diff --git a/tests/test_transport.py b/tests/test_transport.py
index c5baaea..950123b 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -1,5 +1,6 @@
 """Tests for Claude SDK transport layer."""
 
+import json
 import os
 import uuid
 from unittest.mock import AsyncMock, MagicMock, patch
@@ -317,7 +318,6 @@ class TestSubprocessCLITransport:
 
     def test_build_command_with_mcp_servers(self):
         """Test building CLI command with mcp_servers option."""
-        import json
 
         mcp_servers = {
             "test-server": {
@@ -503,7 +503,6 @@ class TestSubprocessCLITransport:
 
     def test_concurrent_writes_are_serialized(self):
         """Test that concurrent writes to stdin are serialized by the lock."""
-        import json
 
         async def _test():
             with patch("anyio.open_process") as mock_exec:
@@ -520,27 +519,11 @@ class TestSubprocessCLITransport:
                 mock_process = MagicMock()
                 mock_process.returncode = None
                 mock_process.stdout = MagicMock()
+                mock_process.stdin = MagicMock()
+                mock_process.stdin.aclose = AsyncMock()
 
-                # Track write calls to verify serialization
-                write_events: list[tuple[str, str]] = []  # (event_type, data_preview)
-
-                async def mock_send(data: str):
-                    """Mock send that tracks start/end of each write."""
-                    preview = data[:20]
-                    write_events.append(("start", preview))
-                    # Small delay to increase chance of interleaving if lock is broken
-                    await anyio.sleep(0.001)
-                    write_events.append(("end", preview))
-
-                mock_stdin = MagicMock()
-                mock_stdin.send = mock_send
-                mock_stdin.aclose = AsyncMock()
-                mock_process.stdin = mock_stdin
-
-                # Return version process first, then main process
                 mock_exec.side_effect = [mock_version_process, mock_process]
 
-                # Create transport in streaming mode (required for stdin writes)
                 async def dummy_stream():
                     yield {"type": "user"}
 
@@ -552,30 +535,32 @@ class TestSubprocessCLITransport:
                 await transport.connect()
                 assert transport.is_ready()
 
-                # Spawn multiple concurrent writes
+                # Track write events to verify serialization
+                write_events: list[tuple[str, int]] = []
+
+                async def tracked_send(data: str):
+                    msg_id = json.loads(data.strip())["id"]
+                    write_events.append(("start", msg_id))
+                    await anyio.sleep(0.001)  # Yield to expose interleaving
+                    write_events.append(("end", msg_id))
+
+                transport._stdin_stream.send = tracked_send
+
+                # Spawn concurrent writes
                 num_writes = 5
-                messages = [
-                    json.dumps({"id": i, "data": "x" * 50}) + "\n"
-                    for i in range(num_writes)
-                ]
+                messages = [json.dumps({"id": i}) + "\n" for i in range(num_writes)]
 
                 async with anyio.create_task_group() as tg:
                     for msg in messages:
                         tg.start_soon(transport.write, msg)
 
-                # Verify serialization: events should be strictly alternating start/end pairs
-                # If writes interleaved, we'd see patterns like: start, start, end, end
+                # Verify: each start must be immediately followed by its end
+                # If interleaved, we'd see: start(0), start(1), end(0), end(1)
                 assert len(write_events) == num_writes * 2
-
                 for i in range(0, len(write_events), 2):
-                    assert write_events[i][0] == "start", f"Expected start at index {i}"
-                    assert write_events[i + 1][0] == "end", (
-                        f"Expected end at index {i + 1}"
-                    )
-                    # Each start should be followed by its corresponding end
-                    assert write_events[i][1] == write_events[i + 1][1], (
-                        f"Mismatched start/end at index {i}"
-                    )
+                    assert write_events[i][0] == "start"
+                    assert write_events[i + 1][0] == "end"
+                    assert write_events[i][1] == write_events[i + 1][1]
 
                 await transport.close()