeInt32))
 		})
 
 		It("should handle alias collisions in NBIRTH", func() {