diff --git a/capnp/helpers/helpers.pxd b/capnp/helpers/helpers.pxd
index 9cdf503..69c0a01 100644
--- a/capnp/helpers/helpers.pxd
+++ b/capnp/helpers/helpers.pxd
@@ -1,5 +1,7 @@
 from .capnp.includes.capnp_cpp cimport Maybe, DynamicStruct, Request, PyPromise, VoidPromise, PyPromiseArray, RemotePromise, DynamicCapability, InterfaceSchema, EnumSchema, StructSchema, DynamicValue, Capability, RpcSystem, MessageBuilder, MessageReader, TwoPartyVatNetwork, PyRestorer, AnyPointer, DynamicStruct_Builder
 
+from .capnp.includes.schema_cpp cimport ByteArray
+
 from non_circular cimport reraise_kj_exception
 
 from cpython.ref cimport PyObject
@@ -30,3 +32,6 @@ cdef extern from "../helpers/rpcHelper.h":
     Capability.Client restoreHelper(RpcSystem&, AnyPointer.Reader&)
     Capability.Client restoreHelper(RpcSystem&, AnyPointer.Builder&)
     RpcSystem makeRpcClientWithRestorer(TwoPartyVatNetwork&, PyRestorer&)
+
+cdef extern from "../helpers/serialize.h":
+    ByteArray messageToPackedBytes(MessageBuilder &)
\ No newline at end of file
diff --git a/capnp/helpers/serialize.h b/capnp/helpers/serialize.h
new file mode 100644
index 0000000..8393c7d
--- /dev/null
+++ b/capnp/helpers/serialize.h
@@ -0,0 +1,20 @@
+#pragma once
+
+#include "kj/io.h"
+#include "capnp/dynamic.h"
+#include "capnp/serialize-packed.h"
+
+kj::Array< ::capnp::byte> messageToPackedBytes(capnp::MessageBuilder & message)
+{
+    auto segments = message.getSegmentsForOutput();
+    size_t totalSize = segments.size() / 2 + 1;
+
+    for (auto& segment: segments) {
+        totalSize += segment.size();
+    }
+
+    kj::Array<capnp::byte> result = kj::heapArray<capnp::byte>(totalSize * 8);
+    kj::ArrayOutputStream out(result.asPtr());
+    capnp::writePackedMessage(out, message);
+    return heapArray(out.getArray()); // TODO: make this non-copying somehow
+}
\ No newline at end of file
diff --git a/capnp/includes/schema_cpp.pxd b/capnp/includes/schema_cpp.pxd
index 2c06ed2..1e75042 100644
--- a/capnp/includes/schema_cpp.pxd
+++ b/capnp/includes/schema_cpp.pxd
@@ -716,6 +716,9 @@ cdef extern from "kj/array.h" namespace " ::kj":
     cdef cppclass WordArray " ::kj::Array< ::capnp::word>":
         word* begin()
         size_t size()
+    cdef cppclass ByteArray " ::kj::Array< ::capnp::byte>":
+        char* begin()
+        size_t size()
 
 cdef extern from "kj/array.h" namespace " ::kj":
     cdef cppclass InputStream:
diff --git a/capnp/lib/capnp.pyx b/capnp/lib/capnp.pyx
index a169864..6e840a1 100644
--- a/capnp/lib/capnp.pyx
+++ b/capnp/lib/capnp.pyx
@@ -940,7 +940,11 @@ cdef class _DynamicStructBuilder:
     cpdef to_bytes_packed(_DynamicStructBuilder self) except +reraise_kj_exception:
         self._check_write()
         cdef _MessageBuilder builder = self._parent
-        return _message_to_packed_bytes(builder)
+        array = helpers.messageToPackedBytes(deref(builder.thisptr))
+        cdef const char* ptr = <const char *>array.begin()
+        cdef bytes ret = ptr[:array.size()]
+        self._is_written = True
+        return ret
 
     cdef _get(self, field):
         cdef C_DynamicValue.Builder value = self.thisptr.get(field)