Add support for Client ID Metadata Documents to enable URL-based client registration

src/ModelContextProtocol.Core/Authentication/AuthorizationServerMetadata.cs

@@ -66,4 +66,10 @@ internal sealed class AuthorizationServerMetadata
     /// </summary>
     [JsonPropertyName("scopes_supported")]
     public List<string>? ScopesSupported { get; set; }
+
+    /// <summary>
+    /// Indicates if the server supports OAuth Client ID Metadata Documents.
+    /// </summary>
+    [JsonPropertyName("client_id_metadata_document_supported")]
+    public bool ClientIdMetadataDocumentSupported { get; set; }
 }

src/ModelContextProtocol.Core/Authentication/ClientOAuthOptions.cs

@@ -23,6 +23,17 @@ public sealed class ClientOAuthOptions
     /// </remarks>
     public string? ClientSecret { get; set; }
 
+    /// <summary>
+    /// Gets or sets the HTTPS URL pointing to this client's metadata document.
+    /// </summary>
+    /// <remarks>
+    /// When specified, and when the authorization server metadata reports
+    /// <c>client_id_metadata_document_supported = true</c>, the OAuth client will respond to
+    /// challenges by sending this URL as the client identifier instead of performing dynamic
+    /// client registration.
+    /// </remarks>
+    public Uri? ClientMetadataDocumentUri { get; set; }
+
     /// <summary>
     /// Gets or sets the OAuth scopes to request.
     /// </summary>

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -30,6 +30,7 @@ internal sealed partial class ClientOAuthProvider
     private readonly IDictionary<string, string> _additionalAuthorizationParameters;
     private readonly Func<IReadOnlyList<Uri>, Uri?> _authServerSelector;
     private readonly AuthorizationRedirectDelegate _authorizationRedirectDelegate;
+    private readonly Uri? _clientMetadataDocumentUri;
 
     // _dcrClientName, _dcrClientUri, _dcrInitialAccessToken and _dcrResponseDelegate are used for dynamic client registration (RFC 7591)
     private readonly string? _dcrClientName;
@@ -74,6 +75,7 @@ public ClientOAuthProvider(
         _redirectUri = options.RedirectUri ?? throw new ArgumentException("ClientOAuthOptions.RedirectUri must configured.", nameof(options));
         _scopes = options.Scopes?.ToArray();
         _additionalAuthorizationParameters = options.AdditionalAuthorizationParameters;
+        _clientMetadataDocumentUri = options.ClientMetadataDocumentUri;
 
         // Set up authorization server selection strategy
         _authServerSelector = options.AuthServerSelector ?? DefaultAuthServerSelector;
@@ -221,7 +223,7 @@ private async Task PerformOAuthAuthorizationAsync(
         _authServerMetadata = authServerMetadata;
 
         // The existing access token must be invalid to have resulted in a 401 response, but refresh might still work.
-        if (await _tokenCache.GetTokensAsync(cancellationToken).ConfigureAwait(false) is { RefreshToken: {} refreshToken })
+        if (await _tokenCache.GetTokensAsync(cancellationToken).ConfigureAwait(false) is { RefreshToken: { } refreshToken })
         {
             var refreshedTokens = await RefreshTokenAsync(refreshToken, protectedResourceMetadata.Resource, authServerMetadata, cancellationToken).ConfigureAwait(false);
             if (refreshedTokens is not null)
@@ -231,10 +233,18 @@ private async Task PerformOAuthAuthorizationAsync(
             }
         }
 
-        // Perform dynamic client registration if needed
+        // Assign a client ID if necessary
         if (string.IsNullOrEmpty(_clientId))
         {
-            await PerformDynamicClientRegistrationAsync(authServerMetadata, cancellationToken).ConfigureAwait(false);
+            // Try using a client metadata document before falling back to dynamic client registration
+            if (authServerMetadata.ClientIdMetadataDocumentSupported && _clientMetadataDocumentUri is not null)
+            {
+                ApplyClientIdMetadataDocument(_clientMetadataDocumentUri);
+            }
+            else
+            {
+                await PerformDynamicClientRegistrationAsync(authServerMetadata, cancellationToken).ConfigureAwait(false);
+            }
         }
 
         // Perform the OAuth flow
@@ -243,6 +253,23 @@ private async Task PerformOAuthAuthorizationAsync(
         LogOAuthAuthorizationCompleted();
     }
 
+    private void ApplyClientIdMetadataDocument(Uri metadataUri)
+    {
+        if (!IsValidClientMetadataDocumentUri(metadataUri))
+        {
+            ThrowFailedToHandleUnauthorizedResponse(
+                $"{nameof(ClientOAuthOptions.ClientMetadataDocumentUri)} must be an HTTPS URL with a non-root absolute path. Value: '{metadataUri}'.");
+        }
+
+        _clientId = metadataUri.AbsoluteUri;
+
+        // See: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-client-id-metadata-document-00#section-3
+        static bool IsValidClientMetadataDocumentUri(Uri uri)
+            => uri.IsAbsoluteUri
+            && string.Equals(uri.Scheme, Uri.UriSchemeHttps, StringComparison.OrdinalIgnoreCase)
+            && uri.AbsolutePath.Length > 1; // AbsolutePath always starts with "/"
+    }
+
     private async Task<AuthorizationServerMetadata> GetAuthServerMetadataAsync(Uri authServerUri, CancellationToken cancellationToken)
     {
         if (authServerUri.OriginalString.Length == 0 ||

tests/ModelContextProtocol.AspNetCore.Tests/OAuth/AuthTests.cs

@@ -9,8 +9,10 @@ namespace ModelContextProtocol.AspNetCore.Tests.OAuth;
 
 public class AuthTests : OAuthTestBase
 {
-   public AuthTests(ITestOutputHelper outputHelper)
-        : base(outputHelper)
+    private const string ClientMetadataDocumentUrl = $"{OAuthServerUrl}/client-metadata/cimd-client.json";
+
+    public AuthTests(ITestOutputHelper outputHelper)
+         : base(outputHelper)
     {
     }
 
@@ -98,6 +100,105 @@ public async Task CanAuthenticate_WithDynamicClientRegistration()
             transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
     }
 
+    [Fact]
+    public async Task CanAuthenticate_WithClientMetadataDocument()
+    {
+        await using var app = await StartMcpServerAsync();
+
+        await using var transport = new HttpClientTransport(new()
+        {
+            Endpoint = new(McpServerUrl),
+            OAuth = new ClientOAuthOptions()
+            {
+                RedirectUri = new Uri("http://localhost:1179/callback"),
+                AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+                ClientMetadataDocumentUri = new Uri(ClientMetadataDocumentUrl)
+            },
+        }, HttpClient, LoggerFactory);
+
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+    }
+
+    [Fact]
+    public async Task UsesDynamicClientRegistration_WhenCimdNotSupported()
+    {
+        // Disable CIMD support on the test OAuth server so the client
+        // falls back to dynamic registration even if a CIMD URL is provided.
+        TestOAuthServer.ClientIdMetadataDocumentSupported = false;
+
+        await using var app = await StartMcpServerAsync();
+
+        // Provide an invalid CIMD URL; if CIMD were used, auth would fail.
+        await using var transport = new HttpClientTransport(new()
+        {
+            Endpoint = new(McpServerUrl),
+            OAuth = new ClientOAuthOptions()
+            {
+                RedirectUri = new Uri("http://localhost:1179/callback"),
+                AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+                ClientMetadataDocumentUri = new Uri("http://invalid-cimd.example.com"),
+                Scopes = ["mcp:tools"],
+                DynamicClientRegistration = new()
+                {
+                    ClientName = "Test MCP Client (No CIMD)",
+                    ClientUri = new Uri("https://example.com/no-cimd"),
+                },
+            },
+        }, HttpClient, LoggerFactory);
+
+        // Should succeed via dynamic client registration.
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+    }
+
+    [Fact]
+    public async Task DoesNotUseClientMetadataDocument_WhenClientIdIsSpecified()
+    {
+        await using var app = await StartMcpServerAsync();
+
+        // Provide an invalid CIMD URL; if CIMD were used, auth would fail.
+        await using var transport = new HttpClientTransport(new()
+        {
+            Endpoint = new(McpServerUrl),
+            OAuth = new ClientOAuthOptions()
+            {
+                ClientId = "demo-client",
+                ClientSecret = "demo-secret",
+                RedirectUri = new Uri("http://localhost:1179/callback"),
+                AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+                ClientMetadataDocumentUri = new Uri("http://invalid-cimd.example.com"),
+            },
+        }, HttpClient, LoggerFactory);
+
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+    }
+
+    [Theory]
+    [InlineData("http://localhost:7029/client-metadata/cimd-client.json")] // Non-HTTPS Scheme
+    [InlineData("http://localhost:7029")] // Missing path
+    public async Task CannotAuthenticate_WithInvalidClientMetadataDocument(string uri)
+    {
+        await using var app = await StartMcpServerAsync();
+
+        await using var transport = new HttpClientTransport(new()
+        {
+            Endpoint = new(McpServerUrl),
+            OAuth = new ClientOAuthOptions()
+            {
+                RedirectUri = new Uri("http://localhost:1179/callback"),
+                AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+                ClientMetadataDocumentUri = new Uri(uri),
+            },
+        }, HttpClient, LoggerFactory);
+
+        var ex = await Assert.ThrowsAsync<McpException>(() => McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
+
+        Assert.StartsWith("Failed to handle unauthorized response", ex.Message);
+    }
+
     [Fact]
     public async Task CanAuthenticate_WithTokenRefresh()
     {

tests/ModelContextProtocol.TestOAuthServer/AuthorizationServerMetadata.cs

@@ -60,4 +60,10 @@ internal sealed class AuthorizationServerMetadata
     /// </summary>
     [JsonPropertyName("scopes_supported")]
     public List<string>? ScopesSupported { get; init; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether CIMD client IDs are supported.
+    /// </summary>
+    [JsonPropertyName("client_id_metadata_document_supported")]
+    public bool ClientIdMetadataDocumentSupported { get; init; }
 }

tests/ModelContextProtocol.TestOAuthServer/ClientInfo.cs

@@ -12,10 +12,15 @@ internal sealed class ClientInfo
     /// </summary>
     public required string ClientId { get; init; }
 
+    /// <summary>
+    /// Gets or sets whether a client secret is required.
+    /// </summary>
+    public required bool RequiresClientSecret { get; init; }
+
     /// <summary>
     /// Gets or sets the client secret.
     /// </summary>
-    public required string ClientSecret { get; init; }
+    public string? ClientSecret { get; init; }
 
     /// <summary>
     /// Gets or sets the list of redirect URIs allowed for this client.

tests/ModelContextProtocol.TestOAuthServer/OAuthServerMetadata.cs

@@ -171,4 +171,11 @@ internal sealed class OAuthServerMetadata
     [JsonPropertyName("claims_supported")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public List<string>? ClaimsSupported { get; init; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether CIMD client IDs are supported.
+    /// </summary>
+    [JsonPropertyName("client_id_metadata_document_supported")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public bool? ClientIdMetadataDocumentSupported { get; init; }
 }

tests/ModelContextProtocol.TestOAuthServer/Program.cs

@@ -13,6 +13,7 @@ public sealed class Program
 {
     private const int _port = 7029;
     private static readonly string _url = $"https://localhost:{_port}";
+    private static readonly string _clientMetadataDocumentUrl = $"{_url}/client-metadata/cimd-client.json";
 
     // Port 5000 is used by tests and port 7071 is used by the ProtectedMcpServer sample
     private static readonly string[] ValidResources = ["http://localhost:5000/", "http://localhost:7071/"];
@@ -44,6 +45,16 @@ public Program(ILoggerProvider? loggerProvider = null, IConnectionListenerFactor
     public bool HasIssuedExpiredToken { get; set; }
     public bool HasRefreshedToken { get; set; }
 
+    /// <summary>
+    /// Gets or sets a value indicating whether the authorization server
+    /// advertises support for client ID metadata documents in its discovery
+    /// document. This is used by tests to toggle CIMD support.
+    /// </summary>
+    /// <remarks>
+    /// The default value is <c>true</c>.
+    /// </remarks>
+    public bool ClientIdMetadataDocumentSupported { get; set; } = true;
+
     /// <summary>
     /// Entry point for the application.
     /// </summary>
@@ -102,6 +113,7 @@ public async Task RunServerAsync(string[]? args = null, CancellationToken cancel
         _clients[clientId] = new ClientInfo
         {
             ClientId = clientId,
+            RequiresClientSecret = true,
             ClientSecret = clientSecret,
             RedirectUris = ["http://localhost:1179/callback"],
         };
@@ -111,10 +123,24 @@ public async Task RunServerAsync(string[]? args = null, CancellationToken cancel
         _clients["test-refresh-client"] = new ClientInfo
         {
             ClientId = "test-refresh-client",
+            RequiresClientSecret = true,
             ClientSecret = "test-refresh-secret",
             RedirectUris = ["http://localhost:1179/callback"],
         };
 
+        // This client is pre-registered to support testing Client ID Metadata Documents (CIMD).
+        // A non-test OAuth server implementation would fetch the metadata document from the client-specified
+        // URL during authorization, but we just register the client here to keep the test implementation simple.
+        // We also set 'RequiresClientSecret' to 'false' here because client secrets are disallowed when using CIMD.
+        // See https://datatracker.ietf.org/doc/html/draft-ietf-oauth-client-id-metadata-document-00#section-4.1
+        _clients[_clientMetadataDocumentUrl] = new ClientInfo
+        {
+            ClientId = _clientMetadataDocumentUrl,
+
+            RequiresClientSecret = false,
+            RedirectUris = ["http://localhost:1179/callback"],
+        };
+
         // The MCP spec tells the client to use /.well-known/oauth-authorization-server but AddJwtBearer looks for
         // /.well-known/openid-configuration by default. To make things easier, we support both with the same response
         // which seems to be common. Ex. https://github.com/keycloak/keycloak/pull/29628
@@ -144,7 +170,8 @@ public async Task RunServerAsync(string[]? args = null, CancellationToken cancel
                     CodeChallengeMethodsSupported = ["S256"],
                     GrantTypesSupported = ["authorization_code", "refresh_token"],
                     IntrospectionEndpoint = $"{_url}/introspect",
-                    RegistrationEndpoint = $"{_url}/register"
+                    RegistrationEndpoint = $"{_url}/register",
+                    ClientIdMetadataDocumentSupported = ClientIdMetadataDocumentSupported,
                 };
 
                 return Results.Ok(metadata);
@@ -468,6 +495,7 @@ public async Task RunServerAsync(string[]? args = null, CancellationToken cancel
             _clients[clientId] = new ClientInfo
             {
                 ClientId = clientId,
+                RequiresClientSecret = true,
                 ClientSecret = clientSecret,
                 RedirectUris = registrationRequest.RedirectUris,
             };
@@ -508,17 +536,17 @@ public async Task RunServerAsync(string[]? args = null, CancellationToken cancel
         var clientId = form["client_id"].ToString();
         var clientSecret = form["client_secret"].ToString();
 
-        if (string.IsNullOrEmpty(clientId) || string.IsNullOrEmpty(clientSecret))
+        if (string.IsNullOrEmpty(clientId) || !_clients.TryGetValue(clientId, out var client))
         {
             return null;
         }
 
-        if (_clients.TryGetValue(clientId, out var client) && client.ClientSecret == clientSecret)
+        if (client.RequiresClientSecret && client.ClientSecret != clientSecret)
         {
-            return client;
+            return null;
         }
 
-        return null;
+        return client;
     }
 
     /// <summary>