Add offline_access scope augmentation per SEP-2207

Agent-Logs-Url: https://github.com/modelcontextprotocol/csharp-sdk/sessions/4d13c15e-a0f6-4d0f-9525-5f0b9e0c000b

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -492,6 +492,7 @@ private Uri BuildAuthorizationUrl(
         }
 
         var scope = GetScopeParameter(protectedResourceMetadata);
+        scope = AugmentScopeWithOfflineAccess(scope, authServerMetadata);
         if (!string.IsNullOrEmpty(scope))
         {
             queryParamsDictionary["scope"] = scope!;
@@ -726,6 +727,37 @@ private async Task PerformDynamicClientRegistrationAsync(
         return _configuredScopes;
     }
 
+    /// <summary>
+    /// Augments the scope parameter with <c>offline_access</c> if the authorization server advertises it in
+    /// <c>scopes_supported</c> and it is not already present. This signals to OIDC-flavored authorization servers
+    /// that the client desires a refresh token, per SEP-2207.
+    /// </summary>
+    private static string? AugmentScopeWithOfflineAccess(string? scope, AuthorizationServerMetadata authServerMetadata)
+    {
+        const string offlineAccess = "offline_access";
+
+        if (authServerMetadata.ScopesSupported is null || !authServerMetadata.ScopesSupported.Contains(offlineAccess))
+        {
+            return scope;
+        }
+
+        if (scope is null)
+        {
+            return offlineAccess;
+        }
+
+        // Check if offline_access is already in the scope string (space-separated tokens).
+        foreach (var token in scope.Split(' '))
+        {
+            if (token == offlineAccess)
+            {
+                return scope;
+            }
+        }
+
+        return scope + " " + offlineAccess;
+    }
+
     /// <summary>
     /// Verifies that the resource URI in the metadata exactly matches the original request URL as required by the RFC.
     /// Per RFC: The resource value must be identical to the URL that the client used to make the request to the resource server.

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

@@ -1261,4 +1261,108 @@ public async Task CanAuthenticate_WithLegacyServerUsingDefaultEndpointFallback()
         await using var client = await McpClient.CreateAsync(
             transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
     }
+
+    [Fact]
+    public async Task AuthorizationFlow_AppendsOfflineAccess_WhenServerAdvertisesIt()
+    {
+        TestOAuthServer.IncludeOfflineAccessInMetadata = true;
+        await using var app = await StartMcpServerAsync();
+
+        string? requestedScope = null;
+
+        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"),
+                AuthorizationRedirectDelegate = (uri, redirect, ct) =>
+                {
+                    var query = QueryHelpers.ParseQuery(uri.Query);
+                    requestedScope = query["scope"].ToString();
+                    return HandleAuthorizationUrlAsync(uri, redirect, ct);
+                },
+            },
+        }, HttpClient, LoggerFactory);
+
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.NotNull(requestedScope);
+        Assert.Contains("offline_access", requestedScope!.Split(' '));
+    }
+
+    [Fact]
+    public async Task AuthorizationFlow_DoesNotAppendOfflineAccess_WhenServerDoesNotAdvertiseIt()
+    {
+        // IncludeOfflineAccessInMetadata defaults to false, so the AS will not advertise offline_access.
+        await using var app = await StartMcpServerAsync();
+
+        string? requestedScope = null;
+
+        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"),
+                AuthorizationRedirectDelegate = (uri, redirect, ct) =>
+                {
+                    var query = QueryHelpers.ParseQuery(uri.Query);
+                    requestedScope = query["scope"].ToString();
+                    return HandleAuthorizationUrlAsync(uri, redirect, ct);
+                },
+            },
+        }, HttpClient, LoggerFactory);
+
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.NotNull(requestedScope);
+        Assert.DoesNotContain("offline_access", requestedScope!.Split(' '));
+    }
+
+    [Fact]
+    public async Task AuthorizationFlow_DoesNotDuplicateOfflineAccess_WhenAlreadyPresent()
+    {
+        TestOAuthServer.IncludeOfflineAccessInMetadata = true;
+
+        // Configure the PRM to already include offline_access in its scopes.
+        Builder.Services.Configure<McpAuthenticationOptions>(McpAuthenticationDefaults.AuthenticationScheme, options =>
+        {
+            options.ResourceMetadata!.ScopesSupported = ["mcp:tools", "offline_access"];
+        });
+
+        await using var app = await StartMcpServerAsync();
+
+        string? requestedScope = null;
+
+        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"),
+                AuthorizationRedirectDelegate = (uri, redirect, ct) =>
+                {
+                    var query = QueryHelpers.ParseQuery(uri.Query);
+                    requestedScope = query["scope"].ToString();
+                    return HandleAuthorizationUrlAsync(uri, redirect, ct);
+                },
+            },
+        }, HttpClient, LoggerFactory);
+
+        await using var client = await McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.NotNull(requestedScope);
+        var scopeTokens = requestedScope!.Split(' ');
+        Assert.Single(scopeTokens, t => t == "offline_access");
+    }
 }

tests/ModelContextProtocol.TestOAuthServer/Program.cs

@@ -78,6 +78,16 @@ public Program(ILoggerProvider? loggerProvider = null, IConnectionListenerFactor
     /// </remarks>
     public bool ExpectResource { get; set; } = true;
 
+    /// <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
+    /// authorization server that issues refresh tokens when the client requests the <c>offline_access</c> scope.
+    /// </summary>
+    /// <remarks>
+    /// The default value is <c>false</c>.
+    /// </remarks>
+    public bool IncludeOfflineAccessInMetadata { get; set; }
+
     public HashSet<string> DisabledMetadataPaths { get; } = new(StringComparer.OrdinalIgnoreCase);
     public IReadOnlyCollection<string> MetadataRequests => _metadataRequests.ToArray();
 
@@ -188,7 +198,9 @@ IResult HandleMetadataRequest(HttpContext context, string? issuerPath = null)
                 ResponseTypesSupported = ["code"],
                 SubjectTypesSupported = ["public"],
                 IdTokenSigningAlgValuesSupported = ["RS256"],
-                ScopesSupported = ["openid", "profile", "email", "mcp:tools"],
+                ScopesSupported = IncludeOfflineAccessInMetadata
+                    ? ["openid", "profile", "email", "mcp:tools", "offline_access"]
+                    : ["openid", "profile", "email", "mcp:tools"],
                 TokenEndpointAuthMethodsSupported = ["client_secret_post"],
                 ClaimsSupported = ["sub", "iss", "name", "email", "aud"],
                 CodeChallengeMethodsSupported = ["S256"],