How to use HTTP Streamable transport for MCP server and client in .NET csharp-sdk?

Question

The modelcontextprotocol/csharp-sdk repo (June 2025) had no samples for the HTTP Streamable transport, and the API is confusing: the client transport class is named SseClientTransport. How do I set up a Streamable HTTP MCP server and client in .NET, and can the server return JSON instead of SSE? When I run the same server code with VS Code as the MCP client, I get "Discovered 0 tools" and "Connection state: Error ; will retry with new session ID".

Originally asked on GitHub: https://github.com/modelcontextprotocol/csharp-sdk/issues/449

AgentArmorHQ team · Accepted Answer

The issue is closed (completed); the working setup was shared in the thread by a community member, and a project contributor clarified the JSON vs SSE behavior.

**Server (ASP.NET Core)**

```csharp
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddMcpServer()
    .WithHttpTransport(options => options.Stateless = true)
    .WithToolsFromAssembly();
var app = builder.Build();
app.UseHttpsRedirection();
app.MapMcp();
app.Run();
```

Another user got the same server working by registering tools with `.WithTools<MyControllerOrFileName>()` and mapping `endpoints.MapMcp("/mcp")` for a custom path.

**Client**

Use `SseClientTransport` with `TransportMode = HttpTransportMode.StreamableHttp` and a URI that does not include `/sse`:

```csharp
var clientTransport = new SseClientTransport(new SseClientTransportOptions
{
    Endpoint = new Uri("https://localhost:7042"),
    TransportMode = HttpTransportMode.StreamableHttp
});
var mcpClient = await McpClientFactory.CreateAsync(clientTransport);
```

One commenter noted the SDK's `StreamableHttpClientSessionTransport` is `internal` and guessed this is the intended usage; another commenter stated that with this `TransportMode` the client won't attempt an SSE connection even as a fallback. There is no maintainer confirmation in the thread.

**JSON vs SSE**: a project contributor explained the csharp-sdk client supports both JSON and SSE responses as the spec requires, but the server currently only produces SSE responses (which the spec allows). A JSON-only server response like the TypeScript SDK's `enableJsonResponse: true` is not supported yet and is tracked in issue #545.

The "Discovered 0 tools" / "Connection state: Error" problem with VS Code as the client was reported in this thread but not resolved there. General advice: check that the server is reachable at the exact path you mapped (root vs `/mcp`) and that your client configuration matches.

Note: the sample client stores Azure OpenAI credentials in Program.cs — replace them with your own and keep secrets out of source control.

AgentArmorHQ team · Answer

**What @ahmar-husain tried on GitHub** (Worked)

A commenter shared a complete working setup: server with `WithHttpTransport(options => options.Stateless = true)` and `app.MapMcp()`, client with `SseClientTransport` and `TransportMode = HttpTransportMode.StreamableHttp` pointing at the root URI without `/sse`.

Source: https://github.com/modelcontextprotocol/csharp-sdk/issues/449#issuecomment-2969194835

AgentArmorHQ team · Answer

**What @vandana2015 tried on GitHub** (Worked)

Another user got the server working with `.WithTools<MyControllerOrFileName>()` and `endpoints.MapMcp("/mcp")` to serve on a custom path.

Source: https://github.com/modelcontextprotocol/csharp-sdk/issues/449#issuecomment-2980458372

AgentArmorHQ team · Answer

**What @ahmar-husain tried on GitHub** (Didn't work)

A commenter inspected the SDK source code looking for a JSON-only response option and concluded it is not possible with the current SDK.

Source: https://github.com/modelcontextprotocol/csharp-sdk/issues/449#issuecomment-2980495548

AgentArmorHQ team · Answer

**What @ananthubabu-geodis tried on GitHub** (Outcome unclear)

A user ran the same server code with VS Code as the MCP client and got "Discovered 0 tools" plus "Connection state: Error ; will retry with new session ID"; no fix was provided in the thread.

Source: https://github.com/modelcontextprotocol/csharp-sdk/issues/449#issuecomment-2984058512

How to use HTTP Streamable transport for MCP server and client in .NET csharp-sdk?

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How to use HTTP Streamable transport for MCP server and client in .NET csharp-sdk?

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