diff --git a/tools/lambda-promtail/lambda-promtail/cw_test.go b/tools/lambda-promtail/lambda-promtail/cw_test.go
new file mode 100644
index 0000000000000..735b0d190ca73
--- /dev/null
+++ b/tools/lambda-promtail/lambda-promtail/cw_test.go
@@ -0,0 +1,59 @@
+package main
+
+import (
+	"context"
+	"testing"
+
+	"github.com/aws/aws-lambda-go/events"
+	"github.com/stretchr/testify/require"
+
+	"github.com/grafana/loki/pkg/logproto"
+)
+
+func Test_parseCWEvent(t *testing.T) {
+	tests := []struct {
+		name           string
+		b              *batch
+		expectedStream string
+		keepStream     bool
+	}{
+		{
+			name: "cloudwatch",
+			b: &batch{
+				streams: map[string]*logproto.Stream{},
+			},
+			expectedStream: `{__aws_cloudwatch_log_group="testLogGroup", __aws_cloudwatch_owner="123456789123", __aws_log_type="cloudwatch"}`,
+			keepStream:     false,
+		},
+		{
+			name: "cloudwatch_keepStream",
+			b: &batch{
+				streams: map[string]*logproto.Stream{},
+			},
+			expectedStream: `{__aws_cloudwatch_log_group="testLogGroup", __aws_cloudwatch_log_stream="testLogStream", __aws_cloudwatch_owner="123456789123", __aws_log_type="cloudwatch"}`,
+			keepStream:     true,
+		},
+	}
+
+	for _, tt := range tests {
+		// https://github.com/aws/aws-lambda-go/blob/main/events/cloudwatch_logs_test.go
+		cwevent := &events.CloudwatchLogsEvent{
+			AWSLogs: events.CloudwatchLogsRawData{
+				Data: "H4sIAAAAAAAAAHWPwQqCQBCGX0Xm7EFtK+smZBEUgXoLCdMhFtKV3akI8d0bLYmibvPPN3wz00CJxmQnTO41whwWQRIctmEcB6sQbFC3CjW3XW8kxpOpP+OC22d1Wml1qZkQGtoMsScxaczKN3plG8zlaHIta5KqWsozoTYw3/djzwhpLwivWFGHGpAFe7DL68JlBUk+l7KSN7tCOEJ4M3/qOI49vMHj+zCKdlFqLaU2ZHV2a4Ct/an0/ivdX8oYc1UVX860fQDQiMdxRQEAAA==",
+			},
+		}
+
+		t.Run(tt.name, func(t *testing.T) {
+			batchSize = 131072 // Set large enough we don't send to promtail
+			keepStream = tt.keepStream
+			err := parseCWEvent(context.Background(), tt.b, cwevent)
+			if err != nil {
+				t.Error(err)
+			}
+			require.Len(t, tt.b.streams, 1)
+			stream, ok := tt.b.streams[tt.expectedStream]
+			require.True(t, ok, "batch does not contain stream: %s", tt.expectedStream)
+			require.NotNil(t, stream)
+		})
+	}
+}