diff --git a/server/tests/test_packet_codec.py b/server/tests/test_packet_codec.py
new file mode 100644
index 0000000..f3202e1
--- /dev/null
+++ b/server/tests/test_packet_codec.py
@@ -0,0 +1,48 @@
+import json
+
+import pytest
+
+from packet_codec import MAX_MESSAGE_BYTES, PacketCodecError, decode_packet, encode_packet
+from transport_policy import Delivery
+
+
+def test_encoded_packet_has_no_tcp_line_framing():
+    encoded = encode_packet({"type": "transform", "x": 1.0})
+    assert encoded.packet_type == "transform"
+    assert encoded.delivery is Delivery.UNRELIABLE_SEQUENCED
+    assert encoded.payload == b'{"type":"transform","x":1.0}'
+    assert not encoded.payload.endswith(b"\n")
+
+
+def test_reliable_packet_carries_delivery_policy_separately_from_json():
+    encoded = encode_packet({"type": "playerState", "characterName": "Nomad"})
+    assert encoded.delivery is Delivery.RELIABLE_ORDERED
+    assert json.loads(encoded.payload) == {"type": "playerState", "characterName": "Nomad"}
+
+
+def test_decode_packet_accepts_one_complete_message_without_delimiter():
+    assert decode_packet(b'{"type":"sessionReady","playerId":7}') == {
+        "type": "sessionReady",
+        "playerId": 7,
+    }
+
+
+def test_codec_rejects_nonfinite_invalid_utf8_nonobject_and_missing_type():
+    with pytest.raises(PacketCodecError):
+        encode_packet({"type": "transform", "x": float("nan")})
+    with pytest.raises(PacketCodecError):
+        decode_packet(b'{"type":"transform","x":NaN}')
+    with pytest.raises(PacketCodecError):
+        decode_packet(b"\xff")
+    with pytest.raises(PacketCodecError):
+        decode_packet(b"[]")
+    with pytest.raises(PacketCodecError):
+        decode_packet(b"{}")
+
+
+def test_codec_enforces_64k_message_limit_before_transport():
+    payload = "x" * MAX_MESSAGE_BYTES
+    with pytest.raises(PacketCodecError):
+        encode_packet({"type": "playerState", "characterName": payload})
+    with pytest.raises(PacketCodecError):
+        decode_packet(b"x" * (MAX_MESSAGE_BYTES + 1))