diff --git a/frontend/src/socket/types.spec.ts b/frontend/src/socket/types.spec.ts
new file mode 100644
index 0000000..3411837
--- /dev/null
+++ b/frontend/src/socket/types.spec.ts
@@ -0,0 +1,367 @@
+/**
+ * Tests for WebSocket message types and factory functions.
+ *
+ * Verifies message creation, structure, and unique ID generation.
+ */
+
+import { describe, it, expect } from 'vitest'
+
+import {
+  generateMessageId,
+  createJoinGameMessage,
+  createActionMessage,
+  createResignMessage,
+  createHeartbeatMessage,
+} from './types'
+import type { GameAction } from './types'
+
+describe('Socket Message Types', () => {
+  describe('generateMessageId', () => {
+    it('generates a unique message ID', () => {
+      /**
+       * Test that generateMessageId returns a UUID.
+       *
+       * Message IDs must be unique for tracking and deduplication.
+       */
+      const id = generateMessageId()
+
+      expect(id).toBeTruthy()
+      expect(typeof id).toBe('string')
+      // UUID format: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
+      expect(id).toMatch(/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i)
+    })
+
+    it('generates different IDs on successive calls', () => {
+      /**
+       * Test that generateMessageId produces unique values.
+       *
+       * Each message must have a unique ID for proper tracking.
+       */
+      const id1 = generateMessageId()
+      const id2 = generateMessageId()
+      const id3 = generateMessageId()
+
+      expect(id1).not.toBe(id2)
+      expect(id2).not.toBe(id3)
+      expect(id1).not.toBe(id3)
+    })
+  })
+
+  describe('createJoinGameMessage', () => {
+    it('creates a join game message with game ID', () => {
+      /**
+       * Test createJoinGameMessage basic structure.
+       *
+       * Join messages must include type, message_id, and game_id.
+       */
+      const gameId = 'game-123'
+
+      const message = createJoinGameMessage(gameId)
+
+      expect(message.type).toBe('join_game')
+      expect(message.game_id).toBe(gameId)
+      expect(message.message_id).toBeTruthy()
+      expect(message.message_id).toMatch(/^[0-9a-f-]+$/i)
+    })
+
+    it('includes last_event_id when provided', () => {
+      /**
+       * Test createJoinGameMessage with last_event_id for reconnection.
+       *
+       * When reconnecting, clients send the last event ID they received
+       * to resume from that point.
+       */
+      const gameId = 'game-123'
+      const lastEventId = 'event-456'
+
+      const message = createJoinGameMessage(gameId, lastEventId)
+
+      expect(message.type).toBe('join_game')
+      expect(message.game_id).toBe(gameId)
+      expect(message.last_event_id).toBe(lastEventId)
+    })
+
+    it('omits last_event_id when not provided', () => {
+      /**
+       * Test createJoinGameMessage without last_event_id for new connections.
+       *
+       * Initial joins don't have a last_event_id.
+       */
+      const gameId = 'game-123'
+
+      const message = createJoinGameMessage(gameId)
+
+      expect(message.last_event_id).toBeUndefined()
+    })
+
+    it('generates unique message IDs', () => {
+      /**
+       * Test that each join message has a unique ID.
+       *
+       * Message IDs must be unique even for the same game.
+       */
+      const gameId = 'game-123'
+
+      const message1 = createJoinGameMessage(gameId)
+      const message2 = createJoinGameMessage(gameId)
+
+      expect(message1.message_id).not.toBe(message2.message_id)
+    })
+  })
+
+  describe('createActionMessage', () => {
+    it('creates an action message with game ID and action', () => {
+      /**
+       * Test createActionMessage basic structure.
+       *
+       * Action messages must include type, message_id, game_id, and action.
+       */
+      const gameId = 'game-123'
+      const action: GameAction = {
+        action_type: 'play_card',
+        card_id: 'card-456',
+      }
+
+      const message = createActionMessage(gameId, action)
+
+      expect(message.type).toBe('action')
+      expect(message.game_id).toBe(gameId)
+      expect(message.action).toEqual(action)
+      expect(message.message_id).toBeTruthy()
+    })
+
+    it('preserves action data structure', () => {
+      /**
+       * Test that action data is preserved without modification.
+       *
+       * The action object should be included as-is in the message.
+       */
+      const gameId = 'game-123'
+      const action: GameAction = {
+        action_type: 'attack',
+        attacker_id: 'attacker-1',
+        defender_id: 'defender-2',
+        attack_index: 0,
+      }
+
+      const message = createActionMessage(gameId, action)
+
+      expect(message.action).toEqual(action)
+      expect(message.action.action_type).toBe('attack')
+      expect(message.action.attacker_id).toBe('attacker-1')
+      expect(message.action.defender_id).toBe('defender-2')
+      expect(message.action.attack_index).toBe(0)
+    })
+
+    it('handles different action types', () => {
+      /**
+       * Test createActionMessage with various action types.
+       *
+       * Different game actions should all be properly wrapped.
+       */
+      const gameId = 'game-123'
+
+      const playCardMessage = createActionMessage(gameId, {
+        action_type: 'play_card',
+        card_id: 'card-1',
+      })
+
+      const attachEnergyMessage = createActionMessage(gameId, {
+        action_type: 'attach_energy',
+        card_id: 'energy-1',
+        target_id: 'pokemon-1',
+      })
+
+      const endTurnMessage = createActionMessage(gameId, {
+        action_type: 'end_turn',
+      })
+
+      expect(playCardMessage.action.action_type).toBe('play_card')
+      expect(attachEnergyMessage.action.action_type).toBe('attach_energy')
+      expect(endTurnMessage.action.action_type).toBe('end_turn')
+    })
+
+    it('generates unique message IDs for different actions', () => {
+      /**
+       * Test that each action message has a unique ID.
+       *
+       * Even identical actions should have different message IDs.
+       */
+      const gameId = 'game-123'
+      const action: GameAction = { action_type: 'end_turn' }
+
+      const message1 = createActionMessage(gameId, action)
+      const message2 = createActionMessage(gameId, action)
+
+      expect(message1.message_id).not.toBe(message2.message_id)
+    })
+  })
+
+  describe('createResignMessage', () => {
+    it('creates a resign message with game ID', () => {
+      /**
+       * Test createResignMessage basic structure.
+       *
+       * Resign messages must include type, message_id, and game_id.
+       */
+      const gameId = 'game-123'
+
+      const message = createResignMessage(gameId)
+
+      expect(message.type).toBe('resign')
+      expect(message.game_id).toBe(gameId)
+      expect(message.message_id).toBeTruthy()
+    })
+
+    it('generates unique message IDs', () => {
+      /**
+       * Test that each resign message has a unique ID.
+       *
+       * Message IDs must be unique even for the same game.
+       */
+      const gameId = 'game-123'
+
+      const message1 = createResignMessage(gameId)
+      const message2 = createResignMessage(gameId)
+
+      expect(message1.message_id).not.toBe(message2.message_id)
+    })
+
+    it('includes only required fields', () => {
+      /**
+       * Test that resign messages don't include extra fields.
+       *
+       * Resign is simple - just type, message_id, and game_id.
+       */
+      const gameId = 'game-123'
+
+      const message = createResignMessage(gameId)
+
+      const keys = Object.keys(message)
+      expect(keys).toContain('type')
+      expect(keys).toContain('message_id')
+      expect(keys).toContain('game_id')
+      expect(keys.length).toBe(3)
+    })
+  })
+
+  describe('createHeartbeatMessage', () => {
+    it('creates a heartbeat message', () => {
+      /**
+       * Test createHeartbeatMessage basic structure.
+       *
+       * Heartbeat messages must include type and message_id.
+       */
+      const message = createHeartbeatMessage()
+
+      expect(message.type).toBe('heartbeat')
+      expect(message.message_id).toBeTruthy()
+    })
+
+    it('does not include game_id', () => {
+      /**
+       * Test that heartbeat messages are game-independent.
+       *
+       * Heartbeats maintain the connection regardless of active game.
+       */
+      const message = createHeartbeatMessage()
+
+      expect('game_id' in message).toBe(false)
+    })
+
+    it('generates unique message IDs', () => {
+      /**
+       * Test that each heartbeat has a unique ID.
+       *
+       * Message IDs must be unique for tracking.
+       */
+      const message1 = createHeartbeatMessage()
+      const message2 = createHeartbeatMessage()
+
+      expect(message1.message_id).not.toBe(message2.message_id)
+    })
+
+    it('includes only required fields', () => {
+      /**
+       * Test that heartbeat messages don't include extra fields.
+       *
+       * Heartbeat is minimal - just type and message_id.
+       */
+      const message = createHeartbeatMessage()
+
+      const keys = Object.keys(message)
+      expect(keys).toContain('type')
+      expect(keys).toContain('message_id')
+      expect(keys.length).toBe(2)
+    })
+  })
+
+  describe('message factories integration', () => {
+    it('all factories generate valid UUID message IDs', () => {
+      /**
+       * Test that all message factories use proper UUID format.
+       *
+       * Consistent ID format ensures reliable message tracking.
+       */
+      const uuidRegex = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i
+
+      const joinMessage = createJoinGameMessage('game-1')
+      const actionMessage = createActionMessage('game-1', { action_type: 'end_turn' })
+      const resignMessage = createResignMessage('game-1')
+      const heartbeatMessage = createHeartbeatMessage()
+
+      expect(joinMessage.message_id).toMatch(uuidRegex)
+      expect(actionMessage.message_id).toMatch(uuidRegex)
+      expect(resignMessage.message_id).toMatch(uuidRegex)
+      expect(heartbeatMessage.message_id).toMatch(uuidRegex)
+    })
+
+    it('messages for the same game have different IDs', () => {
+      /**
+       * Test that multiple messages for one game have unique IDs.
+       *
+       * This ensures proper message tracking within a game session.
+       */
+      const gameId = 'game-123'
+
+      const message1 = createJoinGameMessage(gameId)
+      const message2 = createActionMessage(gameId, { action_type: 'end_turn' })
+      const message3 = createResignMessage(gameId)
+
+      const ids = [message1.message_id, message2.message_id, message3.message_id]
+      const uniqueIds = new Set(ids)
+
+      expect(uniqueIds.size).toBe(3) // All IDs should be unique
+    })
+
+    it('message structure matches expected types', () => {
+      /**
+       * Test that factory output matches TypeScript type definitions.
+       *
+       * This validates that the factories produce correctly-typed messages.
+       */
+      const gameId = 'game-123'
+
+      const joinMessage = createJoinGameMessage(gameId)
+      expect(joinMessage).toHaveProperty('type')
+      expect(joinMessage).toHaveProperty('message_id')
+      expect(joinMessage).toHaveProperty('game_id')
+
+      const actionMessage = createActionMessage(gameId, { action_type: 'end_turn' })
+      expect(actionMessage).toHaveProperty('type')
+      expect(actionMessage).toHaveProperty('message_id')
+      expect(actionMessage).toHaveProperty('game_id')
+      expect(actionMessage).toHaveProperty('action')
+
+      const resignMessage = createResignMessage(gameId)
+      expect(resignMessage).toHaveProperty('type')
+      expect(resignMessage).toHaveProperty('message_id')
+      expect(resignMessage).toHaveProperty('game_id')
+
+      const heartbeatMessage = createHeartbeatMessage()
+      expect(heartbeatMessage).toHaveProperty('type')
+      expect(heartbeatMessage).toHaveProperty('message_id')
+    })
+  })
+
+})