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Multi Round-Trip Requests (MRTR) (#1458)

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Stephen Halter 2026-06-03 16:19:31 -07:00 • committed by GitHub
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40
docs/concepts/roots/roots.md
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@ -103,3 +103,43 @@ server.RegisterNotificationHandler(
Console.WriteLine($"Roots updated. {result.Roots.Count} roots available.");
});
```
### Multi Round-Trip Requests (MRTR)
[MRTR](xref:mrtr) is the SEP-2322 mechanism for server-driven input requests, finalized in protocol revision `DRAFT-2026-v1`. Under the draft protocol, the server-to-client `roots/list` request method is removed; the recommended way to ask the client for its roots from a server handler is to throw <xref:ModelContextProtocol.Protocol.InputRequiredException> and let the SDK emit an <xref:ModelContextProtocol.Protocol.InputRequiredResult> on the wire.
> [!IMPORTANT]
> `RequestRootsAsync` throws `InvalidOperationException("Roots are not supported in stateless mode.")` whenever the server is running stateless — which includes every Streamable HTTP server under `DRAFT-2026-v1` once that revision is forced to stateless-only in a future PR. Stdio servers and current-protocol stateful Streamable HTTP servers continue to work via the legacy server-to-client `roots/list` request flow. For code that needs to run on stateless servers — including all `DRAFT-2026-v1` Streamable HTTP servers going forward — throw `InputRequiredException` from your handler instead. It works under both protocols and both session modes.
For example:
```csharp
[McpServerTool, Description("Tool that requests roots via MRTR")]
public static string ListRootsWithMrtr(
McpServer server,
RequestContext<CallToolRequestParams> context)
{
// On retry, process the client's roots response
if (context.Params!.InputResponses?.TryGetValue("get_roots", out var response) is true)
{
var roots = response.Deserialize(InputResponse.ListRootsResultJsonTypeInfo)?.Roots ?? [];
return $"Found {roots.Count} roots: {string.Join(", ", roots.Select(r => r.Uri))}";
}
if (!server.IsMrtrSupported)
{
return "This tool requires MRTR support (DRAFT-2026-v1, or a stateful current-protocol session).";
}
// First call — request the client's root list
throw new InputRequiredException(
inputRequests: new Dictionary<string, InputRequest>
{
["get_roots"] = InputRequest.ForRootsList(new ListRootsRequestParams())
},
requestState: "awaiting-roots");
}
```
> [!TIP]
> See [Multi Round-Trip Requests (MRTR)](xref:mrtr) for the full protocol details, including load shedding, multiple round trips, and the compatibility matrix.