diff --git a/dev/managedservicesplatform/internal/resource/alertpolicy/BUILD.bazel b/dev/managedservicesplatform/internal/resource/alertpolicy/BUILD.bazel
index 45dd352aba5..9204dcba39d 100644
--- a/dev/managedservicesplatform/internal/resource/alertpolicy/BUILD.bazel
+++ b/dev/managedservicesplatform/internal/resource/alertpolicy/BUILD.bazel
@@ -29,6 +29,7 @@ go_test(
     name = "alertpolicy_test",
     srcs = [
         "conditionbuilder_test.go",
+        "customalert_test.go",
         "responsecode_test.go",
     ],
     embed = [":alertpolicy"],
diff --git a/dev/managedservicesplatform/internal/resource/alertpolicy/customalert.go b/dev/managedservicesplatform/internal/resource/alertpolicy/customalert.go
index 04beac6a606..5f7ac9bc933 100644
--- a/dev/managedservicesplatform/internal/resource/alertpolicy/customalert.go
+++ b/dev/managedservicesplatform/internal/resource/alertpolicy/customalert.go
@@ -1,6 +1,9 @@
 package alertpolicy
 
 import (
+	"fmt"
+	"strings"
+
 	"github.com/sourcegraph/managed-services-platform-cdktf/gen/google/monitoringalertpolicy"
 
 	"github.com/sourcegraph/sourcegraph/dev/managedservicesplatform/spec"
@@ -26,11 +29,18 @@ func newCustomAlertCondition(config *Config) *monitoringalertpolicy.MonitoringAl
 		return &monitoringalertpolicy.MonitoringAlertPolicyConditions{
 			DisplayName: pointers.Ptr(config.Name),
 			ConditionPrometheusQueryLanguage: &monitoringalertpolicy.MonitoringAlertPolicyConditionsConditionPrometheusQueryLanguage{
-				Query:    pointers.Ptr(config.CustomAlert.Query),
+				Query:    pointers.Ptr(flattenPromQLQuery(config.CustomAlert.Query)),
 				Duration: pointers.Stringf("%ds", *config.CustomAlert.DurationMinutes*60),
 			},
 		}
+	default:
+		panic(fmt.Sprintf("unknown custom alert type %q", config.CustomAlert.Type))
 	}
-
-	return nil
+}
+
+// GCP monitoring expects PromQL queries to be on a single line, so we do that
+// automatically and also strip extra spaces for readability of the condensed
+// version.
+func flattenPromQLQuery(query string) string {
+	return strings.Join(strings.Fields(strings.ReplaceAll(query, "\n", " ")), " ")
 }
diff --git a/dev/managedservicesplatform/internal/resource/alertpolicy/customalert_test.go b/dev/managedservicesplatform/internal/resource/alertpolicy/customalert_test.go
new file mode 100644
index 00000000000..3357f704ca6
--- /dev/null
+++ b/dev/managedservicesplatform/internal/resource/alertpolicy/customalert_test.go
@@ -0,0 +1,53 @@
+package alertpolicy
+
+import (
+	"testing"
+
+	"github.com/hexops/autogold/v2"
+)
+
+func TestFlattenPromQLQuery(t *testing.T) {
+	for _, tc := range []struct {
+		name  string
+		query string
+		want  autogold.Value
+	}{{
+		name:  "simple query",
+		query: "sum(rate(foo[1m]))",
+		want:  autogold.Expect("sum(rate(foo[1m]))"),
+	}, {
+		name: "complex query",
+		query: `            (
+				sum by (rpc_error_service_method) (
+				  label_join(
+					rate(workload_googleapis_com:rpc_server_requests_per_rpc_count{
+					  monitored_resource="generic_task",
+					  rpc_connect_rpc_error_code!=""
+					}[30m]),
+				  "rpc_error_service_method",
+				  "/",
+				  "rpc_connect_rpc_error_code", "rpc_service", "rpc_method"
+				  )
+				)
+				/
+				ignoring(rpc_error_service_method) group_left
+				sum by (rpc_error_service_method) (
+				  label_join(
+					rate(workload_googleapis_com:rpc_server_requests_per_rpc_count{
+					  monitored_resource="generic_task",
+					  rpc_connect_rpc_error_code=""
+					}[30m]),
+				  "rpc_service_method_error",
+				  "/",
+				  "rpc_service", "rpc_method", "rpc_connect_rpc_error_code"
+				  )
+				)
+			  ) > 0.2`,
+		want: autogold.Expect(`( sum by (rpc_error_service_method) ( label_join( rate(workload_googleapis_com:rpc_server_requests_per_rpc_count{ monitored_resource="generic_task", rpc_connect_rpc_error_code!="" }[30m]), "rpc_error_service_method", "/", "rpc_connect_rpc_error_code", "rpc_service", "rpc_method" ) ) / ignoring(rpc_error_service_method) group_left sum by (rpc_error_service_method) ( label_join( rate(workload_googleapis_com:rpc_server_requests_per_rpc_count{ monitored_resource="generic_task", rpc_connect_rpc_error_code="" }[30m]), "rpc_service_method_error", "/", "rpc_service", "rpc_method", "rpc_connect_rpc_error_code" ) ) ) > 0.2`),
+	}} {
+		t.Run(tc.name, func(t *testing.T) {
+			got := flattenPromQLQuery(tc.query)
+			tc.want.Equal(t, got)
+		})
+	}
+}