diff --git a/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs b/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
index 75126556b..312814784 100644
--- a/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
+++ b/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
@@ -24,6 +24,7 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
     /// </summary>
     private const string BearerScheme = "Bearer";
     private const string ProtectedResourceMetadataWellKnownPath = "/.well-known/oauth-protected-resource";
+    private static readonly string HttpsPlusHttpScheme = $"{Uri.UriSchemeHttps}+{Uri.UriSchemeHttp}";
 
     private readonly Uri _serverUrl;
     private readonly Uri _redirectUri;
@@ -342,10 +343,13 @@ private async Task<AuthorizationServerMetadata> GetAuthServerMetadataAsync(Uri a
                     ThrowFailedToHandleUnauthorizedResponse($"No authorization_endpoint was provided via '{wellKnownEndpoint}'.");
                 }
 
-                if (metadata.AuthorizationEndpoint.Scheme != Uri.UriSchemeHttp &&
-                    metadata.AuthorizationEndpoint.Scheme != Uri.UriSchemeHttps)
+                string authorizationEndpointScheme = metadata.AuthorizationEndpoint.Scheme;
+
+                if (authorizationEndpointScheme != Uri.UriSchemeHttp &&
+                    authorizationEndpointScheme != Uri.UriSchemeHttps &&
+                    authorizationEndpointScheme != HttpsPlusHttpScheme)
                 {
-                    ThrowFailedToHandleUnauthorizedResponse($"AuthorizationEndpoint must use HTTP or HTTPS. '{metadata.AuthorizationEndpoint}' does not meet this requirement.");
+                    ThrowFailedToHandleUnauthorizedResponse($"AuthorizationEndpoint must use HTTP, HTTPS, or HTTPS+HTTP. '{metadata.AuthorizationEndpoint}' does not meet this requirement.");
                 }
 
                 metadata.ResponseTypesSupported ??= ["code"];
diff --git a/src/ModelContextProtocol.Core/Client/HttpClientTransportOptions.cs b/src/ModelContextProtocol.Core/Client/HttpClientTransportOptions.cs
index 73eaae700..2168310a5 100644
--- a/src/ModelContextProtocol.Core/Client/HttpClientTransportOptions.cs
+++ b/src/ModelContextProtocol.Core/Client/HttpClientTransportOptions.cs
@@ -7,6 +7,8 @@ namespace ModelContextProtocol.Client;
 /// </summary>
 public sealed class HttpClientTransportOptions
 {
+    private static readonly string HttpsPlusHttpScheme = $"{Uri.UriSchemeHttps}+{Uri.UriSchemeHttp}";
+
     /// <summary>
     /// Gets or sets the base address of the server for SSE connections.
     /// </summary>
@@ -25,9 +27,13 @@ public required Uri Endpoint
             {
                 throw new ArgumentException("Endpoint must be an absolute URI.", nameof(value));
             }
-            if (value.Scheme != Uri.UriSchemeHttp && value.Scheme != Uri.UriSchemeHttps)
+            string scheme = value.Scheme;
+
+            if (scheme != Uri.UriSchemeHttp &&
+                scheme != Uri.UriSchemeHttps &&
+                scheme != HttpsPlusHttpScheme)
             {
-                throw new ArgumentException("Endpoint must use HTTP or HTTPS scheme.", nameof(value));
+                throw new ArgumentException("Endpoint must use HTTP, HTTPS, or HTTPS+HTTP scheme.", nameof(value));
             }
 
             field = value;