diff --git a/modal/io_streams.py b/modal/io_streams.py
index 867707599d..46edac36b8 100644
--- a/modal/io_streams.py
+++ b/modal/io_streams.py
@@ -1,6 +1,7 @@
 # Copyright Modal Labs 2022
 import asyncio
 import time
+import uuid
 from collections.abc import AsyncGenerator, AsyncIterator
 from typing import (
     TYPE_CHECKING,
@@ -370,6 +371,7 @@ def __init__(self, object_id: str, object_type: Literal["sandbox", "container_pr
         self._client = client
         self._is_closed = False
         self._buffer = bytearray()
+        self.writer_id = str(uuid.uuid4())
 
     def _get_next_index(self) -> int:
         index = self._index
@@ -449,7 +451,11 @@ async def drain(self) -> None:
                 await retry_transient_errors(
                     self._client.stub.SandboxStdinWrite,
                     api_pb2.SandboxStdinWriteRequest(
-                        sandbox_id=self._object_id, index=index, eof=self._is_closed, input=data
+                        sandbox_id=self._object_id,
+                        index=index,
+                        eof=self._is_closed,
+                        input=data,
+                        writer_id=self.writer_id,
                     ),
                 )
             else:
diff --git a/modal_proto/api.proto b/modal_proto/api.proto
index 5c571b886c..3dad58ff84 100644
--- a/modal_proto/api.proto
+++ b/modal_proto/api.proto
@@ -2606,6 +2606,7 @@ message RuntimeInputMessage {
   bytes message = 1;
   uint64 message_index = 2;
   bool eof = 3;
+  string writer_id = 4;
 }
 
 message RuntimeOutputBatch {
@@ -2876,6 +2877,7 @@ message SandboxStdinWriteRequest {
   bytes input = 2;
   uint32 index = 3;
   bool eof = 4;
+  string writer_id = 5;
 }
 
 message SandboxStdinWriteResponse {