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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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docs/concepts/elicitation/elicitation.md
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@ -170,6 +170,61 @@ Here's an example implementation of how a console application might handle elici
[!code-csharp[](samples/client/Program.cs?name=snippet_ElicitationHandler)]
### 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 `elicitation/create` request method is removed; the recommended way to ask the user for input 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]
> `ElicitAsync` throws `InvalidOperationException("Elicitation is 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 `elicitation/create` 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 elicits via MRTR")]
public static string ElicitWithMrtr(
McpServer server,
RequestContext<CallToolRequestParams> context)
{
// On retry, process the client's elicitation response
if (context.Params!.InputResponses?.TryGetValue("user_input", out var response) is true)
{
var elicitResult = response.Deserialize(InputResponse.ElicitResultJsonTypeInfo);
return elicitResult?.Action == "accept"
? $"User accepted: {elicitResult.Content?.FirstOrDefault().Value}"
: "User declined.";
}
if (!server.IsMrtrSupported)
{
return "This tool requires MRTR support (DRAFT-2026-v1, or a stateful current-protocol session).";
}
// First call — request user input
throw new InputRequiredException(
inputRequests: new Dictionary<string, InputRequest>
{
["user_input"] = InputRequest.ForElicitation(new ElicitRequestParams
{
Message = "Please confirm the action",
RequestedSchema = new()
{
Properties = new Dictionary<string, ElicitRequestParams.PrimitiveSchemaDefinition>
{
["confirm"] = new ElicitRequestParams.BooleanSchema
{
Description = "Confirm the action"
}
}
}
})
},
requestState: "awaiting-confirmation");
}
```
> [!TIP]
> See [Multi Round-Trip Requests (MRTR)](xref:mrtr) for the full protocol details, including multiple round trips, concurrent input requests, and the compatibility matrix.
### URL Elicitation Required Error
When a tool cannot proceed without first completing a URL-mode elicitation (for example, when third-party OAuth authorization is needed), and calling `ElicitAsync` is not practical (for example in [stateless](xref:stateless) mode where server-to-client requests are disabled), the server may throw a <xref:ModelContextProtocol.UrlElicitationRequiredException>. This is a specialized error (JSON-RPC error code `-32042`) that signals to the client that one or more URL-mode elicitations must be completed before the original request can be retried.