diff --git a/src/mcp/client/streamable_http.py b/src/mcp/client/streamable_http.py
index aa3e50e0..9cdf717c 100644
--- a/src/mcp/client/streamable_http.py
+++ b/src/mcp/client/streamable_http.py
@@ -120,7 +120,7 @@ class StreamableHTTPTransport:
             try:
                 # Parse the result as InitializeResult for type safety
                 init_result = InitializeResult.model_validate(message.result, by_name=False)
-                self.protocol_version = str(init_result.protocol_version)
+                self.protocol_version = init_result.protocol_version
                 logger.info(f"Negotiated protocol version: {self.protocol_version}")
             except Exception:  # pragma: no cover
                 logger.warning("Failed to parse initialization response as InitializeResult", exc_info=True)
diff --git a/src/mcp/types/_types.py b/src/mcp/types/_types.py
index 34800ba1..e9d39ef6 100644
--- a/src/mcp/types/_types.py
+++ b/src/mcp/types/_types.py
@@ -304,7 +304,7 @@ class ServerCapabilities(MCPModel):
 class InitializeRequestParams(RequestParams):
     """Parameters for the initialize request."""
 
-    protocol_version: str | int
+    protocol_version: str
     """The latest version of the Model Context Protocol that the client supports."""
     capabilities: ClientCapabilities
     client_info: Implementation
@@ -322,7 +322,7 @@ class InitializeRequest(Request[InitializeRequestParams, Literal["initialize"]])
 class InitializeResult(Result):
     """After receiving an initialize request from the client, the server sends this."""
 
-    protocol_version: str | int
+    protocol_version: str
     """The version of the Model Context Protocol that the server wants to use."""
     capabilities: ServerCapabilities
     server_info: Implementation