diff --git a/CHANGELOG.md b/CHANGELOG.md
index d31ea8f2f..909d8c361 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,11 @@ Format based on [Keep a Changelog](https://keepachangelog.com/).
 - 8-directional movement — diagonal PlayerAction variants (MoveNortheast/Northwest/Southeast/Southwest) + TilePosition::all_neighbors()
 - Entity-entity collision in validate_movement — spatial occupancy check prevents multiple entities on same tile
 - 25 new tests (4 framing + 2 IPC integration + 17 movement unit + 2 movement integration), total 45
+- MessagePack serialization for GDScript (ticket #77) — Protocol codec decoding ObserverSnapshot/PlayerInput from Rust wire format, encoding PlayerInput for server
+- Godot4MessagePack library (pure GDScript) for MessagePack encode/decode
+- Rust fixture generator (gen_fixtures.rs) producing canonical .msgpack test fixtures with rmp_serde
+- 8 cross-language protocol tests verifying Rust↔GDScript MessagePack compatibility (D-030 Layer 1)
+- SimBridge wired to Protocol codec with receive_bytes()/drain_outbound() for transport layer
 - `db/connectors/ticket` CLI — ergonomic ticket management (list, show, create, assign, sprint, deps, search, epics, children, count) with JSON output
 - Sprint 1 "Run" created with 8 stories targeting moving character on screen via IPC bridge
 - Ticket skill rewritten to use ticket CLI instead of raw SQL wrapper scripts
diff --git a/Makefile b/Makefile
index 5daf2e895..d799989bc 100644
--- a/Makefile
+++ b/Makefile
@@ -73,6 +73,9 @@ test: test-server test-client
 test-server:
 	cd server && cargo nextest run
 
+fixtures:
+	cd server && cargo test --test gen_fixtures -- --ignored
+
 test-client:
 	@test -n "$(GODOT)" || { echo "Godot not found. Run 'make setup' first."; exit 1; }
 	$(GODOT) --headless --path client -s res://addons/gdUnit4/bin/GdUnitCmdTool.gd --ignoreHeadlessMode -a res://tests/
diff --git a/client/addons/messagepack/LICENSE b/client/addons/messagepack/LICENSE
new file mode 100755
index 000000000..6929b7c84
--- /dev/null
+++ b/client/addons/messagepack/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025 Luis Chirlaque Hernández
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
\ No newline at end of file
diff --git a/client/addons/messagepack/messagepack.gd b/client/addons/messagepack/messagepack.gd
new file mode 100755
index 000000000..3a7318db6
--- /dev/null
+++ b/client/addons/messagepack/messagepack.gd
@@ -0,0 +1,368 @@
+class_name Messagepack
+## Messagepack implementation for Godot 4 in GDScript
+##
+## You can find the full spec at: https://github.com/msgpack/msgpack/blob/master/spec.md
+
+const FIRST3 = 0xe0
+const FIRST4 = 0xf0
+const LAST4 = 0x0f
+const LAST5 = 0x1f
+
+const types = {
+	"nil": 0xc0,
+	"false": 0xc2,
+	"true": 0xc3,
+	"positive_fixint": [0x00, 0x7f],
+	"negative_fixint": [0xe0, 0xff],
+	"uint_8": 0xcc,
+	"uint_16": 0xcd,
+	"uint_32": 0xce,
+	"uint_64": 0xcf,
+	"int_8": 0xd0,
+	"int_16": 0xd1,
+	"int_32": 0xd2,
+	"int_64": 0xd3,
+	"float_32": 0xca,
+	"float_64": 0xcb,
+	"fixstr": [0xa0, 0xbf],
+	"str_8": 0xd9,
+	"str_16": 0xda,
+	"str_32": 0xdb,
+	"fixarray": [0x90, 0x9f],
+	"array_16": 0xdc,
+	"array_32": 0xdd,
+	"fixmap": [0x80, 0x8f],
+	"map_16": 0xde,
+	"map_32": 0xdf,
+	"bin_8": 0xc4,
+	"bin_16": 0xc5,
+	"bin_32": 0xc6
+}
+
+## This function takes a Variant and encodes it according to the Messagepack spec
+##
+## Parameters:
+##   - value: Variant to be encoded
+##
+## Returns:
+##   A dictionary containing the status of the encoding and the value as a PackedByteArray
+static func encode(value) -> Dictionary:
+	var buffer = StreamPeerBuffer.new()
+	buffer.set_big_endian(true)
+	var err = _encode_message(buffer, value)
+	return {
+		value = buffer.data_array,
+		status = err
+	}
+
+static func _encode_message(buffer: StreamPeerBuffer, value):
+	match typeof(value):
+		TYPE_NIL:
+			buffer.put_u8(types["nil"])
+
+		TYPE_BOOL:
+			if value == true:
+				buffer.put_u8(types["true"])
+			else:
+				buffer.put_u8(types["false"])
+
+		TYPE_INT:
+			if - (1 << 5) <= value and value <= (1 << 7) - 1:
+				buffer.put_8(value)
+			elif - (1 << 7) <= value and value <= (1 << 7):
+				buffer.put_u8(types["int_8"])
+				buffer.put_8(value)
+			elif 0 <= value and value <= (1 << 8) - 1:
+				buffer.put_u8(types["uint_8"])
+				buffer.put_u8(value)
+			elif - (1 << 15) <= value and value <= (1 << 15):
+				buffer.put_u8(types["int_16"])
+				buffer.put_16(value)
+			elif 0 <= value and value <= (1 << 16) - 1:
+				buffer.put_u8(types["uint_16"])
+				buffer.put_u16(value)
+			elif - (1 << 31) <= value and value <= (1 << 31):
+				buffer.put_u8(types["int_32"])
+				buffer.put_32(value)
+			elif 0 <= value and value <= (1 << 32) - 1:
+				buffer.put_u8(types["uint_32"])
+				buffer.put_u32(value)
+			elif - (1 << 63) <= value and value <= (1 << 63):
+				buffer.put_u8(types["int_64"])
+				buffer.put_64(value)
+			else:
+				buffer.put_u8(types["uint_64"])
+				buffer.put_u64(value)
+
+		TYPE_FLOAT:
+			buffer.put_u8(types["float_32"])
+			buffer.put_float(value)
+
+		TYPE_STRING:
+			var bytes = value.to_utf8_buffer()
+			var size = bytes.size()
+			if size <= (1 << 5) - 1:
+				buffer.put_u8(types["fixstr"][0]|size)
+			elif size <= (1 << 8) - 1:
+				buffer.put_u8(types["str_8"])
+				buffer.put_u8(size)
+			elif size <= (1 << 16) - 1:
+				buffer.put_u8(types["str_16"])
+				buffer.put_u16(size)
+			elif size <= (1 << 32) - 1:
+				buffer.put_u8(types["str_32"])
+				buffer.put_u32(size)
+			else:
+				printerr("Unsupported string: string is too big")
+				return ERR_INVALID_DATA
+
+			buffer.put_data(bytes)
+
+		TYPE_ARRAY:
+			var size = value.size()
+			if size <= 15:
+				buffer.put_u8(types["fixarray"][0]|size)
+			elif size <= (1 << 16) - 1:
+				buffer.put_u8(types["array_16"])
+				buffer.put_u16(size)
+			elif size <= (1 << 32) - 1:
+				buffer.put_u8(types["array_32"])
+				buffer.put_u32(size)
+			else:
+				printerr("Unsupported array: array is too long")
+				return ERR_INVALID_DATA
+			
+			for obj in value:
+				_encode_message(buffer, obj)
+
+		TYPE_DICTIONARY:
+			var size = value.size()
+			if size <= 15:
+				buffer.put_u8(types["fixmap"][0]|size)
+			elif size <= (1 << 16) - 1:
+				buffer.put_u8(types["map_16"])
+				buffer.put_u16(size)
+			elif size <= (1 << 32) - 1:
+				buffer.put_u8(types["map_32"])
+				buffer.put_u32(size)
+			else:
+				printerr("Unsupported dictionary: dictionary is too big")
+				return ERR_INVALID_DATA
+			
+			for key in value:
+				_encode_message(buffer, key)
+				_encode_message(buffer, value[key])
+		
+		TYPE_PACKED_BYTE_ARRAY:
+			var size = value.size()
+			if size <= (1 << 8) - 1:
+				buffer.put_u8(types["bin_8"])
+				buffer.put_u8(size)
+			elif size <= (1 << 16) - 1:
+				buffer.put_u8(types["bin_16"])
+				buffer.put_u16(size)
+			elif size <= (1 << 32) - 1:
+				buffer.put_u8(types["bin_32"])
+				buffer.put_u32(size)
+			else:
+				printerr("Unsupported packed byte array: packed byte array is too big")
+				return ERR_INVALID_DATA
+			
+			buffer.put_data(value)
+
+		_:
+			printerr("Unsupported data type: %s" % typeof(value))
+			return ERR_UNAVAILABLE
+
+
+## This function takes a PackedByteArray and decodes it according to the Messagepack spec
+##
+## Parameters:
+##   - bytes: PackedByteArray to be decoded
+##
+## Returns:
+##   A dictionary containing the status of the decoding and the value as Godot Variants
+static func decode(bytes: PackedByteArray):
+	var buffer = StreamPeerBuffer.new()
+	buffer.set_big_endian(true)
+	buffer.set_data_array(bytes)
+	
+	var err = {
+		error = null
+	}
+	var message = _decode_message(buffer, err)
+	return {
+		value = message,
+		status = err.error
+	}
+
+static func _decode_message(buffer: StreamPeerBuffer, err: Dictionary):
+	var buffer_size = buffer.get_size()
+	var first_byte = buffer.get_u8()
+	
+	if first_byte & 0x80 == 0: # positive fixint
+		return first_byte
+		
+	elif first_byte & FIRST4 == 0x80: # fixmap
+		var size = first_byte & 0x0f
+		var dict = {}
+		for _x in range(size):
+			var key = _decode_message(buffer, err)
+			var val = _decode_message(buffer, err)
+			dict[key] = val
+		return dict
+			
+	elif first_byte & FIRST4 == 0x90: # fixarray
+		var size = first_byte & 0x0f
+		var array = []
+		for _x in range(size):
+			var val = _decode_message(buffer, err)
+			array.append(val)
+		return array
+		
+	elif first_byte & FIRST3 == 0xa0: # fixstr
+		var size = first_byte & 0x1f
+		return buffer.get_utf8_string(size)
+		
+	elif first_byte == types["nil"]: # nil
+		print("null size:%s"%buffer_size)
+		return null
+		
+	elif first_byte == types["false"]: # false
+		print("false size:%s"%buffer_size)		
+		return false
+		
+	elif first_byte == types["true"]: # true
+		return true
+		
+	elif first_byte == types["bin_8"]: # bin 8
+		var length = buffer.get_u8()
+		return buffer.get_partial_data(length)
+		
+	elif first_byte == types["bin_16"]: # bin 16
+		var length = buffer.get_u16()
+		return buffer.get_partial_data(length)
+		
+	elif first_byte == types["bin_32"]: # bin 32
+		var length = buffer.get_u32()
+		return buffer.get_partial_data(length)
+		
+	elif first_byte == 0xc7: # ext 8
+		print("Ext 8 type not implemented")
+		return null
+
+	elif first_byte == 0xc8: # ext 16
+		print("Ext 16 type not implemented")
+		return null
+
+	elif first_byte == 0xc9: # ext 32
+		print("Ext 32 type not implemented")
+		return null
+		
+	elif first_byte == types["float_32"]: # float 32
+		return buffer.get_float()
+		
+	elif first_byte == types["float_64"]: # float 64
+		return buffer.get_double()
+		
+	elif first_byte == types["uint_8"]: # uint 8
+		return buffer.get_u8()
+		
+	elif first_byte == types["uint_16"]: # uint 16
+		return buffer.get_u16()
+		
+	elif first_byte == types["uint_32"]: # uint 32
+		return buffer.get_u32()
+		
+	elif first_byte == types["uint_64"]: # uint 64
+		return buffer.get_u64()
+		
+	elif first_byte == types["int_8"]: # int 8
+		return buffer.get_8()
+		
+	elif first_byte == types["int_16"]: # int 16
+		return buffer.get_16()
+		
+	elif first_byte == types["int_32"]: # int 32
+		return buffer.get_32()
+		
+	elif first_byte == types["int_64"]: # int 64
+		return buffer.get_64()
+		
+	elif first_byte == 0xd4: # fixext 1
+		print("Fixext 1 type not implemented")
+		err.error = ERR_UNAVAILABLE
+		return null
+
+	elif first_byte == 0xd5: # fixext 2
+		print("Fixext 2 type not implemented")
+		err.error = ERR_UNAVAILABLE
+		return null
+
+	elif first_byte == 0xd6: # fixext 4
+		print("Fixext 4 type not implemented")
+		err.error = ERR_UNAVAILABLE
+		return null
+
+	elif first_byte == 0xd7: # fixext 8
+		print("Fixext 8 type not implemented")
+		err.error = ERR_UNAVAILABLE
+		return null
+
+	elif first_byte == 0xd8: # fixext 16
+		print("Fixext 16 type not implemented")
+		err.error = ERR_UNAVAILABLE
+		return null
+		
+	elif first_byte == types["str_8"]: # str 8
+		var size = buffer.get_u8()
+		return buffer.get_utf8_string(size)
+		
+	elif first_byte == types["str_16"]: # str 16
+		var size = buffer.get_u16()
+		return buffer.get_utf8_string(size)
+		
+	elif first_byte == types["str_32"]: # str 32
+		var size = buffer.get_u32()
+		return buffer.get_utf8_string(size)
+		
+	elif first_byte == types["array_16"]: # array 16
+		var length = buffer.get_u16()
+		var array = []
+		for _x in range(length):
+			var val = _decode_message(buffer, err)
+			array.append(val)
+		return array
+		
+	elif first_byte == types["array_32"]: # array 32
+		var length = buffer.get_u32()
+		var array = []
+		for _x in range(length):
+			var val = _decode_message(buffer, err)
+			array.append(val)
+		return array
+		
+	elif first_byte == types["map_16"]: # map 16
+		var length = buffer.get_u16()
+		var dict = {}
+		for _x in range(length):
+			var key = _decode_message(buffer, err)
+			var val = _decode_message(buffer, err)
+			dict[key] = val
+		return dict
+		
+	elif first_byte == types["map_32"]: # map 32
+		var length = buffer.get_u32()
+		var dict = {}
+		for _x in range(length):
+			var key = _decode_message(buffer, err)
+			var val = _decode_message(buffer, err)
+			dict[key] = val
+		return dict
+		
+	elif first_byte & FIRST3 == 0xe0: # negative fixint
+		return first_byte - 256
+	else:
+		printerr("Unknown header")
+		err.error = ERR_UNAVAILABLE
+		return null
diff --git a/client/addons/messagepack/messagepack.gd.uid b/client/addons/messagepack/messagepack.gd.uid
new file mode 100644
index 000000000..f2186b8f0
--- /dev/null
+++ b/client/addons/messagepack/messagepack.gd.uid
@@ -0,0 +1 @@
+uid://komeseatyar0
diff --git a/client/scripts/autoloads/game_state.gd b/client/scripts/autoloads/game_state.gd
index badb8800a..ad08a89ad 100644
--- a/client/scripts/autoloads/game_state.gd
+++ b/client/scripts/autoloads/game_state.gd
@@ -1,31 +1,26 @@
 extends Node
 
-# Updated each frame from ObserverSnapshot data
+# Updated each frame from ObserverSnapshot data (Protocol format: {tick, entities}).
+# Entities use Protocol decoded format: {entity_id, x, y, z, kind: {variant, data}}.
 var current_snapshot: Dictionary = {}
+var current_tick: int = 0
 var player_position: Vector2 = Vector2.ZERO
 var visible_entities: Array = []
-var fog_state: Dictionary = {}
-var hud_data: Dictionary = {}
+
+# Player entity ID — the first entity is assumed to be the player (will be
+# refined when the server assigns explicit player entity IDs).
+var player_entity_id: int = 1
 
 func apply_snapshot(snapshot: Dictionary) -> void:
 	current_snapshot = snapshot
 
-	# Parse player data
-	if snapshot.has("player") and snapshot.player.has("position"):
-		var pos = snapshot.player.position
-		if pos is Array and pos.size() >= 2:
-			player_position = Vector2(pos[0], pos[1])
-		else:
-			push_warning("GameState: malformed player position in snapshot")
+	if snapshot.has("tick"):
+		current_tick = snapshot.tick
 
-	# Parse entities
 	if snapshot.has("entities"):
 		visible_entities = snapshot.entities
-
-	# Parse fog state
-	if snapshot.has("fog"):
-		fog_state = snapshot.fog
-
-	# Parse HUD data
-	if snapshot.has("hud"):
-		hud_data = snapshot.hud
+		# Derive player position from the player entity
+		for entity in visible_entities:
+			if entity.has("entity_id") and entity.entity_id == player_entity_id:
+				player_position = Vector2(entity.x, entity.y)
+				break
diff --git a/client/scripts/autoloads/sim_bridge.gd b/client/scripts/autoloads/sim_bridge.gd
index d3dca48f5..aa3794948 100644
--- a/client/scripts/autoloads/sim_bridge.gd
+++ b/client/scripts/autoloads/sim_bridge.gd
@@ -6,6 +6,8 @@ enum ConnectionState { DISCONNECTED, CONNECTING, CONNECTED, ERROR }
 var state: ConnectionState = ConnectionState.DISCONNECTED
 var test_mode: bool = true  # Enable test mode for development without Rust server
 var _test_tick: int = 0
+var _last_snapshot: Variant = null  # Most recent decoded snapshot (consumed by poll_snapshot)
+var _outbound_buffer: Array[PackedByteArray] = []  # Encoded inputs awaiting transport
 
 # Signals
 signal connection_state_changed(old_state: ConnectionState, new_state: ConnectionState)
@@ -35,14 +37,26 @@ func connect_to_sim() -> void:
 func disconnect_from_sim() -> void:
 	_set_state(ConnectionState.DISCONNECTED)
 
-# Send input to simulation (real implementation comes later)
+# Send input to simulation server.
+# player_input: Dictionary with "action" (int from InputMapper.Action enum) and "timestamp_msec".
+# In test mode, inputs are silently dropped. Transport (ticket #79) will send the bytes.
 func send_input(player_input: Dictionary) -> void:
 	if state != ConnectionState.CONNECTED:
 		return
-	# TODO: Serialize and send via MessagePack when IPC is implemented
-	pass
+	if test_mode:
+		return
+	var action_name := _action_enum_to_wire(player_input.get("action", -1))
+	if action_name.is_empty():
+		return
+	var tick: int = player_input.get("timestamp_msec", 0)
+	var encoded := Protocol.encode_player_input(tick, action_name)
+	if encoded.size() == 0:
+		push_error("SimBridge: failed to encode player input (action=%s)" % action_name)
+		return
+	_outbound_buffer.append(encoded)
 
-# Poll for snapshot from simulation
+# Poll for snapshot from simulation.
+# In test mode returns hardcoded data. In live mode, returns the last decoded snapshot (if any).
 func poll_snapshot() -> Variant:
 	if state != ConnectionState.CONNECTED:
 		return null
@@ -52,31 +66,62 @@ func poll_snapshot() -> Variant:
 		snapshot_received.emit(snapshot)
 		return snapshot
 
-	# TODO: Actual polling logic when IPC/MessagePack is implemented
+	if _last_snapshot != null:
+		var snapshot = _last_snapshot
+		_last_snapshot = null
+		snapshot_received.emit(snapshot)
+		return snapshot
+
 	return null
 
-# Hardcoded test snapshot for development (deterministic per D-010 principle 4)
+# Called by transport layer (ticket #79) when raw bytes arrive from the server.
+# Latest-wins semantics: newer snapshots replace unconsumed ones. This is correct
+# for real-time rendering (stale frames are worthless). Upgrade to queue if needed.
+func receive_bytes(bytes: PackedByteArray) -> void:
+	var snapshot = Protocol.decode_snapshot(bytes)
+	if snapshot != null:
+		if _last_snapshot != null:
+			push_warning("SimBridge: overwriting unconsumed snapshot (tick %s replaced by %s)" % [_last_snapshot.tick, snapshot.tick])
+		_last_snapshot = snapshot
+
+# Drain the outbound buffer. Called by transport layer (ticket #79) to get encoded messages.
+func drain_outbound() -> Array[PackedByteArray]:
+	var messages = _outbound_buffer.duplicate()
+	_outbound_buffer.clear()
+	return messages
+
+# Map InputMapper.Action enum values to wire-format action names (matching Rust PlayerAction).
+# OPEN_MENU is client-only — no Rust equivalent, not sent over the wire.
+static func _action_enum_to_wire(action: int) -> String:
+	match action:
+		InputMapper.Action.MOVE_NORTH: return "MoveNorth"
+		InputMapper.Action.MOVE_SOUTH: return "MoveSouth"
+		InputMapper.Action.MOVE_EAST: return "MoveEast"
+		InputMapper.Action.MOVE_WEST: return "MoveWest"
+		InputMapper.Action.INTERACT: return "Interact"
+		InputMapper.Action.USE_PERCEPTION_MODE: return "UsePerceptionMode"
+		InputMapper.Action.PAUSE: return "Pause"
+		InputMapper.Action.OPEN_MENU:
+			# Client-only action, not part of wire protocol
+			push_warning("SimBridge: OPEN_MENU is client-only, not sent to server")
+			return ""
+		_:
+			push_warning("SimBridge: unknown action enum %s" % action)
+			return ""
+
+# Hardcoded test snapshot matching Protocol format (deterministic per D-010 principle 4).
+# Uses the same {tick, entities} schema as Protocol.decode_snapshot() returns.
 func _test_snapshot() -> Dictionary:
 	_test_tick += 1
 	return {
 		"tick": _test_tick,
-		"player": {
-			"position": [10, 10],
-			"health": 100
-		},
 		"entities": [
 			{
-				"id": 1,
-				"type": "npc",
-				"position": [12, 8],
-				"name": "Test NPC"
+				"entity_id": 1,
+				"x": 10.0,
+				"y": 10.0,
+				"z": 0,
+				"kind": { "variant": "Npc", "data": null },
 			},
 		],
-		"fog": {
-			"radius": 8
-		},
-		"hud": {
-			"perception_mode": "baseline",
-			"time": "08:00"
-		}
 	}
diff --git a/client/scripts/main.gd b/client/scripts/main.gd
index 31f803d95..d23871ed1 100644
--- a/client/scripts/main.gd
+++ b/client/scripts/main.gd
@@ -24,16 +24,6 @@ func _process(_delta: float) -> void:
 		# Track camera to player position (D-015)
 		camera.position = GameState.player_position * 32  # tile-space to pixel-space
 
-		# Update HUD
-		if hud and hud.has_method("update_from_hud_data"):
-			hud.update_from_hud_data(GameState.hud_data)
-			if snapshot.has("player") and snapshot.player.has("health"):
-				hud.update_health(snapshot.player.health)
-
-		# Wire monologue display (D-016 perception data path)
-		if snapshot.has("monologue") and monologue_display:
-			monologue_display.show_monologue(snapshot.monologue)
-
 	# Send queued input to simulation
 	var inputs = InputMapper.flush_queue()
 	for input in inputs:
diff --git a/client/scripts/protocol/protocol.gd b/client/scripts/protocol/protocol.gd
new file mode 100644
index 000000000..be31cece7
--- /dev/null
+++ b/client/scripts/protocol/protocol.gd
@@ -0,0 +1,127 @@
+class_name Protocol
+## MessagePack codec for the Rust↔Godot wire protocol (D-020).
+##
+## Encodes/decodes ObserverSnapshot and PlayerInput to match
+## rmp_serde's named-field encoding of server/src/bridge/types.rs.
+##
+## Wire format notes (rmp_serde with to_vec_named):
+##   Structs  → msgpack maps with string keys
+##   Unit enum variants (no data) → bare strings ("MoveNorth", "Npc")
+##   Data enum variants → single-element maps ({"UsePerceptionMode": "thermal"})
+
+
+# -- Decode: bytes from server → GDScript types --------------------------------
+
+## Decode an ObserverSnapshot from MessagePack bytes.
+## Returns { "tick": int, "entities": Array[Dictionary] } or null on error.
+static func decode_snapshot(bytes: PackedByteArray) -> Variant:
+	var result = Messagepack.decode(bytes)
+	if result.status != null:
+		push_error("Protocol: msgpack decode failed: %s" % result.status)
+		return null
+
+	var raw = result.value
+	if not raw is Dictionary or not raw.has("tick") or not raw.has("entities"):
+		push_error("Protocol: snapshot missing required fields")
+		return null
+
+	var entities: Array[Dictionary] = []
+	var raw_entities: Array = raw["entities"]
+	var dropped := 0
+	for raw_entity in raw_entities:
+		var entity = _decode_entity(raw_entity)
+		if entity != null:
+			entities.append(entity)
+		else:
+			dropped += 1
+
+	if dropped > 0:
+		push_error("Protocol: %d/%d entities failed to decode (D-010 information boundary violation)" % [dropped, raw_entities.size()])
+
+	# GDScript int is signed 64-bit. Rust tick is u64 but will not exceed 2^63
+	# in any realistic scenario (would require ~29 billion years at 10 ticks/game-minute per D-031).
+	var tick: int = raw["tick"]
+	return {
+		"tick": tick,
+		"entities": entities,
+		"decode_errors": dropped,
+	}
+
+
+## Decode a single VisibleEntity from a raw msgpack map.
+static func _decode_entity(raw: Dictionary) -> Variant:
+	if not raw.has("entity_id") or not raw.has("x") or not raw.has("y") \
+			or not raw.has("z") or not raw.has("kind"):
+		push_warning("Protocol: entity missing required fields: %s" % str(raw.keys()))
+		return null
+
+	var entity_id: int = raw["entity_id"]
+	return {
+		"entity_id": entity_id,
+		"x": float(raw["x"]),
+		"y": float(raw["y"]),
+		"z": int(raw["z"]),
+		"kind": _decode_enum_variant(raw["kind"]),
+	}
+
+
+## Decode an enum variant from rmp_serde's encoding.
+## Unit variants are bare strings, data variants are single-element maps.
+## Returns { "variant": String, "data": Variant } in both cases.
+static func _decode_enum_variant(raw) -> Dictionary:
+	if raw is String:
+		return { "variant": raw, "data": null }
+	elif raw is Dictionary and raw.size() == 1:
+		var variant_name: String = raw.keys()[0]
+		return { "variant": variant_name, "data": raw[variant_name] }
+	else:
+		push_warning("Protocol: unexpected enum encoding: %s" % str(raw))
+		return { "variant": "Unknown", "data": raw }
+
+
+# -- Encode: GDScript types → bytes to server ----------------------------------
+
+## Encode a PlayerInput to MessagePack bytes.
+## action_name: one of "MoveNorth", "MoveSouth", "MoveEast", "MoveWest",
+##              "Interact", "UsePerceptionMode", "Pause", "Unpause"
+## action_data: null for unit variants, String for UsePerceptionMode
+static func encode_player_input(tick: int, action_name: String, action_data: Variant = null) -> PackedByteArray:
+	var action_value: Variant
+	if action_data != null:
+		# Data variant → single-element map
+		action_value = { action_name: action_data }
+	else:
+		# Unit variant → bare string
+		action_value = action_name
+
+	var input := {
+		"tick": tick,
+		"action": action_value,
+	}
+
+	var result = Messagepack.encode(input)
+	if result.status != null:
+		push_error("Protocol: msgpack encode failed: %s" % result.status)
+		return PackedByteArray()
+
+	return result.value
+
+
+## Decode a PlayerInput from MessagePack bytes (used in tests / echo scenarios).
+## Returns { "tick": int, "action": { "variant": String, "data": Variant } } or null.
+static func decode_player_input(bytes: PackedByteArray) -> Variant:
+	var result = Messagepack.decode(bytes)
+	if result.status != null:
+		push_error("Protocol: msgpack decode failed: %s" % result.status)
+		return null
+
+	var raw = result.value
+	if not raw is Dictionary or not raw.has("tick") or not raw.has("action"):
+		push_error("Protocol: player_input missing required fields")
+		return null
+
+	var tick: int = raw["tick"]
+	return {
+		"tick": tick,
+		"action": _decode_enum_variant(raw["action"]),
+	}
diff --git a/client/scripts/protocol/protocol.gd.uid b/client/scripts/protocol/protocol.gd.uid
new file mode 100644
index 000000000..f4c3493eb
--- /dev/null
+++ b/client/scripts/protocol/protocol.gd.uid
@@ -0,0 +1 @@
+uid://fkmd537xvwxe
diff --git a/client/scripts/rendering/world_renderer.gd b/client/scripts/rendering/world_renderer.gd
index 419090e40..12155ead8 100644
--- a/client/scripts/rendering/world_renderer.gd
+++ b/client/scripts/rendering/world_renderer.gd
@@ -15,6 +15,6 @@ func update_from_state() -> void:
 	if entity_renderer and entity_renderer.has_method("update_entities"):
 		entity_renderer.update_entities(GameState.visible_entities)
 
-	# Update fog overlay
+	# Update fog overlay (fog data will come in D-020 expansion)
 	if fog_renderer and fog_renderer.has_method("update_fog"):
-		fog_renderer.update_fog(GameState.fog_state, GameState.player_position)
+		fog_renderer.update_fog({}, GameState.player_position)
diff --git a/client/tests/fixtures/msgpack/input_move_north.msgpack b/client/tests/fixtures/msgpack/input_move_north.msgpack
new file mode 100644
index 000000000..937f385e1
--- /dev/null
+++ b/client/tests/fixtures/msgpack/input_move_north.msgpack
@@ -0,0 +1 @@
+��tickd�action�MoveNorth
\ No newline at end of file
diff --git a/client/tests/fixtures/msgpack/input_perception_mode.msgpack b/client/tests/fixtures/msgpack/input_perception_mode.msgpack
new file mode 100644
index 000000000..3f922c7e3
--- /dev/null
+++ b/client/tests/fixtures/msgpack/input_perception_mode.msgpack
@@ -0,0 +1 @@
+��tick�Ȧaction��UsePerceptionMode�thermal
\ No newline at end of file
diff --git a/client/tests/fixtures/msgpack/snapshot_empty.msgpack b/client/tests/fixtures/msgpack/snapshot_empty.msgpack
new file mode 100644
index 000000000..5435b2917
Binary files /dev/null and b/client/tests/fixtures/msgpack/snapshot_empty.msgpack differ
diff --git a/client/tests/fixtures/msgpack/snapshot_multi_entity.msgpack b/client/tests/fixtures/msgpack/snapshot_multi_entity.msgpack
new file mode 100644
index 000000000..2c0217a45
Binary files /dev/null and b/client/tests/fixtures/msgpack/snapshot_multi_entity.msgpack differ
diff --git a/client/tests/fixtures/msgpack/snapshot_one_npc.msgpack b/client/tests/fixtures/msgpack/snapshot_one_npc.msgpack
new file mode 100644
index 000000000..f98303206
Binary files /dev/null and b/client/tests/fixtures/msgpack/snapshot_one_npc.msgpack differ
diff --git a/client/tests/test_protocol.gd b/client/tests/test_protocol.gd
new file mode 100644
index 000000000..e574bf318
--- /dev/null
+++ b/client/tests/test_protocol.gd
@@ -0,0 +1,169 @@
+## D-030 Layer 1: Cross-language MessagePack tests
+## Validates that Protocol.gd correctly decodes fixtures generated by Rust (rmp_serde).
+## Fixtures generated by: cargo test --test gen_fixtures -- --ignored
+class_name TestProtocol
+extends GdUnitTestSuite
+
+const FIXTURE_DIR = "res://tests/fixtures/msgpack/"
+
+
+func _load_fixture(name: String) -> PackedByteArray:
+	var path = FIXTURE_DIR + name + ".msgpack"
+	var file = FileAccess.open(path, FileAccess.READ)
+	assert_that(file).is_not_null()
+	return file.get_buffer(file.get_length())
+
+
+# -- Snapshot decoding ----------------------------------------------------------
+
+func test_decode_snapshot_one_npc() -> void:
+	var bytes = _load_fixture("snapshot_one_npc")
+	var snapshot = Protocol.decode_snapshot(bytes)
+
+	assert_that(snapshot).is_not_null()
+	assert_that(snapshot.tick).is_equal(42)
+	assert_that(snapshot.entities.size()).is_equal(1)
+
+	var entity = snapshot.entities[0]
+	assert_that(entity.entity_id).is_equal(1)
+	assert_float(entity.x).is_equal_approx(10.0, 0.001)
+	assert_float(entity.y).is_equal_approx(20.0, 0.001)
+	assert_that(entity.z).is_equal(0)
+	assert_that(entity.kind.variant).is_equal("Npc")
+	assert_that(entity.kind.data).is_null()
+
+
+func test_decode_snapshot_empty() -> void:
+	var bytes = _load_fixture("snapshot_empty")
+	var snapshot = Protocol.decode_snapshot(bytes)
+
+	assert_that(snapshot).is_not_null()
+	assert_that(snapshot.tick).is_equal(0)
+	assert_that(snapshot.entities.size()).is_equal(0)
+
+
+func test_decode_snapshot_multi_entity() -> void:
+	var bytes = _load_fixture("snapshot_multi_entity")
+	var snapshot = Protocol.decode_snapshot(bytes)
+
+	assert_that(snapshot).is_not_null()
+	assert_that(snapshot.tick).is_equal(999)
+	assert_that(snapshot.entities.size()).is_equal(3)
+
+	# NPC at (5, 10, 0)
+	var npc = snapshot.entities[0]
+	assert_that(npc.entity_id).is_equal(1)
+	assert_float(npc.x).is_equal_approx(5.0, 0.001)
+	assert_float(npc.y).is_equal_approx(10.0, 0.001)
+	assert_that(npc.z).is_equal(0)
+	assert_that(npc.kind.variant).is_equal("Npc")
+
+	# Object at (15.5, 3, 1)
+	var obj = snapshot.entities[1]
+	assert_that(obj.entity_id).is_equal(2)
+	assert_float(obj.x).is_equal_approx(15.5, 0.001)
+	assert_float(obj.y).is_equal_approx(3.0, 0.001)
+	assert_that(obj.z).is_equal(1)
+	assert_that(obj.kind.variant).is_equal("Object")
+
+	# Terrain at (0, 0, -1)
+	var terrain = snapshot.entities[2]
+	assert_that(terrain.entity_id).is_equal(3)
+	assert_float(terrain.x).is_equal_approx(0.0, 0.001)
+	assert_float(terrain.y).is_equal_approx(0.0, 0.001)
+	assert_that(terrain.z).is_equal(-1)
+	assert_that(terrain.kind.variant).is_equal("Terrain")
+
+
+# -- PlayerInput decoding -------------------------------------------------------
+
+func test_decode_input_move_north() -> void:
+	var bytes = _load_fixture("input_move_north")
+	var input = Protocol.decode_player_input(bytes)
+
+	assert_that(input).is_not_null()
+	assert_that(input.tick).is_equal(100)
+	assert_that(input.action.variant).is_equal("MoveNorth")
+	assert_that(input.action.data).is_null()
+
+
+func test_decode_input_perception_mode() -> void:
+	var bytes = _load_fixture("input_perception_mode")
+	var input = Protocol.decode_player_input(bytes)
+
+	assert_that(input).is_not_null()
+	assert_that(input.tick).is_equal(200)
+	assert_that(input.action.variant).is_equal("UsePerceptionMode")
+	assert_that(input.action.data).is_equal("thermal")
+
+
+# -- PlayerInput encoding -------------------------------------------------------
+
+func test_encode_decode_roundtrip_unit_variant() -> void:
+	var bytes = Protocol.encode_player_input(50, "MoveEast")
+	assert_that(bytes.size()).is_greater(0)
+
+	var decoded = Protocol.decode_player_input(bytes)
+	assert_that(decoded).is_not_null()
+	assert_that(decoded.tick).is_equal(50)
+	assert_that(decoded.action.variant).is_equal("MoveEast")
+	assert_that(decoded.action.data).is_null()
+
+
+func test_encode_decode_roundtrip_data_variant() -> void:
+	var bytes = Protocol.encode_player_input(75, "UsePerceptionMode", "infrared")
+	assert_that(bytes.size()).is_greater(0)
+
+	var decoded = Protocol.decode_player_input(bytes)
+	assert_that(decoded).is_not_null()
+	assert_that(decoded.tick).is_equal(75)
+	assert_that(decoded.action.variant).is_equal("UsePerceptionMode")
+	assert_that(decoded.action.data).is_equal("infrared")
+
+
+# -- Cross-language roundtrip: GDScript encode matches Rust decode ---------------
+
+func test_gdscript_encode_matches_rust_fixture() -> void:
+	# Encode the same MoveNorth input as the Rust fixture
+	var bytes = Protocol.encode_player_input(100, "MoveNorth")
+
+	# Decode and verify the content matches the Rust fixture
+	var rust_bytes = _load_fixture("input_move_north")
+	var from_gd = Protocol.decode_player_input(bytes)
+	var from_rust = Protocol.decode_player_input(rust_bytes)
+
+	assert_that(from_gd.tick).is_equal(from_rust.tick)
+	assert_that(from_gd.action.variant).is_equal(from_rust.action.variant)
+	assert_that(from_gd.action.data).is_equal(from_rust.action.data)
+
+
+# -- Negative tests: malformed/truncated input -----------------------------------
+
+func test_decode_snapshot_truncated_bytes() -> void:
+	var truncated := PackedByteArray([0x82, 0xa4])  # Incomplete msgpack map
+	var result = Protocol.decode_snapshot(truncated)
+	assert_that(result).is_null()
+
+
+func test_decode_snapshot_wrong_type() -> void:
+	# Encode an array instead of a map — should fail validation
+	var encoded = Messagepack.encode([1, 2, 3])
+	var result = Protocol.decode_snapshot(encoded.value)
+	assert_that(result).is_null()
+
+
+func test_decode_snapshot_missing_fields() -> void:
+	# Map with wrong keys
+	var encoded = Messagepack.encode({"foo": "bar"})
+	var result = Protocol.decode_snapshot(encoded.value)
+	assert_that(result).is_null()
+
+
+func test_decode_player_input_empty_bytes() -> void:
+	var result = Protocol.decode_player_input(PackedByteArray())
+	assert_that(result).is_null()
+
+
+func test_encode_produces_nonempty_bytes() -> void:
+	var bytes = Protocol.encode_player_input(1, "MoveNorth")
+	assert_that(bytes.size()).is_greater(0)
diff --git a/client/tests/test_protocol.gd.uid b/client/tests/test_protocol.gd.uid
new file mode 100644
index 000000000..42e5f4aea
--- /dev/null
+++ b/client/tests/test_protocol.gd.uid
@@ -0,0 +1 @@
+uid://cj0et712osytn
diff --git a/client/tests/test_snapshot_parsing.gd b/client/tests/test_snapshot_parsing.gd
index dcc5c49d5..af025e14c 100644
--- a/client/tests/test_snapshot_parsing.gd
+++ b/client/tests/test_snapshot_parsing.gd
@@ -1,30 +1,24 @@
 ## D-030 Layer 1: Fixture-based tests for snapshot parsing
-## Validates that GameState correctly parses ObserverSnapshot data
+## Validates that GameState correctly parses Protocol-format ObserverSnapshot data
 class_name TestSnapshotParsing
 extends GdUnitTestSuite
 
-# Valid snapshot fixture
+# Valid snapshot in Protocol format (matches Protocol.decode_snapshot output)
 var _valid_snapshot: Dictionary = {
 	"tick": 1,
-	"player": {
-		"position": [10, 15],
-		"health": 85
-	},
 	"entities": [
-		{"id": 1, "type": "npc", "position": [12, 8], "name": "Test NPC"},
-		{"id": 2, "type": "npc", "position": [5, 20], "name": "Second NPC"},
+		{"entity_id": 1, "x": 10.0, "y": 15.0, "z": 0, "kind": {"variant": "Npc", "data": null}},
+		{"entity_id": 2, "x": 5.0, "y": 20.0, "z": 0, "kind": {"variant": "Npc", "data": null}},
 	],
-	"fog": {"radius": 8},
-	"hud": {"perception_mode": "baseline", "time": "14:30"}
 }
 
 func test_apply_valid_snapshot() -> void:
+	GameState.player_entity_id = 1
 	GameState.apply_snapshot(_valid_snapshot)
 
+	assert_that(GameState.current_tick).is_equal(1)
 	assert_that(GameState.player_position).is_equal(Vector2(10, 15))
 	assert_that(GameState.visible_entities.size()).is_equal(2)
-	assert_that(GameState.fog_state).is_equal({"radius": 8})
-	assert_that(GameState.hud_data).is_equal({"perception_mode": "baseline", "time": "14:30"})
 
 func test_empty_snapshot_no_crash() -> void:
 	# Reset state
@@ -37,26 +31,25 @@ func test_empty_snapshot_no_crash() -> void:
 	assert_that(GameState.player_position).is_equal(Vector2.ZERO)
 	assert_that(GameState.visible_entities.size()).is_equal(0)
 
-func test_malformed_position_no_crash() -> void:
-	var bad_snapshot: Dictionary = {
-		"player": {"position": "not_an_array"},
-	}
-	# Reset
-	GameState.player_position = Vector2.ZERO
+func test_no_player_entity_position_unchanged() -> void:
+	GameState.player_entity_id = 999  # No entity with this ID
+	GameState.player_position = Vector2(5, 5)
 
-	# Should not crash — logs warning instead
-	GameState.apply_snapshot(bad_snapshot)
-	assert_that(GameState.player_position).is_equal(Vector2.ZERO)
+	GameState.apply_snapshot(_valid_snapshot)
+
+	# Position stays at previous value since no matching entity
+	assert_that(GameState.player_position).is_equal(Vector2(5, 5))
 
 func test_missing_fields_partial_update() -> void:
 	# First apply valid snapshot
+	GameState.player_entity_id = 1
 	GameState.apply_snapshot(_valid_snapshot)
 	assert_that(GameState.player_position).is_equal(Vector2(10, 15))
 
-	# Apply snapshot with only HUD data — player position unchanged
-	GameState.apply_snapshot({"hud": {"perception_mode": "thermal", "time": "22:00"}})
+	# Apply snapshot with no entities — player position unchanged (no matching entity)
+	GameState.apply_snapshot({"tick": 2})
 	assert_that(GameState.player_position).is_equal(Vector2(10, 15))
-	assert_that(GameState.hud_data.perception_mode).is_equal("thermal")
+	assert_that(GameState.current_tick).is_equal(2)
 
 func test_sim_bridge_test_snapshot_deterministic() -> void:
 	SimBridge._test_tick = 0
@@ -65,8 +58,9 @@ func test_sim_bridge_test_snapshot_deterministic() -> void:
 
 	assert_that(snap1.tick).is_equal(1)
 	assert_that(snap2.tick).is_equal(2)
-	# Snapshot structure is stable
-	assert_that(snap1.has("player")).is_true()
+	# Snapshot matches Protocol format
+	assert_that(snap1.has("tick")).is_true()
 	assert_that(snap1.has("entities")).is_true()
-	assert_that(snap1.has("fog")).is_true()
-	assert_that(snap1.has("hud")).is_true()
+	assert_that(snap1.entities.size()).is_greater(0)
+	assert_that(snap1.entities[0].has("entity_id")).is_true()
+	assert_that(snap1.entities[0].has("kind")).is_true()
diff --git a/client/tests/test_snapshot_parsing.gd.uid b/client/tests/test_snapshot_parsing.gd.uid
new file mode 100644
index 000000000..50d26428c
--- /dev/null
+++ b/client/tests/test_snapshot_parsing.gd.uid
@@ -0,0 +1 @@
+uid://ct5bcdvoyo65p
diff --git a/server/tests/gen_fixtures.rs b/server/tests/gen_fixtures.rs
new file mode 100644
index 000000000..4cd63aff2
--- /dev/null
+++ b/server/tests/gen_fixtures.rs
@@ -0,0 +1,64 @@
+//! Generate MessagePack fixture files for cross-language testing (D-030 Layer 1).
+//! Run with: cargo test --test gen_fixtures -- --ignored
+
+use settled_reach_server::bridge::types::*;
+use std::fs;
+use std::path::Path;
+
+fn write_fixture(name: &str, bytes: &[u8]) {
+    // Write directly into the Godot project's test fixtures (single source of truth)
+    let dir = Path::new("../client/tests/fixtures/msgpack");
+    fs::create_dir_all(dir).expect("create fixture dir");
+    let path = dir.join(format!("{}.msgpack", name));
+    fs::write(&path, bytes).expect("write fixture");
+    eprintln!("Wrote {} ({} bytes)", path.display(), bytes.len());
+}
+
+#[test]
+#[ignore] // Run manually: cargo test --test gen_fixtures -- --ignored
+fn generate_msgpack_fixtures() {
+    // Snapshot with one NPC entity
+    let snapshot = ObserverSnapshot {
+        tick: 42,
+        entities: vec![VisibleEntity {
+            entity_id: 1,
+            x: 10.0,
+            y: 20.0,
+            z: 0,
+            kind: EntityKind::Npc,
+        }],
+    };
+    write_fixture("snapshot_one_npc", &rmp_serde::to_vec_named(&snapshot).unwrap());
+
+    // Empty snapshot
+    let empty = ObserverSnapshot {
+        tick: 0,
+        entities: vec![],
+    };
+    write_fixture("snapshot_empty", &rmp_serde::to_vec_named(&empty).unwrap());
+
+    // PlayerInput: MoveNorth
+    let input_north = PlayerInput {
+        tick: 100,
+        action: PlayerAction::MoveNorth,
+    };
+    write_fixture("input_move_north", &rmp_serde::to_vec_named(&input_north).unwrap());
+
+    // PlayerInput: UsePerceptionMode
+    let input_perception = PlayerInput {
+        tick: 200,
+        action: PlayerAction::UsePerceptionMode("thermal".to_string()),
+    };
+    write_fixture("input_perception_mode", &rmp_serde::to_vec_named(&input_perception).unwrap());
+
+    // Snapshot with multiple entities and all EntityKind variants
+    let snapshot_multi = ObserverSnapshot {
+        tick: 999,
+        entities: vec![
+            VisibleEntity { entity_id: 1, x: 5.0, y: 10.0, z: 0, kind: EntityKind::Npc },
+            VisibleEntity { entity_id: 2, x: 15.5, y: 3.0, z: 1, kind: EntityKind::Object },
+            VisibleEntity { entity_id: 3, x: 0.0, y: 0.0, z: -1, kind: EntityKind::Terrain },
+        ],
+    };
+    write_fixture("snapshot_multi_entity", &rmp_serde::to_vec_named(&snapshot_multi).unwrap());
+}