diff --git a/client/src/telethon/_impl/mtproto/transport/abcs.py b/client/src/telethon/_impl/mtproto/transport/abcs.py
index 432ed322..de24ebc0 100644
--- a/client/src/telethon/_impl/mtproto/transport/abcs.py
+++ b/client/src/telethon/_impl/mtproto/transport/abcs.py
@@ -1,9 +1,15 @@
 from abc import ABC, abstractmethod
+from typing import Callable
+
+
+OutFn = Callable[[bytes | bytearray | memoryview], None]
 
 
 class Transport(ABC):
+    # Python's stream writer has a synchronous write (buffer append) followed
+    # by drain. The buffer is externally managed, so `write` is used as input.
     @abstractmethod
-    def pack(self, input: bytes, output: bytearray) -> None:
+    def pack(self, input: bytes, write: OutFn) -> None:
         pass
 
     @abstractmethod
diff --git a/client/src/telethon/_impl/mtproto/transport/abridged.py b/client/src/telethon/_impl/mtproto/transport/abridged.py
index fda75100..9d43fe32 100644
--- a/client/src/telethon/_impl/mtproto/transport/abridged.py
+++ b/client/src/telethon/_impl/mtproto/transport/abridged.py
@@ -1,6 +1,6 @@
 import struct
 
-from .abcs import MissingBytes, Transport
+from .abcs import MissingBytes, OutFn, Transport
 
 
 class Abridged(Transport):
@@ -22,19 +22,19 @@ class Abridged(Transport):
     def __init__(self) -> None:
         self._init = False
 
-    def pack(self, input: bytes, output: bytearray) -> None:
+    def pack(self, input: bytes, write: OutFn) -> None:
         assert len(input) % 4 == 0
 
         if not self._init:
-            output += b"\xef"
+            write(b"\xef")
             self._init = True
 
         length = len(input) // 4
         if length < 127:
-            output += struct.pack("<b", length)
+            write(struct.pack("<b", length))
         else:
-            output += struct.pack("<i", 0x7F | (length << 8))
-        output += input
+            write(struct.pack("<i", 0x7F | (length << 8)))
+        write(input)
 
     def unpack(self, input: bytes, output: bytearray) -> int:
         if not input:
diff --git a/client/src/telethon/_impl/mtproto/transport/full.py b/client/src/telethon/_impl/mtproto/transport/full.py
index 9a6cde7f..fd59860f 100644
--- a/client/src/telethon/_impl/mtproto/transport/full.py
+++ b/client/src/telethon/_impl/mtproto/transport/full.py
@@ -1,7 +1,7 @@
 import struct
 from zlib import crc32
 
-from .abcs import MissingBytes, Transport
+from .abcs import MissingBytes, OutFn, Transport
 
 
 class Full(Transport):
@@ -24,13 +24,15 @@ class Full(Transport):
         self._send_seq = 0
         self._recv_seq = 0
 
-    def pack(self, input: bytes, output: bytearray) -> None:
+    def pack(self, input: bytes, write: OutFn) -> None:
         assert len(input) % 4 == 0
 
         length = len(input) + 12
-        output += struct.pack("<ii", length, self._send_seq)
-        output += input
-        output += struct.pack("<i", crc32(memoryview(output)[-(length - 4) :]))
+        # Unfortunately there's no hasher that can be updated multiple times,
+        # so a temporary buffer must be used to hash it all in one go.
+        tmp = struct.pack("<ii", length, self._send_seq) + input
+        write(tmp)
+        write(struct.pack("<I", crc32(tmp)))
         self._send_seq += 1
 
     def unpack(self, input: bytes, output: bytearray) -> int:
diff --git a/client/src/telethon/_impl/mtproto/transport/intermediate.py b/client/src/telethon/_impl/mtproto/transport/intermediate.py
index 9dd8e188..241d094f 100644
--- a/client/src/telethon/_impl/mtproto/transport/intermediate.py
+++ b/client/src/telethon/_impl/mtproto/transport/intermediate.py
@@ -1,6 +1,6 @@
 import struct
 
-from .abcs import MissingBytes, Transport
+from .abcs import MissingBytes, OutFn, Transport
 
 
 class Intermediate(Transport):
@@ -22,15 +22,15 @@ class Intermediate(Transport):
     def __init__(self) -> None:
         self._init = False
 
-    def pack(self, input: bytes, output: bytearray) -> None:
+    def pack(self, input: bytes, write: OutFn) -> None:
         assert len(input) % 4 == 0
 
         if not self._init:
-            output += b"\xee\xee\xee\xee"
+            write(b"\xee\xee\xee\xee")
             self._init = True
 
-        output += struct.pack("<i", len(input))
-        output += input
+        write(struct.pack("<i", len(input)))
+        write(input)
 
     def unpack(self, input: bytes, output: bytearray) -> int:
         if len(input) < 4:
diff --git a/client/tests/transport/abridged_test.py b/client/tests/transport/abridged_test.py
index 7733babf..71af5dc1 100644
--- a/client/tests/transport/abridged_test.py
+++ b/client/tests/transport/abridged_test.py
@@ -4,9 +4,16 @@ from pytest import raises
 from telethon._impl.mtproto.transport.abridged import Abridged
 
 
+class Output(bytearray):
+    __slots__ = ()
+
+    def __call__(self, data: bytes) -> None:
+        self += data
+
+
 def setup_pack(n: int) -> Tuple[Abridged, bytes, bytearray]:
     input = bytes(x & 0xFF for x in range(n))
-    return Abridged(), input, bytearray()
+    return Abridged(), input, Output()
 
 
 def test_pack_empty() -> None:
diff --git a/client/tests/transport/full_test.py b/client/tests/transport/full_test.py
index 65d9c28e..b85827fc 100644
--- a/client/tests/transport/full_test.py
+++ b/client/tests/transport/full_test.py
@@ -4,9 +4,16 @@ from pytest import raises
 from telethon._impl.mtproto.transport.full import Full
 
 
+class Output(bytearray):
+    __slots__ = ()
+
+    def __call__(self, data: bytes) -> None:
+        self += data
+
+
 def setup_pack(n: int) -> Tuple[Full, bytes, bytearray]:
     input = bytes(x & 0xFF for x in range(n))
-    return Full(), input, bytearray()
+    return Full(), input, Output()
 
 
 def setup_unpack(n: int) -> Tuple[bytes, Full, bytes, bytearray]:
diff --git a/client/tests/transport/intermediate_test.py b/client/tests/transport/intermediate_test.py
index 657f1fe0..b3a6efe8 100644
--- a/client/tests/transport/intermediate_test.py
+++ b/client/tests/transport/intermediate_test.py
@@ -4,9 +4,16 @@ from pytest import raises
 from telethon._impl.mtproto.transport.intermediate import Intermediate
 
 
+class Output(bytearray):
+    __slots__ = ()
+
+    def __call__(self, data: bytes) -> None:
+        self += data
+
+
 def setup_pack(n: int) -> Tuple[Intermediate, bytes, bytearray]:
     input = bytes(x & 0xFF for x in range(n))
-    return Intermediate(), input, bytearray()
+    return Intermediate(), input, Output()
 
 
 def test_pack_empty() -> None: