modelcontextprotocol · halter73 · Jul 23, 2026 · May 27, 2026 · May 28, 2026 · Jun 10, 2026
diff --git a/docs/list-of-diagnostics.md b/docs/list-of-diagnostics.md
@@ -45,3 +45,4 @@ When APIs are marked as obsolete, a diagnostic is emitted to warn users that the
 | `MCP9004` | In place | <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.EnableLegacySse> opts into the legacy SSE transport which has no built-in HTTP-level backpressure. Use Streamable HTTP instead. See [Stateless — Legacy SSE transport](xref:stateless#legacy-sse-transport) for details. |
 | `MCP9005` | In place | The Roots, Sampling, and Logging features are deprecated as of specification version 2026-07-28 and may be removed in a future version. See SEP-2577 for more information. |
 | `MCP9006` | In place | The stateful Streamable HTTP configuration knobs on <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions> — `EventStreamStore`, `SessionMigrationHandler`, `PerSessionExecutionContext`, `IdleTimeout`, and `MaxIdleSessionCount` — only apply when `Stateless = false`. Starting with the `2026-07-28` protocol revision, Streamable HTTP no longer supports sessions, and the SDK now defaults `Stateless` to `true`. These knobs remain available for back-compat with the legacy stateful Streamable HTTP transport but new code should target the stateless path. |
+| `MCP9007` | In place | `AuthorizationRedirectDelegate` and `ClientOAuthOptions.AuthorizationRedirectDelegate` are retained for source and binary compatibility but cannot provide the RFC 9207 authorization-response issuer. Use `ClientOAuthOptions.AuthorizationCallbackHandler` for issuer-aware authorization flows. |
diff --git a/samples/ProtectedMcpClient/Program.cs b/samples/ProtectedMcpClient/Program.cs
@@ -1,4 +1,5 @@
 using Microsoft.Extensions.Logging;
+using ModelContextProtocol.Authentication;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
 using System.Diagnostics;
@@ -32,7 +33,7 @@
     OAuth = new()
     {
         RedirectUri = new Uri("http://localhost:1179/callback"),
-        AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+        AuthorizationCallbackHandler = HandleAuthorizationUrlAsync,
         DynamicClientRegistration = new()
         {
             ClientName = "ProtectedMcpClient",
@@ -68,12 +69,16 @@
 /// Handles the OAuth authorization URL by starting a local HTTP server and opening a browser.
 /// This implementation demonstrates how SDK consumers can provide their own authorization flow.
 /// </summary>
-/// <param name="authorizationUrl">The authorization URL to open in the browser.</param>
-/// <param name="redirectUri">The redirect URI where the authorization code will be sent.</param>
+/// <param name="authorizationContext">The context containing the authorization and redirect URIs.</param>
 /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
-/// <returns>The authorization code extracted from the callback, or null if the operation failed.</returns>
-static async Task<string?> HandleAuthorizationUrlAsync(Uri authorizationUrl, Uri redirectUri, CancellationToken cancellationToken)
+/// <returns>The authorization result extracted from the callback, or null if the operation failed.</returns>
+static async Task<AuthorizationResult?> HandleAuthorizationUrlAsync(
+    AuthorizationCallbackContext authorizationContext,
+    CancellationToken cancellationToken)
 {
+    var authorizationUrl = authorizationContext.AuthorizationUri;
+    var redirectUri = authorizationContext.RedirectUri;
+
     Console.WriteLine("Starting OAuth authorization flow...");
     Console.WriteLine($"Opening browser to: {authorizationUrl}");
 
@@ -93,6 +98,7 @@
         var context = await listener.GetContextAsync();
         var query = HttpUtility.ParseQueryString(context.Request.Url?.Query ?? string.Empty);
         var code = query["code"];
+        var iss = query["iss"];
         var error = query["error"];
 
         string responseHtml = "<html><body><h1>Authentication complete</h1><p>You can close this window now.</p></body></html>";
@@ -115,7 +121,7 @@
         }
 
         Console.WriteLine("Authorization code received successfully.");
-        return code;
+        return new AuthorizationResult { Code = code, Iss = iss };
     }
     catch (Exception ex)
     {

diff --git a/src/Common/Obsoletions.cs b/src/Common/Obsoletions.cs
@@ -46,4 +46,8 @@ internal static class Obsoletions
     public const string LegacyStatefulHttp_DiagnosticId = "MCP9006";
     public const string LegacyStatefulHttp_Message = "Stateful Streamable HTTP mode is a back-compat-only escape hatch for legacy clients. Set HttpServerTransportOptions.Stateless = true (the default as of the 2026-07-28 protocol revision) for new code. See SEP-2567.";
     public const string LegacyStatefulHttp_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#obsolete-apis";
+
+    public const string AuthorizationRedirectDelegate_DiagnosticId = "MCP9007";
+    public const string AuthorizationRedirectDelegate_Message = "AuthorizationRedirectDelegate cannot provide the RFC 9207 issuer and is retained for compatibility only. Use AuthorizationCallbackHandler instead.";
+    public const string AuthorizationRedirectDelegate_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#obsolete-apis";
 }
diff --git a/src/ModelContextProtocol.Core/Authentication/AuthorizationCallbackContext.cs b/src/ModelContextProtocol.Core/Authentication/AuthorizationCallbackContext.cs
@@ -0,0 +1,17 @@
+namespace ModelContextProtocol.Authentication;
+
+/// <summary>
+/// Provides the information needed to complete an OAuth authorization request.
+/// </summary>
+public sealed class AuthorizationCallbackContext
+{
+    /// <summary>
+    /// Gets the authorization URI that the user needs to visit.
+    /// </summary>
+    public required Uri AuthorizationUri { get; init; }
+
+    /// <summary>
+    /// Gets the redirect URI where the authorization response will be sent.
+    /// </summary>
+    public required Uri RedirectUri { get; init; }
+}
diff --git a/src/ModelContextProtocol.Core/Authentication/AuthorizationRedirectDelegate.cs b/src/ModelContextProtocol.Core/Authentication/AuthorizationRedirectDelegate.cs
@@ -1,4 +1,3 @@
-
 namespace ModelContextProtocol.Authentication;
 
 /// <summary>
@@ -9,20 +8,13 @@ namespace ModelContextProtocol.Authentication;
 /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
 /// <returns>A task that represents the asynchronous operation. The task result contains the authorization code if successful, or null if the operation failed or was cancelled.</returns>
 /// <remarks>
-/// <para>
-/// This delegate provides SDK consumers with full control over how the OAuth authorization flow is handled.
-/// Implementers can choose to:
-/// </para>
-/// <list type="bullet">
-/// <item><description>Start a local HTTP server and open a browser (default behavior)</description></item>
-/// <item><description>Display the authorization URL to the user for manual handling</description></item>
-/// <item><description>Integrate with a custom UI or authentication flow</description></item>
-/// <item><description>Use a different redirect mechanism altogether</description></item>
-/// </list>
-/// <para>
-/// The implementation should handle user interaction to visit the authorization URL and extract
-/// the authorization code from the callback. The authorization code is typically provided as
-/// a query parameter in the redirect URI callback.
-/// </para>
+/// This delegate cannot return the <c>iss</c> parameter from the authorization response, so
+/// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see> issuer validation is
+/// skipped when it is used. Use <see cref="ClientOAuthOptions.AuthorizationCallbackHandler"/> for
+/// issuer-aware authorization flows.
 /// </remarks>
-public delegate Task<string?> AuthorizationRedirectDelegate(Uri authorizationUri, Uri redirectUri, CancellationToken cancellationToken);
+[Obsolete(
+    ModelContextProtocol.Obsoletions.AuthorizationRedirectDelegate_Message,
+    DiagnosticId = ModelContextProtocol.Obsoletions.AuthorizationRedirectDelegate_DiagnosticId,
+    UrlFormat = ModelContextProtocol.Obsoletions.AuthorizationRedirectDelegate_Url)]
+public delegate Task<string?> AuthorizationRedirectDelegate(Uri authorizationUri, Uri redirectUri, CancellationToken cancellationToken);
diff --git a/src/ModelContextProtocol.Core/Authentication/AuthorizationResult.cs b/src/ModelContextProtocol.Core/Authentication/AuthorizationResult.cs
@@ -0,0 +1,39 @@
+namespace ModelContextProtocol.Authentication;
+
+/// <summary>
+/// Represents the result of an OAuth authorization redirect, containing the authorization code
+/// and optionally the issuer identifier from the authorization response.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The <see cref="Iss"/> property should be populated from the <c>iss</c> query parameter in the
+/// redirect URI when present, as specified by
+/// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>.
+/// This enables the SDK to validate that the authorization response originated from the expected
+/// authorization server, mitigating mix-up attacks.
+/// </para>
+/// </remarks>
+public sealed class AuthorizationResult
+{
+    /// <summary>
+    /// Gets the authorization code returned by the authorization server.
+    /// </summary>
+    public string? Code { get; init; }
+
+    /// <summary>
+    /// Gets the issuer identifier returned in the authorization response per
+    /// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This value should be extracted from the <c>iss</c> query parameter of the redirect URI.
+    /// When present, the SDK validates it against the expected authorization server issuer to
+    /// prevent mix-up attacks.
+    /// </para>
+    /// <para>
+    /// Implementations of <see cref="ClientOAuthOptions.AuthorizationCallbackHandler"/> should populate this
+    /// property whenever the <c>iss</c> parameter is present in the redirect URI callback.
+    /// </para>
+    /// </remarks>
+    public string? Iss { get; init; }
+}
diff --git a/src/ModelContextProtocol.Core/Authentication/AuthorizationServerMetadata.cs b/src/ModelContextProtocol.Core/Authentication/AuthorizationServerMetadata.cs
@@ -72,4 +72,11 @@ internal sealed class AuthorizationServerMetadata
     /// </summary>
     [JsonPropertyName("client_id_metadata_document_supported")]
     public bool ClientIdMetadataDocumentSupported { get; set; }
+
+    /// <summary>
+    /// Indicates whether the authorization server includes the <c>iss</c> parameter in authorization responses
+    /// as defined in <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>.
+    /// </summary>
+    [JsonPropertyName("authorization_response_iss_parameter_supported")]
+    public bool AuthorizationResponseIssParameterSupported { get; set; }
 }
diff --git a/src/ModelContextProtocol.Core/Authentication/ClientOAuthOptions.cs b/src/ModelContextProtocol.Core/Authentication/ClientOAuthOptions.cs
@@ -70,18 +70,44 @@ public sealed class ClientOAuthOptions
     public ScopeSelectorDelegate? ScopeSelector { get; set; }
 
     /// <summary>
-    /// Gets or sets the authorization redirect delegate for handling the OAuth authorization flow.
+    /// Gets or sets the callback that handles the OAuth authorization flow.
     /// </summary>
     /// <remarks>
     /// <para>
-    /// This delegate is responsible for handling the OAuth authorization URL and obtaining the authorization code.
-    /// If not specified, a default implementation will be used that prompts the user to enter the code manually.
+    /// This callback receives the authorization and redirect URIs in an
+    /// <see cref="AuthorizationCallbackContext"/> and returns the authorization response.
+    /// If not specified, a default implementation prompts the user to enter the full redirect URL manually.
     /// </para>
     /// <para>
     /// Custom implementations might open a browser, start an HTTP listener, or use other mechanisms to capture
-    /// the authorization code from the OAuth redirect.
+    /// the authorization response. They should return both the <c>code</c> and <c>iss</c> query parameters
+    /// from the redirect URI callback. This enables the SDK to validate the <c>iss</c> parameter per
+    /// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>, which mitigates
+    /// mix-up attacks.
+    /// </para>
+    /// <para>
+    /// This property cannot be configured together with <see cref="AuthorizationRedirectDelegate"/>.
+    /// </para>
+    /// </remarks>
+    public Func<AuthorizationCallbackContext, CancellationToken, Task<AuthorizationResult?>>? AuthorizationCallbackHandler { get; set; }
+
+    /// <summary>
+    /// Gets or sets the legacy authorization redirect delegate for handling the OAuth authorization flow.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This delegate returns only the authorization code and cannot provide the <c>iss</c> parameter from
+    /// the authorization response. Consequently, RFC 9207 issuer validation is skipped when this delegate
+    /// is used. Use <see cref="AuthorizationCallbackHandler"/> for issuer-aware authorization flows.
+    /// </para>
+    /// <para>
+    /// This property cannot be configured together with <see cref="AuthorizationCallbackHandler"/>.
     /// </para>
     /// </remarks>
+    [Obsolete(
+        ModelContextProtocol.Obsoletions.AuthorizationRedirectDelegate_Message,
+        DiagnosticId = ModelContextProtocol.Obsoletions.AuthorizationRedirectDelegate_DiagnosticId,
+        UrlFormat = ModelContextProtocol.Obsoletions.AuthorizationRedirectDelegate_Url)]
     public AuthorizationRedirectDelegate? AuthorizationRedirectDelegate { get; set; }
 
     /// <summary>