feature: Added configurable token endpoint auth method selection

src/ModelContextProtocol.Core/Authentication/ClientOAuthOptions.cs

@@ -113,6 +113,20 @@ public sealed class ClientOAuthOptions
         UrlFormat = ModelContextProtocol.Obsoletions.AuthorizationRedirectDelegate_Url)]
     public AuthorizationRedirectDelegate? AuthorizationRedirectDelegate { get; set; }
 
+    /// <summary>
+    /// Gets or sets the token endpoint authentication method selector function.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This function is used to select which token endpoint authentication method to use when multiple methods are available.
+    /// If not specified, the first available method will be selected.
+    /// </para>
+    /// <para>
+    /// The function receives a list of supported authentication methods from the authorization server metadata and should return the selected method, or null if no suitable method is found.
+    /// </para>
+    /// </remarks>
+    public Func<IReadOnlyList<string>?, string?>? TokenEndpointAuthMethodSelector { get; set; }
+
     /// <summary>
     /// Gets or sets the authorization server selector function.
     /// </summary>

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -30,6 +30,7 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
     private readonly string? _configuredScopes;
     private readonly ScopeSelectorDelegate? _scopeSelector;
     private readonly IDictionary<string, string> _additionalAuthorizationParameters;
+    private readonly Func<IReadOnlyList<string>?, string?> _tokenEndpointAuthMethodSelector;
     private readonly Func<IReadOnlyList<Uri>, Uri?> _authServerSelector;
     private readonly Func<AuthorizationCallbackContext, CancellationToken, Task<AuthorizationResult?>> _authorizationCallbackHandler;
     private readonly bool _validateAuthorizationResponseState;
@@ -106,6 +107,9 @@ public ClientOAuthProvider(
         _additionalAuthorizationParameters = options.AdditionalAuthorizationParameters;
         _clientMetadataDocumentUri = options.ClientMetadataDocumentUri;
 
+        // Set up token endpoint authentication method selector (use default if not provided)
+        _tokenEndpointAuthMethodSelector = options.TokenEndpointAuthMethodSelector ?? DefaultTokenEndpointAuthMethodSelector;
+
         // Set up authorization server selection strategy
         _authServerSelector = options.AuthServerSelector ?? DefaultAuthServerSelector;
 
@@ -154,6 +158,13 @@ public ClientOAuthProvider(
         _tokenCache = options.TokenCache ?? new InMemoryTokenCache();
     }
 
+    /// <summary>
+    /// Default token endpoint authentication method selector that selects the first supported method from the authorization server metadata.
+    /// </summary>
+    /// <param name="tokenEndpointAuthMethodsSupported">The list of supported token endpoint authentication methods.</param>
+    /// <returns>The selected token endpoint authentication method, or null if none are available.</returns>
+    private static string? DefaultTokenEndpointAuthMethodSelector(IReadOnlyList<string>? tokenEndpointAuthMethodsSupported) => tokenEndpointAuthMethodsSupported?.FirstOrDefault();
+
     /// <summary>
     /// Default authorization server selection strategy that selects the first available server.
     /// </summary>
@@ -475,7 +486,7 @@ private async Task<string> GetAccessTokenCoreAsync(HttpResponseMessage response,
         _clientCredentialsAuthorizationServer = selectedAuthServer.OriginalString;
 
         // Determine the token endpoint auth method from server metadata if not already set by DCR.
-        _tokenEndpointAuthMethod ??= authServerMetadata.TokenEndpointAuthMethodsSupported?.FirstOrDefault();
+        _tokenEndpointAuthMethod ??= _tokenEndpointAuthMethodSelector(authServerMetadata.TokenEndpointAuthMethodsSupported);
 
         // Store auth server metadata for future refresh operations
         _authServerMetadata = authServerMetadata;