diff --git a/tests/integration/run_instances_malformed_test.go b/tests/integration/run_instances_malformed_test.go
new file mode 100644
index 000000000..aaf85f2a1
--- /dev/null
+++ b/tests/integration/run_instances_malformed_test.go
@@ -0,0 +1,52 @@
+//go:build integration
+
+package integration
+
+import (
+	"testing"
+
+	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/service/ec2"
+)
+
+// TestRunInstances_MalformedAMIIDRejected asserts that an ImageId lacking the
+// "ami-" prefix is rejected by ValidateRunInstancesInput
+// (gateway/ec2/instance/RunInstances.go) before any NATS hop. No StubSubject
+// is registered for spinifex.node.status or any per-node launch subject: if
+// the gateway ever dispatched this request instead of rejecting it up front,
+// the call would hang on utils.Gather's timeout rather than fail fast, making
+// this test itself a tripwire for that regression.
+func TestRunInstances_MalformedAMIIDRejected(t *testing.T) {
+	gw := StartGateway(t)
+
+	_, err := gw.EC2Client(t).RunInstances(&ec2.RunInstancesInput{
+		ImageId:      aws.String("not-an-ami-id"),
+		InstanceType: aws.String("t3.micro"),
+		MinCount:     aws.Int64(1),
+		MaxCount:     aws.Int64(1),
+	})
+	requireAWSErrorCode(t, err, "InvalidAMIID.Malformed")
+}
+
+// TestRunInstances_MalformedCapacityReservationIDRejected asserts that a
+// targeted-launch CapacityReservationId lacking the "cr-" prefix is rejected
+// by runInstancesInner's gateway-side-only check (RunInstances.go) before the
+// launch reaches the capacity-reservation NATS subject. As above, no daemon
+// stub is registered, so a regression that let this request through would
+// hang instead of failing fast.
+func TestRunInstances_MalformedCapacityReservationIDRejected(t *testing.T) {
+	gw := StartGateway(t)
+
+	_, err := gw.EC2Client(t).RunInstances(&ec2.RunInstancesInput{
+		ImageId:      aws.String("ami-0123456789abcdef0"),
+		InstanceType: aws.String("t3.micro"),
+		MinCount:     aws.Int64(1),
+		MaxCount:     aws.Int64(1),
+		CapacityReservationSpecification: &ec2.CapacityReservationSpecification{
+			CapacityReservationTarget: &ec2.CapacityReservationTarget{
+				CapacityReservationId: aws.String("not-a-cr-id"),
+			},
+		},
+	})
+	requireAWSErrorCode(t, err, "InvalidCapacityReservationId.Malformed")
+}