McpClient.CreateAsync times out behind APIM in .NET SDK while MCP Inspector/Azure AI Foundry succeed

Summary

I am seeing a consistent timeout when initialising McpClient from the MCP .NET SDK.

The same MCP server works successfully in:

• MCP Inspector
• Azure AI Foundry

The MCP server is exposed behind Azure API Management (APIM).

Environment

• .NET: net10.0
• ModelContextProtocol package: 1.1.0
• MCP transport mode: HttpTransportMode.StreamableHttp

Observed behaviour

McpClient.CreateAsync(...) times out and throws:

• System.TimeoutException: The operation has timed out.

Expected behavior

McpClient.CreateAsync(...) should initialize successfully with the same endpoint/auth that works in MCP Inspector and Azure AI Foundry.

Repro steps

1. Set MCP endpoint and APIM subscription key.
2. Call McpClient.CreateAsync(...) directly with HttpClientTransport (StreamableHttp).
3. Immediately call ListToolsAsync().
4. Timeout occurs during McpClient.CreateAsync(...) (initialization), before tool listing.

using ModelContextProtocol.Client;
using HttpClientTransport = ModelContextProtocol.Client.HttpClientTransport;

var key = Environment.GetEnvironmentVariable("APIM_SUBSCRIPTION_KEY")
          ?? throw new InvalidOperationException("Set APIM_SUBSCRIPTION_KEY");

await using McpClient mcpClient = await McpClient.CreateAsync(
    new HttpClientTransport(new HttpClientTransportOptions
    {
        TransportMode = ModelContextProtocol.Client.HttpTransportMode.StreamableHttp,
        Endpoint = new Uri("https://<redacted>.azure-api.net/apim-mcp/mcp"),
        AdditionalHeaders = new Dictionary<string, string>
        {
            ["Ocp-Apim-Subscription-Key"] = key
        }
    }));

var tools = await mcpClient.ListToolsAsync();
Console.WriteLine($"Tools: {tools.Count}");

Start by reproducing the provided net10.0 example with HttpClientTransport in StreamableHttp mode, focusing on McpClient.CreateAsync initialization before ListToolsAsync. Compare the SDK's initialization behavior with the same APIM endpoint and authentication in MCP Inspector and Azure AI Foundry; done when CreateAsync completes and tool listing succeeds without a timeout.