Preserve per-request client capability extensions when the session declares its own

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -587,12 +587,27 @@ internal static void InjectRequestMeta(
         // extensions/io.modelcontextprotocol/tasks. Blindly overwriting the node would drop those
         // additions, so merge instead: set the standard capability fields from the session
         // capabilities while preserving any extra keys (extensions) the request envelope already had.
+        // The "extensions" map is merged by extension identifier, so a session that declares its own
+        // extensions does not replace the request's map. Every other capability, and the settings
+        // object of an extension declared on both sides, is taken whole from the session.
         var serializedCapabilities = (JsonObject)JsonSerializer.SerializeToNode(clientCapabilities, McpJsonUtilities.JsonContext.Default.ClientCapabilities)!;
         if (metaObj[MetaKeys.ClientCapabilities] is JsonObject existingCapabilities)
         {
             foreach (var property in serializedCapabilities.ToArray())
             {
-                existingCapabilities[property.Key] = property.Value?.DeepClone();
+                if (property.Key == "extensions" &&
+                    property.Value is JsonObject sessionExtensions &&
+                    existingCapabilities["extensions"] is JsonObject requestExtensions)
+                {
+                    foreach (var extension in sessionExtensions.ToArray())
+                    {
+                        requestExtensions[extension.Key] = extension.Value?.DeepClone();
+                    }
+                }
+                else
+                {
+                    existingCapabilities[property.Key] = property.Value?.DeepClone();
+                }
             }
         }
         else

tests/ModelContextProtocol.Tests/Client/McpClientTaskMethodsTests.cs

@@ -5,6 +5,7 @@
 using ModelContextProtocol.Server;
 using System.Runtime.InteropServices;
 using System.Text.Json;
+using System.Text.Json.Nodes;
 
 #pragma warning disable MCPEXP001
 
@@ -41,6 +42,13 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
             {
                 Name = "test-tool",
                 Description = "A test tool"
+            }),
+            McpServerTool.Create(
+            (McpServer server) => JsonSerializer.Serialize(server.ClientCapabilities, McpJsonUtilities.DefaultOptions),
+            new McpServerToolCreateOptions
+            {
+                Name = "echo-client-capabilities",
+                Description = "Returns the client capabilities the server received with the request"
             })]);
     }
 
@@ -114,6 +122,89 @@ public async Task CallToolAsTaskAsync_WithTaskStore_ReturnsCreatedTask()
         Assert.True(augmented.TaskCreated.PollIntervalMs > 0);
     }
 
+    [Fact]
+    public async Task CallToolAsTaskAsync_WithSessionLevelExtensions_StillReturnsCreatedTask()
+    {
+        // A client declaring its own extensions at session level must not lose the per-request
+        // tasks opt-in that CallToolAsTaskAsync adds to the request's client capabilities.
+        await using var client = await CreateMcpClientForServer(new McpClientOptions
+        {
+            Capabilities = new ClientCapabilities
+            {
+                Extensions = new Dictionary<string, object>
+                {
+                    ["io.modelcontextprotocol/ui"] = new JsonObject(),
+                },
+            },
+        });
+        var ct = TestContext.Current.CancellationToken;
+
+        var augmented = await client.CallToolAsTaskAsync(
+            new CallToolRequestParams
+            {
+                Name = "test-tool",
+                Arguments = CreateArguments("input", "test"),
+            }, ct);
+
+        Assert.True(augmented.IsTask);
+        Assert.NotNull(augmented.TaskCreated);
+    }
+
+    [Fact]
+    public async Task CallToolAsync_WithRequestAndSessionCapabilities_MergesExtensionsByIdentifierOnly()
+    {
+        // The session declares a standard capability and two extensions. The request declares the
+        // same standard capability with another shape, one extension of its own, and other settings
+        // for an extension the session also declares.
+        await using var client = await CreateMcpClientForServer(new McpClientOptions
+        {
+            Capabilities = new ClientCapabilities
+            {
+                Elicitation = new ElicitationCapability { Form = new FormElicitationCapability() },
+                Extensions = new Dictionary<string, object>
+                {
+                    ["com.example/session-only"] = new JsonObject(),
+                    ["com.example/shared"] = new JsonObject { ["mode"] = "session" },
+                },
+            },
+        });
+        var ct = TestContext.Current.CancellationToken;
+
+        var result = await client.CallToolAsync(
+            new CallToolRequestParams
+            {
+                Name = "echo-client-capabilities",
+                Meta = new JsonObject
+                {
+                    [MetaKeys.ClientCapabilities] = new JsonObject
+                    {
+                        ["elicitation"] = new JsonObject { ["url"] = new JsonObject() },
+                        ["extensions"] = new JsonObject
+                        {
+                            ["com.example/request-only"] = new JsonObject(),
+                            ["com.example/shared"] = new JsonObject { ["mode"] = "request", ["limit"] = 10 },
+                        },
+                    },
+                },
+            }, ct);
+
+        var text = Assert.IsType<TextContentBlock>(Assert.Single(result.Content)).Text;
+        var received = JsonNode.Parse(text)!.AsObject();
+
+        // A standard capability is taken whole from the session.
+        Assert.True(JsonNode.DeepEquals(new JsonObject { ["form"] = new JsonObject() }, received["elicitation"]));
+
+        // Extensions are merged by identifier, and the settings of an extension declared on both
+        // sides are taken whole from the session, not merged field by field.
+        var expectedExtensions = new JsonObject
+        {
+            ["com.example/request-only"] = new JsonObject(),
+            ["com.example/session-only"] = new JsonObject(),
+            ["com.example/shared"] = new JsonObject { ["mode"] = "session" },
+        };
+        Assert.True(JsonNode.DeepEquals(expectedExtensions, received["extensions"]), received.ToJsonString());
+    }
+
     [Fact]
     public async Task CallToolAsync_PollsUntilCompletion_ReturnsResult()
     {