Repository navigation
Session-level client extensions overwrite the per-request Tasks opt-in: CallToolAsTaskAsync never creates a task #1920
Description
Describe the bug
On the 2026-07-28 revision, a client whose session capabilities declare any extension (for example MCP Apps, io.modelcontextprotocol/ui) never gets a task back from CallToolAsTaskAsync. The server answers synchronously, as if the client had not opted in to Tasks.
McpTasksClientExtensions.CallToolAsTaskAsync writes _meta["io.modelcontextprotocol/clientCapabilities"].extensions["io.modelcontextprotocol/tasks"] on the request. McpSessionHandler.InjectRequestMeta then merges the session capabilities one top-level key at a time:
foreach (var property in serializedCapabilities.ToArray())
{
existingCapabilities[property.Key] = property.Value?.DeepClone();
}When the session capabilities carry extensions, that key replaces the request's whole extensions object, and the tasks entry is lost. The comment above this block says per-request extensions such as io.modelcontextprotocol/tasks must be preserved, so the shallow merge contradicts the intent.
Reproduced on 2.0.0. The same code ships in 2.2.0 and is on main.
To Reproduce
[TestMethod]
[DataRow(false)] // passes
[DataRow(true)] // fails: IsTask is false, negotiated 2026-07-28
public async Task CallToolAsTaskAsync_CreatesATask(bool declareSessionExtension)
{
var builder = WebApplication.CreateSlimBuilder();
builder.WebHost.UseUrls("http://127.0.0.1:0");
builder.Services
.AddMcpServer(options => options.ToolCollection =
[
McpServerTool.Create(
async (CancellationToken ct) => { await Task.Delay(200, ct); return "done"; },
new McpServerToolCreateOptions { Name = "slow" }),
])
.WithHttpTransport()
.WithTasks(new InMemoryMcpTaskStore());
await using var app = builder.Build();
app.MapMcp("/mcp");
await app.StartAsync();
var options = new McpClientOptions();
if (declareSessionExtension)
{
options.Capabilities = new ClientCapabilities
{
Extensions = new Dictionary<string, object> { ["io.modelcontextprotocol/ui"] = new JsonObject() },
};
}
await using var client = await McpClient.CreateAsync(
new HttpClientTransport(new HttpClientTransportOptions { Endpoint = new Uri($"{app.Urls.First()}/mcp") }),
options);
var answer = await client.CallToolAsTaskAsync(new CallToolRequestParams { Name = "slow" });
Assert.IsTrue(answer.IsTask);
}Expected behavior
The request declares both the session extensions and the per-request io.modelcontextprotocol/tasks extension, and the server answers a CreateTaskResult. extensions (and any other object-valued capability) should be merged key by key, not replaced.
Workaround
Declare io.modelcontextprotocol/tasks in the session capabilities, next to the other extensions.
Environment
- ModelContextProtocol 2.0.0 (reproduced); the merge code is unchanged in 2.2.0 and on
main - ModelContextProtocol.Extensions.Tasks 2.0.0
- .NET 10, Streamable HTTP, stateless server
Note
This issue was AI-generated (Claude Code). The repro was run locally against 2.0.0.
Activity