diff --git a/src/client/index.ts b/src/client/index.ts
index 0e7d6cf4..a8fbdcee 100644
--- a/src/client/index.ts
+++ b/src/client/index.ts
@@ -166,7 +166,9 @@ export class Client<
       this._serverCapabilities = result.capabilities;
       this._serverVersion = result.serverInfo;
       // HTTP transports must set the protocol version in each header after initialization.
-      transport.protocolVersion = result.protocolVersion;
+      if (transport.setProtocolVersion) {
+        transport.setProtocolVersion(result.protocolVersion);
+      }
 
       this._instructions = result.instructions;
 
diff --git a/src/client/sse.ts b/src/client/sse.ts
index 3111cd9d..5aa99abb 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -62,7 +62,7 @@ export class SSEClientTransport implements Transport {
   private _eventSourceInit?: EventSourceInit;
   private _requestInit?: RequestInit;
   private _authProvider?: OAuthClientProvider;
-  protocolVersion?: string;
+  private _protocolVersion?: string;
 
   onclose?: () => void;
   onerror?: (error: Error) => void;
@@ -109,8 +109,8 @@ export class SSEClientTransport implements Transport {
         headers["Authorization"] = `Bearer ${tokens.access_token}`;
       }
     }
-    if (this.protocolVersion) {
-      headers["mcp-protocol-version"] = this.protocolVersion;
+    if (this._protocolVersion) {
+      headers["mcp-protocol-version"] = this._protocolVersion;
     }
 
     return headers;
@@ -255,4 +255,8 @@ export class SSEClientTransport implements Transport {
       throw error;
     }
   }
+
+  setProtocolVersion(version: string): void {
+    this._protocolVersion = version;
+  }
 }
diff --git a/src/client/streamableHttp.ts b/src/client/streamableHttp.ts
index 2a1895f5..4117bb1b 100644
--- a/src/client/streamableHttp.ts
+++ b/src/client/streamableHttp.ts
@@ -124,7 +124,7 @@ export class StreamableHTTPClientTransport implements Transport {
   private _authProvider?: OAuthClientProvider;
   private _sessionId?: string;
   private _reconnectionOptions: StreamableHTTPReconnectionOptions;
-  protocolVersion?: string;
+  private _protocolVersion?: string;
 
   onclose?: () => void;
   onerror?: (error: Error) => void;
@@ -174,8 +174,8 @@ export class StreamableHTTPClientTransport implements Transport {
     if (this._sessionId) {
       headers["mcp-session-id"] = this._sessionId;
     }
-    if (this.protocolVersion) {
-      headers["mcp-protocol-version"] = this.protocolVersion;
+    if (this._protocolVersion) {
+      headers["mcp-protocol-version"] = this._protocolVersion;
     }
 
     return new Headers(
@@ -520,4 +520,11 @@ export class StreamableHTTPClientTransport implements Transport {
       throw error;
     }
   }
+
+  setProtocolVersion(version: string): void {
+    this._protocolVersion = version;
+  }
+  get protocolVersion(): string | undefined {
+    return this._protocolVersion;
+  }
 }
diff --git a/src/server/index.ts b/src/server/index.ts
index d05c5489..caf72f9c 100644
--- a/src/server/index.ts
+++ b/src/server/index.ts
@@ -254,9 +254,6 @@ export class Server<
     const protocolVersion = SUPPORTED_PROTOCOL_VERSIONS.includes(requestedVersion)
         ? requestedVersion
         : LATEST_PROTOCOL_VERSION;
-    if (this.transport) {
-      this.transport.protocolVersion = protocolVersion;
-    }
 
     return {
       protocolVersion,
diff --git a/src/server/sse.ts b/src/server/sse.ts
index b2f85ce0..e9a4d53a 100644
--- a/src/server/sse.ts
+++ b/src/server/sse.ts
@@ -17,8 +17,6 @@ const MAXIMUM_MESSAGE_SIZE = "4mb";
 export class SSEServerTransport implements Transport {
   private _sseResponse?: ServerResponse;
   private _sessionId: string;
-
-  protocolVersion?: string;
   onclose?: () => void;
   onerror?: (error: Error) => void;
   onmessage?: (message: JSONRPCMessage, extra?: { authInfo?: AuthInfo }) => void;
diff --git a/src/server/streamableHttp.ts b/src/server/streamableHttp.ts
index 62503120..34b2ab68 100644
--- a/src/server/streamableHttp.ts
+++ b/src/server/streamableHttp.ts
@@ -111,7 +111,6 @@ export class StreamableHTTPServerTransport implements Transport {
   private _onsessioninitialized?: (sessionId: string) => void;
 
   sessionId?: string;
-  protocolVersion?: string;
   onclose?: () => void;
   onerror?: (error: Error) => void;
   onmessage?: (message: JSONRPCMessage, extra?: { authInfo?: AuthInfo }) => void;
diff --git a/src/shared/transport.ts b/src/shared/transport.ts
index 976be79d..b75e072e 100644
--- a/src/shared/transport.ts
+++ b/src/shared/transport.ts
@@ -77,9 +77,7 @@ export interface Transport {
   sessionId?: string;
 
   /**
-   * The protocol version used for the connection.
-   *
-   * Only set after the initialize response was received.
+   * Sets the protocol version used for the connection (called when the initialize response is received).
    */
-  protocolVersion?: string;
+  setProtocolVersion?: (version: string) => void;
 }