diff --git a/server/gns_snapshot_envelope.py b/server/gns_snapshot_envelope.py
new file mode 100644
index 0000000..20906a0
--- /dev/null
+++ b/server/gns_snapshot_envelope.py
@@ -0,0 +1,66 @@
+from __future__ import annotations
+
+import struct
+from dataclasses import dataclass
+
+from packet_codec import MAX_MESSAGE_BYTES
+
+MAGIC = b"COG2"
+VERSION = 1
+HEADER = struct.Struct(">4sBBHI")
+HEADER_SIZE = HEADER.size
+
+_FAMILY_BY_PACKET_TYPE = {
+    "transform": 1,
+    "npcState": 2,
+}
+_PACKET_TYPE_BY_FAMILY = {value: key for key, value in _FAMILY_BY_PACKET_TYPE.items()}
+
+
+class SnapshotEnvelopeError(ValueError):
+    pass
+
+
+@dataclass(frozen=True)
+class SnapshotEnvelope:
+    packet_type: str
+    sequence: int
+    payload: bytes
+
+
+def is_snapshot_packet_type(packet_type: str) -> bool:
+    return packet_type in _FAMILY_BY_PACKET_TYPE
+
+
+def encode_snapshot(packet_type: str, payload: bytes, sequence: int) -> bytes:
+    family = _FAMILY_BY_PACKET_TYPE.get(packet_type)
+    if family is None:
+        raise SnapshotEnvelopeError(f"packet type {packet_type!r} is not a GNS snapshot family")
+    if not isinstance(sequence, int) or isinstance(sequence, bool) or not 1 <= sequence <= 0xFFFFFFFF:
+        raise SnapshotEnvelopeError("snapshot sequence must be between 1 and 0xffffffff")
+    raw = bytes(payload)
+    if not raw:
+        raise SnapshotEnvelopeError("snapshot payload cannot be empty")
+    if HEADER_SIZE + len(raw) > MAX_MESSAGE_BYTES:
+        raise SnapshotEnvelopeError("snapshot envelope exceeds maximum GNS message size")
+    return HEADER.pack(MAGIC, VERSION, family, 0, sequence) + raw
+
+
+def decode_snapshot(message: bytes | bytearray | memoryview) -> SnapshotEnvelope | None:
+    raw = bytes(message)
+    if not raw.startswith(MAGIC):
+        return None
+    if len(raw) < HEADER_SIZE:
+        raise SnapshotEnvelopeError("truncated GNS snapshot envelope")
+    magic, version, family, reserved, sequence = HEADER.unpack_from(raw)
+    if magic != MAGIC or version != VERSION or reserved != 0:
+        raise SnapshotEnvelopeError("invalid GNS snapshot envelope header")
+    packet_type = _PACKET_TYPE_BY_FAMILY.get(family)
+    if packet_type is None:
+        raise SnapshotEnvelopeError("unknown GNS snapshot family")
+    if sequence == 0:
+        raise SnapshotEnvelopeError("snapshot sequence zero is reserved")
+    payload = raw[HEADER_SIZE:]
+    if not payload:
+        raise SnapshotEnvelopeError("snapshot envelope payload is empty")
+    return SnapshotEnvelope(packet_type, sequence, payload)