1aifanatic · Sep 18, 2026
diff --git a/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs b/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
@@ -690,6 +690,21 @@ private static IEnumerable<Uri> GetWellKnownAuthorizationServerMetadataUris(Uri
 
         if (!httpResponse.IsSuccessStatusCode)
         {
+            // The MCP authorization spec requires the RFC 8707 resource parameter on token requests, but some
+            // authorization servers reject it on refresh_token grants (Microsoft Entra ID v2.0 fails with
+            // AADSTS9010010). Rather than forcing interactive re-authorization every time the access token
+            // expires, retry the refresh once without it. A dead refresh token (invalid_grant) can't be fixed
+            // by dropping the resource, so that falls through to re-authorization as before.
+            if (resourceUri is not null && httpResponse.StatusCode == System.Net.HttpStatusCode.BadRequest)
+            {
+                var error = await ReadOAuthErrorCodeAsync(httpResponse, cancellationToken).ConfigureAwait(false);
+                if (error != "invalid_grant")
+                {
+                    LogOAuthTokenRefreshRetryingWithoutResource(resourceUri, error);
+                    return await RefreshTokensAsync(refreshToken, resourceUri: null, authServerMetadata, cancellationToken).ConfigureAwait(false);
+                }
+            }
+
             return null;
         }
 
@@ -698,6 +713,19 @@ private static IEnumerable<Uri> GetWellKnownAuthorizationServerMetadataUris(Uri
         return tokens.AccessToken;
     }
 
+    private static async Task<string?> ReadOAuthErrorCodeAsync(HttpResponseMessage response, CancellationToken cancellationToken)
+    {
+        try
+        {
+            var body = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
+            return JsonSerializer.Deserialize(body, McpJsonUtilities.JsonContext.Default.OAuthErrorResponse)?.Error;
+        }
+        catch (JsonException)
+        {
+            return null;
+        }
+    }
+
     private async Task<string> InitiateAuthorizationCodeFlowAsync(
         ProtectedResourceMetadata protectedResourceMetadata,
         AuthorizationServerMetadata authServerMetadata,
@@ -1572,6 +1600,9 @@ private static void ThrowFailedToHandleUnauthorizedResponse(string message) =>
     [LoggerMessage(Level = LogLevel.Information, Message = "OAuth token refresh completed successfully")]
     partial void LogOAuthTokenRefreshCompleted();
 
+    [LoggerMessage(Level = LogLevel.Warning, Message = "OAuth token refresh with resource '{Resource}' was rejected with error '{Error}'. Retrying without the resource parameter.")]
+    partial void LogOAuthTokenRefreshRetryingWithoutResource(string resource, string? error);
+
     [LoggerMessage(Level = LogLevel.Error, Message = "Error fetching auth server metadata from {Endpoint}")]
     partial void LogErrorFetchingAuthServerMetadata(Exception ex, Uri endpoint);
 

diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/OAuth/AuthTests.cs b/tests/ModelContextProtocol.AspNetCore.Tests/OAuth/AuthTests.cs
@@ -523,6 +523,60 @@ public async Task CannotAuthenticate_WithInvalidClientMetadataDocument(string ur
 
     [Fact]
     public async Task CanAuthenticate_WithTokenRefresh()
+    {
+        await using var app = await StartMcpServerThatForcesTokenRefreshAsync();
+        await using var transport = CreateOAuthTransport();
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+
+        await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.True(TestOAuthServer.HasRefreshedToken);
+
+        // Conformant authorization servers only see the spec-required refresh request that includes the resource.
+        Assert.Equal([McpServerUrl], TestOAuthServer.RefreshTokenRequestResources);
+        Assert.Equal(1, TestOAuthServer.AuthorizationCodeTokenRequestCount);
+    }
+
+    [Fact]
+    public async Task CanAuthenticate_WithTokenRefresh_WhenAuthServerRejectsResourceOnRefresh()
+    {
+        // Simulates Microsoft Entra ID v2.0, which rejects the RFC 8707 resource parameter on refresh_token grants (AADSTS9010010).
+        TestOAuthServer.RejectRefreshWithResourceError = "invalid_target";
+
+        await using var app = await StartMcpServerThatForcesTokenRefreshAsync();
+        await using var transport = CreateOAuthTransport();
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+
+        await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+
+        // The rejected refresh is retried once without the resource instead of forcing interactive re-authorization.
+        Assert.True(TestOAuthServer.HasRefreshedToken);
+        Assert.Equal([McpServerUrl, null], TestOAuthServer.RefreshTokenRequestResources);
+        Assert.Equal(1, TestOAuthServer.AuthorizationCodeTokenRequestCount);
+    }
+
+    [Fact]
+    public async Task TokenRefresh_RejectedWithInvalidGrant_IsNotRetriedWithoutResource()
+    {
+        // invalid_grant means the refresh token itself is dead, so dropping the resource can't help.
+        TestOAuthServer.RejectRefreshWithResourceError = "invalid_grant";
+
+        await using var app = await StartMcpServerThatForcesTokenRefreshAsync();
+        await using var transport = CreateOAuthTransport();
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+
+        await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+
+        // The client falls back to a new authorization-code flow instead.
+        Assert.False(TestOAuthServer.HasRefreshedToken);
+        Assert.Equal([McpServerUrl], TestOAuthServer.RefreshTokenRequestResources);
+        Assert.Equal(2, TestOAuthServer.AuthorizationCodeTokenRequestCount);
+    }
+
+    private async Task<WebApplication> StartMcpServerThatForcesTokenRefreshAsync()
     {
         var hasForcedRefresh = false;
 
@@ -531,7 +585,7 @@ public async Task CanAuthenticate_WithTokenRefresh()
             options.ToolCollection = new();
         });
 
-        await using var app = await StartMcpServerAsync(configureMiddleware: app =>
+        return await StartMcpServerAsync(configureMiddleware: app =>
         {
             // Add middleware to intercept list tools requests and force a token refresh on the first call
             app.Use(async (context, next) =>
@@ -566,25 +620,6 @@ public async Task CanAuthenticate_WithTokenRefresh()
                 await next(context);
             });
         });
-
-        await using var transport = new HttpClientTransport(new()
-        {
-            Endpoint = new(McpServerUrl),
-            OAuth = new()
-            {
-                ClientId = "demo-client",
-                ClientSecret = "demo-secret",
-                RedirectUri = new Uri("http://localhost:1179/callback"),
-                AuthorizationCallbackHandler = HandleAuthorizationUrlAsync,
-            },
-        }, HttpClient, LoggerFactory);
-
-        await using var client = await McpClient.CreateAsync(
-            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
-
-        await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
-
-        Assert.True(TestOAuthServer.HasRefreshedToken);
     }
 
     [Fact]

diff --git a/tests/ModelContextProtocol.TestOAuthServer/Program.cs b/tests/ModelContextProtocol.TestOAuthServer/Program.cs
@@ -29,6 +29,7 @@ public sealed class Program
 
     private readonly ConcurrentQueue<string> _metadataRequests = new();
     private int _authorizationCodeTokenRequestCount;
+    private readonly ConcurrentQueue<string?> _refreshTokenRequestResources = new();
 
     private readonly RSA _rsa;
     private readonly string _keyId;
@@ -91,6 +92,13 @@ public Program(ILoggerProvider? loggerProvider = null, IConnectionListenerFactor
     /// </remarks>
     public bool ExpectResource { get; set; } = true;
 
+    /// <summary>
+    /// Gets or sets the OAuth error code returned for <c>refresh_token</c> grants that include a resource parameter.
+    /// When set, such grants are rejected and grants without a resource parameter are accepted, simulating
+    /// authorization servers like Microsoft Entra ID v2.0 that reject RFC 8707 resource indicators on refresh (AADSTS9010010).
+    /// </summary>
+    public string? RejectRefreshWithResourceError { get; set; }
+
     /// <summary>
     /// Gets or sets a value indicating whether the authorization server advertises support for
     /// <c>offline_access</c> in its <c>scopes_supported</c> metadata. This simulates an OIDC-flavored
@@ -141,6 +149,9 @@ public Program(ILoggerProvider? loggerProvider = null, IConnectionListenerFactor
     /// <summary>Gets the number of authorization-code token exchange requests received.</summary>
     public int AuthorizationCodeTokenRequestCount => Volatile.Read(ref _authorizationCodeTokenRequestCount);
 
+    /// <summary>Gets the <c>resource</c> field of each <c>refresh_token</c> grant received, or <see langword="null"/> where it was absent.</summary>
+    public IReadOnlyCollection<string?> RefreshTokenRequestResources => _refreshTokenRequestResources.ToArray();
+
     /// <summary>Gets the <c>scope</c> field from the most recent Dynamic Client Registration request.</summary>
     public string? LastRegistrationScope { get; private set; }
 
@@ -452,7 +463,23 @@ IResult HandleMetadataRequest(HttpContext context, string? issuerPath = null)
             // RFC 7523 JWT-bearer assertions carry the target resource inside the JWT itself,
             // so we skip the form-level resource check for that grant type.
             var resource = form["resource"].ToString();
-            if (grant_type != "urn:ietf:params:oauth:grant-type:jwt-bearer" &&
+            if (grant_type == "refresh_token")
+            {
+                _refreshTokenRequestResources.Enqueue(string.IsNullOrEmpty(resource) ? null : resource);
+            }
+
+            if (grant_type == "refresh_token" && RejectRefreshWithResourceError is { } refreshResourceError)
+            {
+                if (!string.IsNullOrEmpty(resource))
+                {
+                    return Results.BadRequest(new OAuthErrorResponse
+                    {
+                        Error = refreshResourceError,
+                        ErrorDescription = "AADSTS9010010: The resource parameter provided in the request doesn't match with the requested scopes."
+                    });
+                }
+            }
+            else if (grant_type != "urn:ietf:params:oauth:grant-type:jwt-bearer" &&
                 (ExpectResource ? (string.IsNullOrEmpty(resource) || !ValidResources.Contains(resource)) : !string.IsNullOrEmpty(resource)))
             {
                 return Results.BadRequest(new OAuthErrorResponse