Ability to set custom protocol version (#117)

src/Schema/Enum/ProtocolVersion.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Schema\Enum;
+
+/**
+ * Available protocol versions for MCP.
+ */
+enum ProtocolVersion: string
+{
+    case V2024_11_05 = '2024-11-05';
+
+    case V2025_03_26 = '2025-03-26';
+
+    case V2025_06_18 = '2025-06-18';
+}

src/Schema/JsonRpc/MessageInterface.php

@@ -11,6 +11,8 @@
 
 namespace Mcp\Schema\JsonRpc;
 
+use Mcp\Schema\Enum\ProtocolVersion;
+
 /**
  * Refers to any valid JSON-RPC object that can be decoded off the wire, or encoded to be sent.
  *
@@ -19,5 +21,5 @@ namespace Mcp\Schema\JsonRpc;
 interface MessageInterface extends \JsonSerializable
 {
     public const JSONRPC_VERSION = '2.0';
-    public const PROTOCOL_VERSION = '2025-06-18';
+    public const PROTOCOL_VERSION = ProtocolVersion::V2025_06_18;
 }

src/Schema/Result/InitializeResult.php

@@ -12,6 +12,7 @@
 namespace Mcp\Schema\Result;
 
 use Mcp\Exception\InvalidArgumentException;
+use Mcp\Schema\Enum\ProtocolVersion;
 use Mcp\Schema\Implementation;
 use Mcp\Schema\JsonRpc\MessageInterface;
 use Mcp\Schema\JsonRpc\Response;
@@ -38,6 +39,7 @@ class InitializeResult implements ResultInterface
         public readonly Implementation $serverInfo,
         public readonly ?string $instructions = null,
         public readonly ?array $_meta = null,
+        public readonly ?ProtocolVersion $protocolVersion = null,
     ) {
     }
 
@@ -66,7 +68,8 @@ class InitializeResult implements ResultInterface
             ServerCapabilities::fromArray($data['capabilities']),
             Implementation::fromArray($data['serverInfo']),
             $data['instructions'] ?? null,
-            $data['_meta'] ?? null
+            $data['_meta'] ?? null,
+            ProtocolVersion::tryFrom($data['protocolVersion']),
         );
     }
 
@@ -81,8 +84,9 @@ class InitializeResult implements ResultInterface
      */
     public function jsonSerialize(): array
     {
+        $protocolVersion = $this->protocolVersion ?? MessageInterface::PROTOCOL_VERSION;
         $data = [
-            'protocolVersion' => MessageInterface::PROTOCOL_VERSION,
+            'protocolVersion' => $protocolVersion->value,
             'capabilities' => $this->capabilities,
             'serverInfo' => $this->serverInfo,
         ];