diff --git a/artiq/runtime/session.c b/artiq/runtime/session.c
index ef4bf5adf..c6e215ace 100644
--- a/artiq/runtime/session.c
+++ b/artiq/runtime/session.c
@@ -16,8 +16,12 @@
 #include "rtiocrg.h"
 #include "session.h"
 
-#define BUFFER_IN_SIZE (1024*1024)
-#define BUFFER_OUT_SIZE (1024*1024)
+// 2.5MiB in payload + 1KiB for headers.
+// We need more than 1MiB to send a 1MiB list due to tags;
+// about 5/4MiB for an 1MiB int32 list, 9/8MiB for an 1MiB int64 list.
+#define BUFFER_SIZE (2560*1024 + 1024)
+#define BUFFER_IN_SIZE  BUFFER_SIZE
+#define BUFFER_OUT_SIZE BUFFER_SIZE
 
 static int process_input();
 static int out_packet_available();
@@ -829,8 +833,10 @@ static int send_rpc_value(const char **tag, void **value)
 
             for(int i = 0; i < list->length; i++) {
                 const char *tag_copy = *tag;
-                if(!send_rpc_value(&tag_copy, &element))
+                if(!send_rpc_value(&tag_copy, &element)) {
+                    log("failed to send list at element %d/%d", i, list->length);
                     return 0;
+                }
             }
             skip_rpc_value(tag);
 
diff --git a/artiq/test/coredevice/embedding.py b/artiq/test/coredevice/embedding.py
index 04a136164..190ec23b9 100644
--- a/artiq/test/coredevice/embedding.py
+++ b/artiq/test/coredevice/embedding.py
@@ -2,7 +2,7 @@ from artiq.language import *
 from artiq.test.hardware_testbench import ExperimentCase
 
 
-class Roundtrip(EnvExperiment):
+class _Roundtrip(EnvExperiment):
     def build(self):
         self.setattr_device("core")
 
@@ -12,7 +12,7 @@ class Roundtrip(EnvExperiment):
 
 class RoundtripTest(ExperimentCase):
     def assertRoundtrip(self, obj):
-        exp = self.create(Roundtrip)
+        exp = self.create(_Roundtrip)
         def callback(objcopy):
             self.assertEqual(obj, objcopy)
         exp.roundtrip(obj, callback)
@@ -42,7 +42,7 @@ class RoundtripTest(ExperimentCase):
         self.assertRoundtrip(obj)
 
 
-class DefaultArg(EnvExperiment):
+class _DefaultArg(EnvExperiment):
     def build(self):
         self.setattr_device("core")
 
@@ -55,5 +55,23 @@ class DefaultArg(EnvExperiment):
 
 class DefaultArgTest(ExperimentCase):
     def test_default_arg(self):
-        exp = self.create(DefaultArg)
+        exp = self.create(_DefaultArg)
         self.assertEqual(exp.run(), 42)
+
+
+class _Payload1MB(EnvExperiment):
+    def build(self):
+        self.setattr_device("core")
+
+    def devnull(self, d):
+        pass
+
+    @kernel
+    def run(self):
+        data = [0 for _ in range(1000000//4)]
+        self.devnull(data)
+
+class LargePayloadTest(ExperimentCase):
+    def test_1MB(self):
+        exp = self.create(_Payload1MB)
+        exp.run()