diff --git a/src/core/BinaryCodec.ts b/src/core/BinaryCodec.ts
new file mode 100644
index 000000000..d62e20a5b
--- /dev/null
+++ b/src/core/BinaryCodec.ts
@@ -0,0 +1,784 @@
+import {
+  BINARY_HEADER_SIZE,
+  BINARY_PROTOCOL_VERSION,
+  BinaryClientGameplayMessage,
+  BinaryMessageType,
+  BinaryProtocolContext,
+  BinaryServerGameplayMessage,
+  INTENT_FLAG_OPTION_A,
+  INTENT_FLAG_OPTION_B,
+  createBinaryProtocolContext,
+  intentTypeToOpcode,
+  opcodeToIntentType,
+  opcodeToUnitType,
+  playerIdToIndex,
+  playerIndexToId,
+  requireClientId,
+  stampedIntentClientIndex,
+  unitTypeToOpcode,
+} from "./BinaryProtocol";
+import {
+  ClientHashMessage,
+  ClientIntentMessage,
+  ClientMessage,
+  ClientPingMessage,
+  GameStartInfo,
+  Intent,
+  QuickChatKeySchema,
+  ServerDesyncMessage,
+  ServerTurnMessage,
+  StampedIntent,
+} from "./Schemas";
+import { UnitType } from "./game/Game";
+
+const textEncoder = new TextEncoder();
+const textDecoder = new TextDecoder();
+
+class BinaryWriter {
+  private readonly chunks: Uint8Array[] = [];
+  private totalLength = 0;
+
+  writeUint8(value: number) {
+    const chunk = new Uint8Array(1);
+    chunk[0] = value;
+    this.push(chunk);
+  }
+
+  writeUint16(value: number) {
+    const chunk = new Uint8Array(2);
+    new DataView(chunk.buffer).setUint16(0, value, true);
+    this.push(chunk);
+  }
+
+  writeUint32(value: number) {
+    const chunk = new Uint8Array(4);
+    new DataView(chunk.buffer).setUint32(0, value, true);
+    this.push(chunk);
+  }
+
+  writeInt32(value: number) {
+    const chunk = new Uint8Array(4);
+    new DataView(chunk.buffer).setInt32(0, value, true);
+    this.push(chunk);
+  }
+
+  writeFloat64(value: number) {
+    const chunk = new Uint8Array(8);
+    new DataView(chunk.buffer).setFloat64(0, value, true);
+    this.push(chunk);
+  }
+
+  writeBoolean(value: boolean) {
+    this.writeUint8(value ? 1 : 0);
+  }
+
+  writeString(value: string) {
+    const encoded = textEncoder.encode(value);
+    this.writeUint16(encoded.length);
+    this.push(encoded);
+  }
+
+  writeFrame(
+    messageType: BinaryMessageType,
+    writePayload: () => void,
+  ): Uint8Array {
+    this.writeUint8(BINARY_PROTOCOL_VERSION);
+    this.writeUint8(messageType);
+    this.writeUint16(0);
+    writePayload();
+    return this.finish();
+  }
+
+  finish(): Uint8Array {
+    const output = new Uint8Array(this.totalLength);
+    let offset = 0;
+    for (const chunk of this.chunks) {
+      output.set(chunk, offset);
+      offset += chunk.length;
+    }
+    return output;
+  }
+
+  private push(chunk: Uint8Array) {
+    this.chunks.push(chunk);
+    this.totalLength += chunk.length;
+  }
+}
+
+class BinaryReader {
+  private readonly view: DataView;
+  private offset = 0;
+
+  constructor(private readonly bytes: Uint8Array) {
+    this.view = new DataView(bytes.buffer, bytes.byteOffset, bytes.byteLength);
+  }
+
+  readHeader(expectedType: BinaryMessageType) {
+    if (this.bytes.byteLength < BINARY_HEADER_SIZE) {
+      throw new Error("Binary frame too short");
+    }
+    const version = this.readUint8();
+    if (version !== BINARY_PROTOCOL_VERSION) {
+      throw new Error(`Unsupported binary protocol version: ${version}`);
+    }
+    const messageType = this.readUint8();
+    if (messageType !== expectedType) {
+      throw new Error(
+        `Unexpected binary message type: expected ${expectedType}, received ${messageType}`,
+      );
+    }
+    const flags = this.readUint16();
+    if (flags !== 0) {
+      throw new Error(`Unsupported binary header flags: ${flags}`);
+    }
+  }
+
+  readUint8(): number {
+    this.ensureAvailable(1);
+    const value = this.view.getUint8(this.offset);
+    this.offset += 1;
+    return value;
+  }
+
+  readUint16(): number {
+    this.ensureAvailable(2);
+    const value = this.view.getUint16(this.offset, true);
+    this.offset += 2;
+    return value;
+  }
+
+  readUint32(): number {
+    this.ensureAvailable(4);
+    const value = this.view.getUint32(this.offset, true);
+    this.offset += 4;
+    return value;
+  }
+
+  readInt32(): number {
+    this.ensureAvailable(4);
+    const value = this.view.getInt32(this.offset, true);
+    this.offset += 4;
+    return value;
+  }
+
+  readFloat64(): number {
+    this.ensureAvailable(8);
+    const value = this.view.getFloat64(this.offset, true);
+    this.offset += 8;
+    return value;
+  }
+
+  readBoolean(): boolean {
+    const value = this.readUint8();
+    if (value !== 0 && value !== 1) {
+      throw new Error(`Invalid boolean value: ${value}`);
+    }
+    return value === 1;
+  }
+
+  readString(): string {
+    const length = this.readUint16();
+    this.ensureAvailable(length);
+    const value = textDecoder.decode(
+      this.bytes.subarray(this.offset, this.offset + length),
+    );
+    this.offset += length;
+    return value;
+  }
+
+  ensureFinished() {
+    if (this.offset !== this.bytes.byteLength) {
+      throw new Error(
+        `Unexpected trailing bytes: ${this.bytes.byteLength - this.offset}`,
+      );
+    }
+  }
+
+  private ensureAvailable(length: number) {
+    if (this.offset + length > this.bytes.byteLength) {
+      throw new Error("Unexpected end of binary frame");
+    }
+  }
+}
+
+export function binaryContextFromGameStartInfo(
+  gameStartInfo: Pick<GameStartInfo, "players">,
+): BinaryProtocolContext {
+  return createBinaryProtocolContext(gameStartInfo);
+}
+
+export function toUint8Array(data: ArrayBuffer | Uint8Array): Uint8Array {
+  if (data instanceof Uint8Array) {
+    return data;
+  }
+  return new Uint8Array(data);
+}
+
+export function encodeBinaryClientGameplayMessage(
+  message: BinaryClientGameplayMessage,
+  context: BinaryProtocolContext,
+): Uint8Array {
+  switch (message.type) {
+    case "intent":
+      return encodeClientIntentMessage(message, context);
+    case "hash":
+      return encodeClientHashMessage(message);
+    case "ping":
+      return encodeClientPingMessage(message);
+  }
+}
+
+export function decodeBinaryClientGameplayMessage(
+  data: ArrayBuffer | Uint8Array,
+  context: BinaryProtocolContext,
+): ClientIntentMessage | ClientHashMessage | ClientPingMessage {
+  const bytes = toUint8Array(data);
+  if (bytes.byteLength < BINARY_HEADER_SIZE) {
+    throw new Error("Binary frame too short");
+  }
+
+  switch (bytes[1]) {
+    case BinaryMessageType.Intent:
+      return decodeClientIntentMessage(bytes, context);
+    case BinaryMessageType.Hash:
+      return decodeClientHashMessage(bytes);
+    case BinaryMessageType.Ping:
+      return decodeClientPingMessage(bytes);
+    default:
+      throw new Error(`Unknown client binary message type: ${bytes[1]}`);
+  }
+}
+
+export function encodeBinaryServerGameplayMessage(
+  message: BinaryServerGameplayMessage,
+  context: BinaryProtocolContext,
+): Uint8Array {
+  switch (message.type) {
+    case "turn":
+      return encodeServerTurnMessage(message, context);
+    case "desync":
+      return encodeServerDesyncMessage(message);
+  }
+}
+
+export function decodeBinaryServerGameplayMessage(
+  data: ArrayBuffer | Uint8Array,
+  context: BinaryProtocolContext,
+): ServerTurnMessage | ServerDesyncMessage {
+  const bytes = toUint8Array(data);
+  if (bytes.byteLength < BINARY_HEADER_SIZE) {
+    throw new Error("Binary frame too short");
+  }
+
+  switch (bytes[1]) {
+    case BinaryMessageType.Turn:
+      return decodeServerTurnMessage(bytes, context);
+    case BinaryMessageType.Desync:
+      return decodeServerDesyncMessage(bytes);
+    default:
+      throw new Error(`Unknown server binary message type: ${bytes[1]}`);
+  }
+}
+
+export function encodeClientIntentMessage(
+  message: ClientIntentMessage,
+  context: BinaryProtocolContext,
+): Uint8Array {
+  const writer = new BinaryWriter();
+  return writer.writeFrame(BinaryMessageType.Intent, () => {
+    encodeIntent(writer, message.intent, context);
+  });
+}
+
+export function decodeClientIntentMessage(
+  data: ArrayBuffer | Uint8Array,
+  context: BinaryProtocolContext,
+): ClientIntentMessage {
+  const reader = new BinaryReader(toUint8Array(data));
+  reader.readHeader(BinaryMessageType.Intent);
+  const intent = decodeIntent(reader, context);
+  reader.ensureFinished();
+  return {
+    type: "intent",
+    intent,
+  };
+}
+
+export function encodeClientHashMessage(
+  message: ClientHashMessage,
+): Uint8Array {
+  const writer = new BinaryWriter();
+  return writer.writeFrame(BinaryMessageType.Hash, () => {
+    writer.writeUint32(message.turnNumber);
+    writer.writeInt32(message.hash);
+  });
+}
+
+export function decodeClientHashMessage(
+  data: ArrayBuffer | Uint8Array,
+): ClientHashMessage {
+  const reader = new BinaryReader(toUint8Array(data));
+  reader.readHeader(BinaryMessageType.Hash);
+  const turnNumber = reader.readUint32();
+  const hash = reader.readInt32();
+  reader.ensureFinished();
+  return {
+    type: "hash",
+    turnNumber,
+    hash,
+  };
+}
+
+export function encodeClientPingMessage(
+  _message: ClientPingMessage,
+): Uint8Array {
+  const writer = new BinaryWriter();
+  return writer.writeFrame(BinaryMessageType.Ping, () => {});
+}
+
+export function decodeClientPingMessage(
+  data: ArrayBuffer | Uint8Array,
+): ClientPingMessage {
+  const reader = new BinaryReader(toUint8Array(data));
+  reader.readHeader(BinaryMessageType.Ping);
+  reader.ensureFinished();
+  return { type: "ping" };
+}
+
+export function encodeServerTurnMessage(
+  message: ServerTurnMessage,
+  context: BinaryProtocolContext,
+): Uint8Array {
+  const writer = new BinaryWriter();
+  return writer.writeFrame(BinaryMessageType.Turn, () => {
+    writer.writeUint32(message.turn.turnNumber);
+    writer.writeUint16(message.turn.intents.length);
+    for (const intent of message.turn.intents) {
+      writer.writeUint16(stampedIntentClientIndex(intent, context));
+      encodeIntent(writer, intent, context);
+    }
+  });
+}
+
+export function decodeServerTurnMessage(
+  data: ArrayBuffer | Uint8Array,
+  context: BinaryProtocolContext,
+): ServerTurnMessage {
+  const reader = new BinaryReader(toUint8Array(data));
+  reader.readHeader(BinaryMessageType.Turn);
+  const turnNumber = reader.readUint32();
+  const intentCount = reader.readUint16();
+  const intents: StampedIntent[] = [];
+  for (let i = 0; i < intentCount; i++) {
+    const clientIndex = reader.readUint16();
+    const clientID = requireClientId(clientIndex, context);
+    const intent = decodeIntent(reader, context);
+    intents.push({
+      ...intent,
+      clientID,
+    } as StampedIntent);
+  }
+  reader.ensureFinished();
+  return {
+    type: "turn",
+    turn: {
+      turnNumber,
+      intents,
+    },
+  };
+}
+
+export function encodeServerDesyncMessage(
+  message: ServerDesyncMessage,
+): Uint8Array {
+  const writer = new BinaryWriter();
+  return writer.writeFrame(BinaryMessageType.Desync, () => {
+    writer.writeUint32(message.turn);
+    writer.writeBoolean(message.correctHash !== null);
+    if (message.correctHash !== null) {
+      writer.writeInt32(message.correctHash);
+    }
+    writer.writeUint16(message.clientsWithCorrectHash);
+    writer.writeUint16(message.totalActiveClients);
+  });
+}
+
+export function decodeServerDesyncMessage(
+  data: ArrayBuffer | Uint8Array,
+): ServerDesyncMessage {
+  const reader = new BinaryReader(toUint8Array(data));
+  reader.readHeader(BinaryMessageType.Desync);
+  const turn = reader.readUint32();
+  const hasCorrectHash = reader.readBoolean();
+  const correctHash = hasCorrectHash ? reader.readInt32() : null;
+  const clientsWithCorrectHash = reader.readUint16();
+  const totalActiveClients = reader.readUint16();
+  reader.ensureFinished();
+  return {
+    type: "desync",
+    turn,
+    correctHash,
+    clientsWithCorrectHash,
+    totalActiveClients,
+  };
+}
+
+function encodeIntent(
+  writer: BinaryWriter,
+  intent: Intent,
+  context: BinaryProtocolContext,
+) {
+  const intentOpcode = intentTypeToOpcode(intent.type);
+  writer.writeUint8(intentOpcode);
+
+  let flags = 0;
+  switch (intent.type) {
+    case "attack":
+      if (intent.targetID !== null) {
+        flags |= INTENT_FLAG_OPTION_A;
+      }
+      if (intent.troops !== null) {
+        flags |= INTENT_FLAG_OPTION_B;
+      }
+      break;
+    case "build_unit":
+      if (intent.rocketDirectionUp !== undefined) {
+        flags |= INTENT_FLAG_OPTION_A;
+      }
+      if (intent.rocketDirectionUp) {
+        flags |= INTENT_FLAG_OPTION_B;
+      }
+      break;
+    case "quick_chat":
+      if (intent.target !== undefined) {
+        flags |= INTENT_FLAG_OPTION_A;
+      }
+      break;
+  }
+  writer.writeUint16(flags);
+
+  switch (intent.type) {
+    case "attack":
+      if (intent.targetID !== null) {
+        writer.writeUint16(playerIdToIndex(intent.targetID, context));
+      }
+      if (intent.troops !== null) {
+        writer.writeFloat64(intent.troops);
+      }
+      return;
+    case "cancel_attack":
+      writer.writeString(intent.attackID);
+      return;
+    case "spawn":
+      writer.writeUint32(intent.tile);
+      return;
+    case "mark_disconnected":
+      writer.writeUint16(playerIdToIndex(intent.clientID, context));
+      writer.writeBoolean(intent.isDisconnected);
+      return;
+    case "boat":
+      writer.writeFloat64(intent.troops);
+      writer.writeUint32(intent.dst);
+      return;
+    case "cancel_boat":
+      writer.writeUint32(intent.unitID);
+      return;
+    case "allianceRequest":
+      writer.writeUint16(playerIdToIndex(intent.recipient, context));
+      return;
+    case "allianceReject":
+      writer.writeUint16(playerIdToIndex(intent.requestor, context));
+      return;
+    case "breakAlliance":
+      writer.writeUint16(playerIdToIndex(intent.recipient, context));
+      return;
+    case "targetPlayer":
+      writer.writeUint16(playerIdToIndex(intent.target, context));
+      return;
+    case "emoji":
+      writer.writeUint16(playerIdToIndex(intent.recipient, context));
+      writer.writeUint16(intent.emoji);
+      return;
+    case "donate_gold":
+      writer.writeUint16(playerIdToIndex(intent.recipient, context));
+      if (intent.gold === null) {
+        writer.writeBoolean(false);
+      } else {
+        writer.writeBoolean(true);
+        writer.writeFloat64(intent.gold);
+      }
+      return;
+    case "donate_troops":
+      writer.writeUint16(playerIdToIndex(intent.recipient, context));
+      if (intent.troops === null) {
+        writer.writeBoolean(false);
+      } else {
+        writer.writeBoolean(true);
+        writer.writeFloat64(intent.troops);
+      }
+      return;
+    case "build_unit":
+      writer.writeUint8(unitTypeToOpcode(intent.unit));
+      writer.writeUint32(intent.tile);
+      return;
+    case "upgrade_structure":
+      writer.writeUint8(unitTypeToOpcode(intent.unit));
+      writer.writeUint32(intent.unitId);
+      return;
+    case "embargo":
+      writer.writeUint16(playerIdToIndex(intent.targetID, context));
+      writer.writeBoolean(intent.action === "start");
+      return;
+    case "embargo_all":
+      writer.writeBoolean(intent.action === "start");
+      return;
+    case "move_warship":
+      writer.writeUint32(intent.unitId);
+      writer.writeUint32(intent.tile);
+      return;
+    case "quick_chat":
+      writer.writeUint16(playerIdToIndex(intent.recipient, context));
+      writer.writeString(intent.quickChatKey);
+      if (intent.target !== undefined) {
+        writer.writeUint16(playerIdToIndex(intent.target, context));
+      }
+      return;
+    case "allianceExtension":
+      writer.writeUint16(playerIdToIndex(intent.recipient, context));
+      return;
+    case "delete_unit":
+      writer.writeUint32(intent.unitId);
+      return;
+    case "toggle_pause":
+      writer.writeBoolean(intent.paused);
+      return;
+    case "kick_player":
+    case "update_game_config":
+      throw new Error(`Unsupported binary intent type: ${intent.type}`);
+  }
+}
+
+function decodeIntent(
+  reader: BinaryReader,
+  context: BinaryProtocolContext,
+): Intent {
+  const intentType = opcodeToIntentType(reader.readUint8());
+  const flags = reader.readUint16();
+
+  switch (intentType) {
+    case "attack": {
+      assertIntentFlags(
+        intentType,
+        flags,
+        INTENT_FLAG_OPTION_A | INTENT_FLAG_OPTION_B,
+      );
+      const hasTarget = (flags & INTENT_FLAG_OPTION_A) !== 0;
+      const hasTroops = (flags & INTENT_FLAG_OPTION_B) !== 0;
+      return {
+        type: "attack",
+        targetID: hasTarget
+          ? requireClientId(reader.readUint16(), context)
+          : null,
+        troops: hasTroops ? reader.readFloat64() : null,
+      };
+    }
+    case "cancel_attack":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "cancel_attack",
+        attackID: reader.readString(),
+      };
+    case "spawn":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "spawn",
+        tile: reader.readUint32(),
+      };
+    case "mark_disconnected":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "mark_disconnected",
+        clientID: requireClientId(reader.readUint16(), context),
+        isDisconnected: reader.readBoolean(),
+      };
+    case "boat":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "boat",
+        troops: reader.readFloat64(),
+        dst: reader.readUint32(),
+      };
+    case "cancel_boat":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "cancel_boat",
+        unitID: reader.readUint32(),
+      };
+    case "allianceRequest":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "allianceRequest",
+        recipient: requireClientId(reader.readUint16(), context),
+      };
+    case "allianceReject":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "allianceReject",
+        requestor: requireClientId(reader.readUint16(), context),
+      };
+    case "breakAlliance":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "breakAlliance",
+        recipient: requireClientId(reader.readUint16(), context),
+      };
+    case "targetPlayer":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "targetPlayer",
+        target: requireClientId(reader.readUint16(), context),
+      };
+    case "emoji": {
+      assertIntentFlags(intentType, flags, 0);
+      const recipient = playerIndexToId(reader.readUint16(), context);
+      if (recipient === null) {
+        throw new Error("Emoji recipient cannot be null");
+      }
+      return {
+        type: "emoji",
+        recipient,
+        emoji: reader.readUint16(),
+      };
+    }
+    case "donate_gold": {
+      assertIntentFlags(intentType, flags, 0);
+      const recipient = requireClientId(reader.readUint16(), context);
+      const hasGold = reader.readBoolean();
+      return {
+        type: "donate_gold",
+        recipient,
+        gold: hasGold ? reader.readFloat64() : null,
+      };
+    }
+    case "donate_troops": {
+      assertIntentFlags(intentType, flags, 0);
+      const recipient = requireClientId(reader.readUint16(), context);
+      const hasTroops = reader.readBoolean();
+      return {
+        type: "donate_troops",
+        recipient,
+        troops: hasTroops ? reader.readFloat64() : null,
+      };
+    }
+    case "build_unit": {
+      assertIntentFlags(
+        intentType,
+        flags,
+        INTENT_FLAG_OPTION_A | INTENT_FLAG_OPTION_B,
+      );
+      const unit = decodeUnit(reader.readUint8());
+      const tile = reader.readUint32();
+      const hasRocketDirection = (flags & INTENT_FLAG_OPTION_A) !== 0;
+      const rocketDirectionUp =
+        hasRocketDirection && (flags & INTENT_FLAG_OPTION_B) !== 0;
+      return {
+        type: "build_unit",
+        unit,
+        tile,
+        rocketDirectionUp: hasRocketDirection ? rocketDirectionUp : undefined,
+      };
+    }
+    case "upgrade_structure":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "upgrade_structure",
+        unit: decodeUnit(reader.readUint8()),
+        unitId: reader.readUint32(),
+      };
+    case "embargo":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "embargo",
+        targetID: requireClientId(reader.readUint16(), context),
+        action: reader.readBoolean() ? "start" : "stop",
+      };
+    case "embargo_all":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "embargo_all",
+        action: reader.readBoolean() ? "start" : "stop",
+      };
+    case "move_warship":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "move_warship",
+        unitId: reader.readUint32(),
+        tile: reader.readUint32(),
+      };
+    case "quick_chat": {
+      assertIntentFlags(intentType, flags, INTENT_FLAG_OPTION_A);
+      const recipient = requireClientId(reader.readUint16(), context);
+      const quickChatKey = reader.readString();
+      const target =
+        (flags & INTENT_FLAG_OPTION_A) !== 0
+          ? requireClientId(reader.readUint16(), context)
+          : undefined;
+      if (!QuickChatKeySchema.safeParse(quickChatKey).success) {
+        throw new Error(`Invalid quick chat key: ${quickChatKey}`);
+      }
+      return {
+        type: "quick_chat",
+        recipient,
+        quickChatKey,
+        target,
+      };
+    }
+    case "allianceExtension":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "allianceExtension",
+        recipient: requireClientId(reader.readUint16(), context),
+      };
+    case "delete_unit":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "delete_unit",
+        unitId: reader.readUint32(),
+      };
+    case "toggle_pause":
+      assertIntentFlags(intentType, flags, 0);
+      return {
+        type: "toggle_pause",
+        paused: reader.readBoolean(),
+      };
+  }
+
+  throw new Error(`Unhandled binary intent type: ${intentType}`);
+}
+
+function decodeUnit(opcode: number): UnitType {
+  return opcodeToUnitType(opcode);
+}
+
+function assertIntentFlags(
+  intentType: Intent["type"],
+  flags: number,
+  allowedFlags: number,
+) {
+  const invalidFlags = flags & ~allowedFlags;
+  if (invalidFlags !== 0) {
+    throw new Error(
+      `Unsupported flags ${invalidFlags} for binary intent type ${intentType}`,
+    );
+  }
+}
+
+export function isBinaryGameplayClientMessage(
+  message: ClientMessage,
+): message is BinaryClientGameplayMessage {
+  return (
+    message.type === "intent" ||
+    message.type === "hash" ||
+    message.type === "ping"
+  );
+}
diff --git a/src/core/BinaryProtocol.ts b/src/core/BinaryProtocol.ts
new file mode 100644
index 000000000..eab80c246
--- /dev/null
+++ b/src/core/BinaryProtocol.ts
@@ -0,0 +1,235 @@
+import {
+  ClientHashMessage,
+  ClientID,
+  ClientIntentMessage,
+  ClientPingMessage,
+  GameStartInfo,
+  Intent,
+  ServerDesyncMessage,
+  ServerTurnMessage,
+  StampedIntent,
+} from "./Schemas";
+import { AllPlayers, UnitType } from "./game/Game";
+
+export const BINARY_PROTOCOL_VERSION = 1;
+
+export const BINARY_HEADER_SIZE = 4;
+export const NO_PLAYER_INDEX = 0xffff;
+export const ALL_PLAYERS_INDEX = 0xfffe;
+
+export enum BinaryMessageType {
+  Intent = 1,
+  Turn = 2,
+  Hash = 3,
+  Ping = 4,
+  Desync = 5,
+}
+
+export enum BinaryIntentType {
+  Attack = 1,
+  CancelAttack = 2,
+  Spawn = 3,
+  MarkDisconnected = 4,
+  BoatAttack = 5,
+  CancelBoat = 6,
+  AllianceRequest = 7,
+  AllianceReject = 8,
+  BreakAlliance = 9,
+  TargetPlayer = 10,
+  Emoji = 11,
+  DonateGold = 12,
+  DonateTroops = 13,
+  BuildUnit = 14,
+  UpgradeStructure = 15,
+  Embargo = 16,
+  EmbargoAll = 17,
+  MoveWarship = 18,
+  QuickChat = 19,
+  AllianceExtension = 20,
+  DeleteUnit = 21,
+  TogglePause = 22,
+}
+
+export const INTENT_FLAG_OPTION_A = 1 << 0;
+export const INTENT_FLAG_OPTION_B = 1 << 1;
+
+export type BinaryClientGameplayMessage =
+  | ClientIntentMessage
+  | ClientHashMessage
+  | ClientPingMessage;
+
+export type BinaryServerGameplayMessage =
+  | ServerTurnMessage
+  | ServerDesyncMessage;
+
+const INTENT_TYPE_TO_OPCODE: Record<
+  Intent["type"],
+  BinaryIntentType | undefined
+> = {
+  attack: BinaryIntentType.Attack,
+  cancel_attack: BinaryIntentType.CancelAttack,
+  spawn: BinaryIntentType.Spawn,
+  mark_disconnected: BinaryIntentType.MarkDisconnected,
+  boat: BinaryIntentType.BoatAttack,
+  cancel_boat: BinaryIntentType.CancelBoat,
+  allianceRequest: BinaryIntentType.AllianceRequest,
+  allianceReject: BinaryIntentType.AllianceReject,
+  breakAlliance: BinaryIntentType.BreakAlliance,
+  targetPlayer: BinaryIntentType.TargetPlayer,
+  emoji: BinaryIntentType.Emoji,
+  donate_gold: BinaryIntentType.DonateGold,
+  donate_troops: BinaryIntentType.DonateTroops,
+  build_unit: BinaryIntentType.BuildUnit,
+  upgrade_structure: BinaryIntentType.UpgradeStructure,
+  embargo: BinaryIntentType.Embargo,
+  embargo_all: BinaryIntentType.EmbargoAll,
+  move_warship: BinaryIntentType.MoveWarship,
+  quick_chat: BinaryIntentType.QuickChat,
+  allianceExtension: BinaryIntentType.AllianceExtension,
+  delete_unit: BinaryIntentType.DeleteUnit,
+  kick_player: undefined,
+  toggle_pause: BinaryIntentType.TogglePause,
+  update_game_config: undefined,
+};
+
+const OPCODE_TO_INTENT_TYPE: Record<BinaryIntentType, Intent["type"]> = {
+  [BinaryIntentType.Attack]: "attack",
+  [BinaryIntentType.CancelAttack]: "cancel_attack",
+  [BinaryIntentType.Spawn]: "spawn",
+  [BinaryIntentType.MarkDisconnected]: "mark_disconnected",
+  [BinaryIntentType.BoatAttack]: "boat",
+  [BinaryIntentType.CancelBoat]: "cancel_boat",
+  [BinaryIntentType.AllianceRequest]: "allianceRequest",
+  [BinaryIntentType.AllianceReject]: "allianceReject",
+  [BinaryIntentType.BreakAlliance]: "breakAlliance",
+  [BinaryIntentType.TargetPlayer]: "targetPlayer",
+  [BinaryIntentType.Emoji]: "emoji",
+  [BinaryIntentType.DonateGold]: "donate_gold",
+  [BinaryIntentType.DonateTroops]: "donate_troops",
+  [BinaryIntentType.BuildUnit]: "build_unit",
+  [BinaryIntentType.UpgradeStructure]: "upgrade_structure",
+  [BinaryIntentType.Embargo]: "embargo",
+  [BinaryIntentType.EmbargoAll]: "embargo_all",
+  [BinaryIntentType.MoveWarship]: "move_warship",
+  [BinaryIntentType.QuickChat]: "quick_chat",
+  [BinaryIntentType.AllianceExtension]: "allianceExtension",
+  [BinaryIntentType.DeleteUnit]: "delete_unit",
+  [BinaryIntentType.TogglePause]: "toggle_pause",
+};
+
+const UNIT_TYPE_TO_OPCODE = new Map<UnitType, number>(
+  Object.values(UnitType).map((type, index) => [type, index + 1]),
+);
+
+const OPCODE_TO_UNIT_TYPE = new Map<number, UnitType>(
+  Object.values(UnitType).map((type, index) => [index + 1, type]),
+);
+
+export interface BinaryProtocolContext {
+  readonly playerIds: readonly ClientID[];
+  readonly playerIdToIndex: ReadonlyMap<ClientID, number>;
+}
+
+export function createBinaryProtocolContext(
+  gameStartInfo: Pick<GameStartInfo, "players">,
+): BinaryProtocolContext {
+  const playerIds = gameStartInfo.players.map((player) => player.clientID);
+  const playerIdToIndex = new Map<ClientID, number>();
+  playerIds.forEach((clientID, index) => {
+    playerIdToIndex.set(clientID, index);
+  });
+  return {
+    playerIds,
+    playerIdToIndex,
+  };
+}
+
+export function intentTypeToOpcode(
+  intentType: Intent["type"],
+): BinaryIntentType {
+  const opcode = INTENT_TYPE_TO_OPCODE[intentType];
+  if (opcode === undefined) {
+    throw new Error(`Unsupported binary intent type: ${intentType}`);
+  }
+  return opcode;
+}
+
+export function opcodeToIntentType(opcode: number): Intent["type"] {
+  const intentType = OPCODE_TO_INTENT_TYPE[opcode as BinaryIntentType];
+  if (intentType === undefined) {
+    throw new Error(`Unknown binary intent opcode: ${opcode}`);
+  }
+  return intentType;
+}
+
+export function unitTypeToOpcode(unitType: UnitType): number {
+  const opcode = UNIT_TYPE_TO_OPCODE.get(unitType);
+  if (opcode === undefined) {
+    throw new Error(`Unknown unit type: ${unitType}`);
+  }
+  return opcode;
+}
+
+export function opcodeToUnitType(opcode: number): UnitType {
+  const unitType = OPCODE_TO_UNIT_TYPE.get(opcode);
+  if (unitType === undefined) {
+    throw new Error(`Unknown unit opcode: ${opcode}`);
+  }
+  return unitType;
+}
+
+export function playerIdToIndex(
+  playerId: ClientID | null | typeof AllPlayers,
+  context: BinaryProtocolContext,
+): number {
+  if (playerId === null) {
+    return NO_PLAYER_INDEX;
+  }
+  if (playerId === AllPlayers) {
+    return ALL_PLAYERS_INDEX;
+  }
+  const index = context.playerIdToIndex.get(playerId);
+  if (index === undefined) {
+    throw new Error(`Unknown player ID: ${playerId}`);
+  }
+  return index;
+}
+
+export function playerIndexToId(
+  playerIndex: number,
+  context: BinaryProtocolContext,
+): ClientID | null | typeof AllPlayers {
+  if (playerIndex === NO_PLAYER_INDEX) {
+    return null;
+  }
+  if (playerIndex === ALL_PLAYERS_INDEX) {
+    return AllPlayers;
+  }
+  const playerId = context.playerIds[playerIndex];
+  if (playerId === undefined) {
+    throw new Error(`Invalid player index: ${playerIndex}`);
+  }
+  return playerId;
+}
+
+export function requireClientId(
+  playerIndex: number,
+  context: BinaryProtocolContext,
+): ClientID {
+  const playerId = playerIndexToId(playerIndex, context);
+  if (playerId === null || playerId === AllPlayers) {
+    throw new Error(`Expected client player index, received ${playerIndex}`);
+  }
+  return playerId;
+}
+
+export function stampedIntentClientIndex(
+  intent: Pick<StampedIntent, "clientID">,
+  context: BinaryProtocolContext,
+): number {
+  const index = context.playerIdToIndex.get(intent.clientID);
+  if (index === undefined) {
+    throw new Error(`Unknown stamped client ID: ${intent.clientID}`);
+  }
+  return index;
+}