diff --git a/packages/shared/src/index.ts b/packages/shared/src/index.ts
index ff5f988..7dc79f5 100644
--- a/packages/shared/src/index.ts
+++ b/packages/shared/src/index.ts
@@ -1,2 +1,3 @@
 export * from "./constants.js";
 export * from "./schemas/game.js";
+export * from "./schemas/lobby.js";
diff --git a/packages/shared/src/schemas/lobby.ts b/packages/shared/src/schemas/lobby.ts
index 93d5563..1e92a6f 100644
--- a/packages/shared/src/schemas/lobby.ts
+++ b/packages/shared/src/schemas/lobby.ts
@@ -1,6 +1,7 @@
 import * as z from "zod";
 
 import { LOBBY_STATUSES } from "../constants.js";
+import { GameQuestionSchema } from "./game.js";
 
 export const LobbyPlayerSchema = z.object({
   lobbyId: z.uuid(),
@@ -19,4 +20,103 @@ export const LobbySchema = z.object({
   createdAt: z.iso.datetime(),
   players: z.array(LobbyPlayerSchema),
 });
+
 export type Lobby = z.infer<typeof LobbySchema>;
+
+export const JoinLobbyResponseSchema = LobbySchema;
+
+export type JoinLobbyResponse = z.infer<typeof JoinLobbyResponseSchema>;
+
+// ----------------------------------------------------------------------------
+// WebSocket: Client → Server
+// ----------------------------------------------------------------------------
+
+export const WsLobbyJoinSchema = z.object({
+  type: z.literal("lobby:join"),
+  code: z.string().min(1).max(10),
+});
+
+export type WsLobbyJoin = z.infer<typeof WsLobbyJoinSchema>;
+
+export const WsLobbyLeaveSchema = z.object({
+  type: z.literal("lobby:leave"),
+  lobbyId: z.uuid(),
+});
+
+export type WsLobbyLeave = z.infer<typeof WsLobbyLeaveSchema>;
+
+export const WsLobbyStartSchema = z.object({
+  type: z.literal("lobby:start"),
+  lobbyId: z.uuid(),
+});
+
+export type WsLobbyStart = z.infer<typeof WsLobbyStartSchema>;
+
+export const WsGameAnswerSchema = z.object({
+  type: z.literal("game:answer"),
+  lobbyId: z.uuid(),
+  questionId: z.uuid(),
+  selectedOptionId: z.number().int().min(0).max(3),
+});
+
+export type WsGameAnswer = z.infer<typeof WsGameAnswerSchema>;
+
+export const WsClientMessageSchema = z.discriminatedUnion("type", [
+  WsLobbyJoinSchema,
+  WsLobbyLeaveSchema,
+  WsLobbyStartSchema,
+  WsGameAnswerSchema,
+]);
+export type WsClientMessage = z.infer<typeof WsClientMessageSchema>;
+
+// ----------------------------------------------------------------------------
+// WebSocket: Server → Client
+// ----------------------------------------------------------------------------
+
+export const WsLobbyStateSchema = z.object({
+  type: z.literal("lobby:state"),
+  lobby: LobbySchema,
+});
+
+export type WsLobbyState = z.infer<typeof WsLobbyStateSchema>;
+
+export const WsGameQuestionSchema = z.object({
+  type: z.literal("game:question"),
+  question: GameQuestionSchema,
+  questionNumber: z.number().int().min(1),
+  totalQuestions: z.number().int().min(1),
+});
+
+export type WsGameQuestion = z.infer<typeof WsGameQuestionSchema>;
+
+export const WsGameAnswerResultSchema = z.object({
+  type: z.literal("game:answer_result"),
+  correctOptionId: z.number().int().min(0).max(3),
+  results: z.array(
+    z.object({
+      userId: z.string(),
+      selectedOptionId: z.number().int().min(0).max(3).nullable(),
+      isCorrect: z.boolean(),
+    }),
+  ),
+  players: z.array(LobbyPlayerSchema),
+});
+
+export type WsGameAnswerResult = z.infer<typeof WsGameAnswerResultSchema>;
+
+export const WsGameFinishedSchema = z.object({
+  type: z.literal("game:finished"),
+  players: z.array(LobbyPlayerSchema),
+  winnerIds: z.array(z.string()),
+});
+
+export type WsGameFinished = z.infer<typeof WsGameFinishedSchema>;
+
+export const WsServerMessageSchema = z.discriminatedUnion("type", [
+  WsLobbyStateSchema,
+  WsGameQuestionSchema,
+  WsGameAnswerResultSchema,
+  WsGameFinishedSchema,
+]);
+
+export type WsServerMessage = z.infer<typeof WsServerMessageSchema>;