diff --git a/server/packet_codec.py b/server/packet_codec.py
new file mode 100644
index 0000000..1bf0c4a
--- /dev/null
+++ b/server/packet_codec.py
@@ -0,0 +1,59 @@
+from __future__ import annotations
+
+import json
+from dataclasses import dataclass
+from typing import Any
+
+from transport_policy import Delivery, delivery_for_packet_type
+
+MAX_MESSAGE_BYTES = 64 * 1024
+
+
+class PacketCodecError(ValueError):
+    pass
+
+
+@dataclass(frozen=True)
+class EncodedPacket:
+    packet_type: str
+    payload: bytes
+    delivery: Delivery
+
+
+def _reject_json_constant(value: str) -> None:
+    raise PacketCodecError(f"non-finite JSON constant is not allowed: {value}")
+
+
+def encode_packet(packet: dict[str, Any]) -> EncodedPacket:
+    if not isinstance(packet, dict):
+        raise PacketCodecError("packet must be a JSON object")
+    packet_type = packet.get("type")
+    if not isinstance(packet_type, str) or not packet_type:
+        raise PacketCodecError("packet type must be a non-empty string")
+    try:
+        payload = json.dumps(packet, separators=(",", ":"), allow_nan=False).encode("utf-8")
+    except (TypeError, ValueError) as error:
+        raise PacketCodecError(f"packet is not JSON serializable: {error}") from error
+    if len(payload) > MAX_MESSAGE_BYTES:
+        raise PacketCodecError("packet exceeds maximum message size")
+    return EncodedPacket(packet_type, payload, delivery_for_packet_type(packet_type))
+
+
+def decode_packet(payload: bytes | bytearray | memoryview) -> dict[str, Any]:
+    raw = bytes(payload)
+    if len(raw) > MAX_MESSAGE_BYTES:
+        raise PacketCodecError("packet exceeds maximum message size")
+    try:
+        text = raw.decode("utf-8", errors="strict")
+    except UnicodeDecodeError as error:
+        raise PacketCodecError("packet is not valid UTF-8") from error
+    try:
+        packet = json.loads(text, parse_constant=_reject_json_constant)
+    except (json.JSONDecodeError, PacketCodecError) as error:
+        raise PacketCodecError(f"invalid JSON packet: {error}") from error
+    if not isinstance(packet, dict):
+        raise PacketCodecError("packet must be a JSON object")
+    packet_type = packet.get("type")
+    if not isinstance(packet_type, str) or not packet_type:
+        raise PacketCodecError("packet type must be a non-empty string")
+    return packet