feat: update tests and add protocol get and set to transport

Author: mattzcarey

src/server/streamableHttp.ts

@@ -184,6 +184,7 @@ export class StreamableHTTPServerTransport implements Transport {
     private _allowedOrigins?: string[];
     private _enableDnsRebindingProtection: boolean;
     private _retryInterval?: number;
+    private _protocolVersion?: string;
 
     sessionId?: string;
     onclose?: () => void;
@@ -213,6 +214,21 @@ export class StreamableHTTPServerTransport implements Transport {
         this._started = true;
     }
 
+    /**
+     * Sets the protocol version after negotiation during initialization.
+     * This is called by the Server class after the initialize handshake completes.
+     */
+    setProtocolVersion(version: string): void {
+        this._protocolVersion = version;
+    }
+
+    /**
+     * Gets the negotiated protocol version, if set.
+     */
+    get protocolVersion(): string | undefined {
+        return this._protocolVersion;
+    }
+
     /**
      * Validates request headers for DNS rebinding protection.
      * @returns Error message if validation fails, undefined if validation passes.
@@ -794,19 +810,32 @@ export class StreamableHTTPServerTransport implements Transport {
         return true;
     }
 
+    /**
+     * Validates the MCP-Protocol-Version header on incoming requests.
+     *
+     * For initialization: Version negotiation handles unknown versions gracefully
+     * (server responds with its supported version).
+     *
+     * For subsequent requests with MCP-Protocol-Version header:
+     * - Accept if in supported list
+     * - 400 if unsupported
+     *
+     * For HTTP requests without the MCP-Protocol-Version header:
+     * - Accept and default to the version negotiated at initialization
+     */
     private validateProtocolVersion(req: IncomingMessage, res: ServerResponse): boolean {
-        let protocolVersion = req.headers['mcp-protocol-version'] ?? DEFAULT_NEGOTIATED_PROTOCOL_VERSION;
+        let protocolVersion = req.headers['mcp-protocol-version'];
         if (Array.isArray(protocolVersion)) {
             protocolVersion = protocolVersion[protocolVersion.length - 1];
         }
 
-        if (!SUPPORTED_PROTOCOL_VERSIONS.includes(protocolVersion)) {
+        if (protocolVersion !== undefined && !SUPPORTED_PROTOCOL_VERSIONS.includes(protocolVersion)) {
             res.writeHead(400).end(
                 JSON.stringify({
                     jsonrpc: '2.0',
                     error: {
                         code: -32000,
-                        message: `Bad Request: Unsupported protocol version (supported versions: ${SUPPORTED_PROTOCOL_VERSIONS.join(', ')})`
+                        message: `Bad Request: Unsupported protocol version: ${protocolVersion} (supported versions: ${SUPPORTED_PROTOCOL_VERSIONS.join(', ')})`
                     },
                     id: null
                 })