diff --git a/docs/HISTORY.md b/docs/HISTORY.md
index ed5e9d30..69faad19 100644
--- a/docs/HISTORY.md
+++ b/docs/HISTORY.md
@@ -1,5 +1,8 @@
 # History
 
+## 1.56.0
+* Add support for Minecraft 1.21.5
+
 ## 1.55.0
 * [Fix `client.end()` (#1376)](https://github.com/PrismarineJS/node-minecraft-protocol/commit/3bd4dc1b2002cd7badfa5b9cf8dda35cd6cc9ac1) (thanks @h5mcbox)
 * [Fix #1369 online-mode error 1.20.5-1.21.4 (#1375)](https://github.com/PrismarineJS/node-minecraft-protocol/commit/5ec3dd4b367fcc039fbcb3edd214fe3cf8178a6d) (thanks @h5mcbox)
diff --git a/docs/README.md b/docs/README.md
index fbcaa436..815241cb 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -13,7 +13,7 @@ Parse and serialize minecraft packets, plus authentication and encryption.
 
  * Supports Minecraft PC version 1.7.10, 1.8.8, 1.9 (15w40b, 1.9, 1.9.1-pre2, 1.9.2, 1.9.4),
   1.10 (16w20a, 1.10-pre1, 1.10, 1.10.1, 1.10.2), 1.11 (16w35a, 1.11, 1.11.2), 1.12 (17w15a, 17w18b, 1.12-pre4, 1.12, 1.12.1, 1.12.2), and 1.13 (17w50a, 1.13, 1.13.1, 1.13.2-pre1, 1.13.2-pre2, 1.13.2), 1.14 (1.14, 1.14.1, 1.14.3, 1.14.4)
-  , 1.15 (1.15, 1.15.1, 1.15.2) and 1.16 (20w13b, 20w14a, 1.16-rc1, 1.16, 1.16.1, 1.16.2, 1.16.3, 1.16.4, 1.16.5), 1.17 (21w07a, 1.17, 1.17.1), 1.18 (1.18, 1.18.1 and 1.18.2), 1.19 (1.19, 1.19.1, 1.19.2, 1.19.3, 1.19.4), 1.20 (1.20, 1.20.1, 1.20.2, 1.20.3, 1.20.4, 1.20.5, 1.20.6), 1.21 (1.21, 1.21.1, 1.21.3, 1.21.4)
+  , 1.15 (1.15, 1.15.1, 1.15.2) and 1.16 (20w13b, 20w14a, 1.16-rc1, 1.16, 1.16.1, 1.16.2, 1.16.3, 1.16.4, 1.16.5), 1.17 (21w07a, 1.17, 1.17.1), 1.18 (1.18, 1.18.1 and 1.18.2), 1.19 (1.19, 1.19.1, 1.19.2, 1.19.3, 1.19.4), 1.20 (1.20, 1.20.1, 1.20.2, 1.20.3, 1.20.4, 1.20.5, 1.20.6), 1.21 (1.21, 1.21.1, 1.21.3, 1.21.4, 1.21.5)
  * Parses all packets and emits events with packet fields as JavaScript
    objects.
  * Send a packet by supplying fields as a JavaScript object.
diff --git a/package.json b/package.json
index 687b1c8b..4403b4fe 100644
--- a/package.json
+++ b/package.json
@@ -53,7 +53,7 @@
     "endian-toggle": "^0.0.0",
     "lodash.get": "^4.1.2",
     "lodash.merge": "^4.3.0",
-    "minecraft-data": "^3.78.0",
+    "minecraft-data": "^3.85.0",
     "minecraft-folder-path": "^1.2.0",
     "node-fetch": "^2.6.1",
     "node-rsa": "^0.4.2",
diff --git a/src/client/chat.js b/src/client/chat.js
index f14269be..d74242e5 100644
--- a/src/client/chat.js
+++ b/src/client/chat.js
@@ -23,6 +23,19 @@ module.exports = function (client, options) {
   client._lastChatSignature = null
   client._lastRejectedMessage = null
 
+  // Chat packet index tracking (for 1.21.5+)
+  client._expectedChatIndex = 0
+
+  client.on('login', () => {
+    // Reset expected chat index on login
+    client._expectedChatIndex = 0
+  })
+
+  // Reset expected chat index when entering configuration state
+  client.on('finish_configuration', () => {
+    client._expectedChatIndex = 0
+  })
+
   // This stores the last n (5 or 20) messages that the player has seen, from unique players
   if (mcData.supportFeature('chainedChatWithHashing')) client._lastSeenMessages = new LastSeenMessages()
   else client._lastSeenMessages = new LastSeenMessagesWithInvalidation()
@@ -45,6 +58,23 @@ module.exports = function (client, options) {
     }
   }()
 
+  // Calculate checksum for the lastSeen message signatures (1.21.5+)
+  const calculateChecksum = (lastSeen) => {
+    if (!mcData.supportFeature('chatChecksum') || !lastSeen || lastSeen.length === 0) {
+      return 0 // Default value for backward compatibility
+    }
+
+    // Simple hash calculation from the last seen signatures
+    let checksum = 0
+    for (const msg of lastSeen) {
+      if (msg.messageSignature) {
+        // XOR the first byte of each signature for a simple checksum
+        checksum ^= msg.messageSignature[0] || 0
+      }
+    }
+    return checksum
+  }
+
   function updateAndValidateSession (uuid, message, currentSignature, index, previousMessages, salt, timestamp) {
     const player = client._players[uuid]
 
@@ -195,6 +225,16 @@ module.exports = function (client, options) {
   })
 
   client.on('player_chat', (packet) => {
+    // Validate chat packet index for 1.21.5+
+    if (mcData.supportFeature('chatIndex') && 'index' in packet) {
+      if (packet.index !== client._expectedChatIndex) {
+        client.emit('error', new Error(`Chat index mismatch. Expected: ${client._expectedChatIndex}, got: ${packet.index}`))
+        client.end('Chat message received out of order')
+        return
+      }
+      client._expectedChatIndex++
+    }
+
     if (mcData.supportFeature('useChatSessions')) {
       const tsDelta = BigInt(Date.now()) - packet.timestamp
       const expired = !packet.timestamp || tsDelta > messageExpireTime || tsDelta < 0
@@ -372,17 +412,27 @@ module.exports = function (client, options) {
       if (mcData.supportFeature('useChatSessions')) { // 1.19.3+
         const { acknowledged, acknowledgements } = getAcknowledgements()
         const canSign = client.profileKeys && client._session
-        client.write((mcData.supportFeature('seperateSignedChatCommandPacket') && canSign) ? 'chat_command_signed' : 'chat_command', {
+
+        // Prepare the packet parameters
+        const params = {
           command,
           timestamp: options.timestamp,
           salt: options.salt,
           argumentSignatures: canSign ? signaturesForCommand(command, options.timestamp, options.salt, options.preview, acknowledgements) : [],
           messageCount: client._lastSeenMessages.pending,
           acknowledged
-        })
+        }
+
+        // Add checksum for 1.21.5+
+        if (mcData.supportFeature('chatChecksum')) {
+          params.checksum = options.checksum !== undefined ? options.checksum : calculateChecksum(client._lastSeenMessages)
+        }
+
+        client.write((mcData.supportFeature('seperateSignedChatCommandPacket') && canSign) ? 'chat_command_signed' : 'chat_command', params)
         client._lastSeenMessages.pending = 0
       } else {
-        client.write('chat_command', {
+        // Prepare the packet parameters
+        const params = {
           command,
           timestamp: options.timestamp,
           salt: options.salt,
@@ -393,7 +443,14 @@ module.exports = function (client, options) {
             messageSignature: e.signature
           })),
           lastRejectedMessage: client._lastRejectedMessage
-        })
+        }
+
+        // Add checksum for 1.21.5+
+        if (mcData.supportFeature('chatChecksum')) {
+          params.checksum = options.checksum !== undefined ? options.checksum : calculateChecksum(client._lastSeenMessages)
+        }
+
+        client.write('chat_command', params)
       }
 
       return
@@ -401,21 +458,31 @@ module.exports = function (client, options) {
 
     if (mcData.supportFeature('useChatSessions')) {
       const { acknowledgements, acknowledged } = getAcknowledgements()
-      client.write('chat_message', {
+
+      // Prepare the packet parameters
+      const params = {
         message,
         timestamp: options.timestamp,
         salt: options.salt,
         signature: (client.profileKeys && client._session) ? client.signMessage(message, options.timestamp, options.salt, undefined, acknowledgements) : undefined,
         offset: client._lastSeenMessages.pending,
         acknowledged
-      })
+      }
+
+      // Add checksum for 1.21.5+
+      if (mcData.supportFeature('chatChecksum')) {
+        params.checksum = options.checksum !== undefined ? options.checksum : calculateChecksum(client._lastSeenMessages)
+      }
+
+      client.write('chat_message', params)
       client._lastSeenMessages.pending = 0
 
       return
     }
 
     if (options.skipPreview || !client.serverFeatures.chatPreview) {
-      client.write('chat_message', {
+      // Prepare the packet parameters
+      const params = {
         message,
         timestamp: options.timestamp,
         salt: options.salt,
@@ -426,7 +493,14 @@ module.exports = function (client, options) {
           messageSignature: e.signature
         })),
         lastRejectedMessage: client._lastRejectedMessage
-      })
+      }
+
+      // Add checksum for 1.21.5+
+      if (mcData.supportFeature('chatChecksum')) {
+        params.checksum = options.checksum !== undefined ? options.checksum : calculateChecksum(client._lastSeenMessages)
+      }
+
+      client.write('chat_message', params)
       client._lastSeenMessages.pending = 0
     } else if (client.serverFeatures.chatPreview) {
       client.write('chat_preview', {
diff --git a/src/client/play.js b/src/client/play.js
index 6e06dc15..bd4c9f5b 100644
--- a/src/client/play.js
+++ b/src/client/play.js
@@ -48,9 +48,13 @@ module.exports = function (client, options) {
 
     function enterConfigState (finishCb) {
       if (client.state === states.CONFIGURATION) return
-      // If we are returning to the configuration state from the play state, we ahve to acknowledge it.
+      // If we are returning to the configuration state from the play state, we have to acknowledge it.
       if (client.state === states.PLAY) {
         client.write('configuration_acknowledged', {})
+        // Reset chat index counter when returning to configuration state
+        if (client._expectedChatIndex !== undefined) {
+          client._expectedChatIndex = 0
+        }
       }
       client.state = states.CONFIGURATION
       client.on('select_known_packs', () => {
@@ -66,6 +70,11 @@ module.exports = function (client, options) {
     }
 
     function onReady () {
+      // Reset chat index on player join
+      if (mcData.supportFeature('chatIndex')) {
+        client._expectedChatIndex = 0
+      }
+
       client.emit('playerJoin')
       if (mcData.supportFeature('signedChat')) {
         if (options.disableChatSigning && client.serverFeatures.enforcesSecureChat) {
diff --git a/src/server/chat.js b/src/server/chat.js
index dc9dc3b0..ca1500a7 100644
--- a/src/server/chat.js
+++ b/src/server/chat.js
@@ -43,6 +43,19 @@ module.exports = function (client, server, options) {
 
   if (!options.generatePreview) options.generatePreview = message => message
 
+  // Chat index tracking for 1.21.5+
+  client._chatMessageIndex = 0
+
+  // Reset chat index when client enters play state
+  client.on('login', () => {
+    client._chatMessageIndex = 0
+  })
+
+  // Reset chat index when entering configuration state
+  client.on('finish_configuration', () => {
+    client._chatMessageIndex = 0
+  })
+
   function validateMessageChain (packet) {
     try {
       validateLastMessages(pending, packet.previousMessages, packet.lastRejectedMessage)
@@ -104,6 +117,16 @@ module.exports = function (client, server, options) {
     }
     lastTimestamp = packet.timestamp
 
+    // Verify checksum if supported
+    if (client.supportFeature('chatChecksum') && 'checksum' in packet) {
+      const calculatedChecksum = calculateChecksum(packet.previousMessages)
+      if (packet.checksum !== 0 && packet.checksum !== calculatedChecksum) {
+        debug('Chat checksum mismatch', packet.checksum, calculatedChecksum)
+        raise('multiplayer.disconnect.chat_validation_failed')
+        return
+      }
+    }
+
     // Checks here: 1) make sure client can chat, 2) chain/session is OK, 3) signature is OK, 4) log if expired
     if (client.settings.disabledChat) return raise('chat.disabled.options')
     if (client.supportFeature('chainedChatWithHashing')) validateMessageChain(packet) // 1.19.1
@@ -112,6 +135,17 @@ module.exports = function (client, server, options) {
     if ((BigInt(Date.now()) - packet.timestamp) > messageExpireTime) debug(client.socket.address(), 'sent expired message TS', packet.timestamp)
   })
 
+  client.on('chat_command', (packet) => {
+    // Verify checksum if supported
+    if (client.supportFeature('chatChecksum') && 'checksum' in packet) {
+      const calculatedChecksum = calculateChecksum(packet.previousMessages)
+      if (packet.checksum !== 0 && packet.checksum !== calculatedChecksum) {
+        debug('Chat command checksum mismatch', packet.checksum, calculatedChecksum)
+        raise('multiplayer.disconnect.chat_validation_failed')
+      }
+    }
+  })
+
   // Client will occasionally send a list of seen messages to the server, here we listen & check chain validity
   client.on('message_acknowledgement', (packet) => {
     if (client.supportFeature('useChatSessions')) {
@@ -170,6 +204,31 @@ module.exports = function (client, server, options) {
     }
     return true
   }
+
+  // Helper function to calculate checksum
+  function calculateChecksum (messages) {
+    if (!messages || messages.length === 0) return 0
+
+    let checksum = 0
+    for (const msg of messages) {
+      if (msg.signature) {
+        // XOR the first byte of each signature for a simple checksum
+        checksum ^= msg.signature[0] || 0
+      }
+    }
+    return checksum
+  }
+
+  // Override and extend logSentMessageFromPeer to increment index
+  const originalLogSent = client.logSentMessageFromPeer
+  client.logSentMessageFromPeer = function (chatPacket) {
+    // Include chat index in outgoing player_chat packets
+    if (client.supportFeature('chatIndex')) {
+      chatPacket.index = client._chatMessageIndex++
+    }
+
+    return originalLogSent ? originalLogSent.call(this, chatPacket) : true
+  }
 }
 
 class LastSeenMessages extends Array {
diff --git a/src/version.js b/src/version.js
index dfed3bb2..c30b95e6 100644
--- a/src/version.js
+++ b/src/version.js
@@ -2,5 +2,5 @@
 
 module.exports = {
   defaultVersion: '1.21.1',
-  supportedVersions: ['1.7', '1.8.8', '1.9.4', '1.10.2', '1.11.2', '1.12.2', '1.13.2', '1.14.4', '1.15.2', '1.16.5', '1.17.1', '1.18.2', '1.19', '1.19.2', '1.19.3', '1.19.4', '1.20', '1.20.1', '1.20.2', '1.20.4', '1.20.6', '1.21.1', '1.21.3', '1.21.4']
+  supportedVersions: ['1.7', '1.8.8', '1.9.4', '1.10.2', '1.11.2', '1.12.2', '1.13.2', '1.14.4', '1.15.2', '1.16.5', '1.17.1', '1.18.2', '1.19', '1.19.2', '1.19.3', '1.19.4', '1.20', '1.20.1', '1.20.2', '1.20.4', '1.20.6', '1.21.1', '1.21.3', '1.21.4', '1.21.5']
 }