diff --git a/server/tests/test_tcp_transport.py b/server/tests/test_tcp_transport.py
new file mode 100644
index 0000000..ae82bc5
--- /dev/null
+++ b/server/tests/test_tcp_transport.py
@@ -0,0 +1,50 @@
+import socket
+
+import pytest
+
+from packet_codec import MAX_MESSAGE_BYTES, encode_packet
+from tcp_transport import LineMessageBuffer, TcpFramingError, frame_message, send_message
+
+
+def test_tcp_framing_adds_delimiter_only_at_transport_boundary():
+    encoded = encode_packet({"type": "playerState", "characterName": "Nomad"})
+    assert not encoded.payload.endswith(b"\n")
+    framed = frame_message(encoded.payload)
+    assert framed == encoded.payload + b"\n"
+
+
+def test_line_buffer_preserves_fragmented_and_coalesced_messages():
+    buffer = LineMessageBuffer()
+    assert buffer.feed(b'{"type":"first"') == []
+    messages = buffer.feed(b'}\n{"type":"second"}\n{"type":"third"')
+    assert messages == [b'{"type":"first"}', b'{"type":"second"}']
+    assert buffer.buffered_bytes > 0
+    assert buffer.feed(b'}\r\n') == [b'{"type":"third"}']
+    assert buffer.buffered_bytes == 0
+
+
+def test_line_buffer_rejects_oversize_unterminated_and_terminated_messages():
+    with pytest.raises(TcpFramingError):
+        LineMessageBuffer().feed(b"x" * (MAX_MESSAGE_BYTES + 1))
+    with pytest.raises(TcpFramingError):
+        LineMessageBuffer().feed((b"x" * (MAX_MESSAGE_BYTES + 1)) + b"\n")
+
+
+def test_frame_rejects_raw_delimiters_and_oversize_payloads():
+    with pytest.raises(TcpFramingError):
+        frame_message(b"bad\nmessage")
+    with pytest.raises(TcpFramingError):
+        frame_message(b"bad\rmessage")
+    with pytest.raises(TcpFramingError):
+        frame_message(b"x" * (MAX_MESSAGE_BYTES + 1))
+
+
+def test_send_message_preserves_existing_tcp_wire_format():
+    reader, writer = socket.socketpair()
+    try:
+        payload = encode_packet({"type": "keepAlive"}).payload
+        send_message(writer, payload)
+        assert reader.recv(4096) == payload + b"\n"
+    finally:
+        reader.close()
+        writer.close()