ae36026695Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: bb4b44f0-f505-49c5-9878-76e2b40c7a15 Copilot-Session: 8eac1204-10ae-4319-9333-aee5dd43dcc8
284 lines
15 KiB
Markdown
284 lines
15 KiB
Markdown
---
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title: Multi Round-Trip Requests (MRTR)
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author: halter73
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description: How servers request client input during tool execution using Multi Round-Trip Requests.
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uid: mrtr
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---
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# Multi Round-Trip Requests (MRTR)
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Multi Round-Trip Requests (MRTR) let a server tool request input from the client — such as [elicitation](xref:elicitation), [sampling](xref:sampling), or [roots](xref:roots) — as part of a single tool call, without requiring a separate server-to-client JSON-RPC request for each interaction. Instead of returning a final result, the server returns an **incomplete result** containing one or more input requests. The client fulfills those requests and retries the original tool call with the responses attached.
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## Overview
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MRTR is useful when:
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- A tool needs user confirmation before proceeding (elicitation).
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- A tool needs LLM reasoning from the client (sampling).
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- A tool needs an updated list of client roots.
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- A tool needs to perform multiple rounds of interaction in a single logical operation.
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- A stateless server needs to orchestrate multi-step flows without keeping handler state in memory between rounds.
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## How MRTR works
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1. The client calls a tool on the server via `tools/call`.
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2. The server tool determines it needs client input and returns an `InputRequiredResult` containing `inputRequests` and/or `requestState`.
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3. The client resolves each input request (for example, by prompting the user for elicitation, calling an LLM for sampling, or listing its roots).
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4. The client retries the original `tools/call` with `inputResponses` (keyed to the input requests) and `requestState` echoed back.
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5. The server processes the responses and either returns a final result or another `InputRequiredResult` for additional rounds.
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## Opting in
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MRTR activates when both peers negotiate protocol revision **`2026-07-28`**. The C# SDK client prefers `2026-07-28` by default — it probes with `server/discover` and falls back to an `initialize` handshake only when the server doesn't support it. Stateless HTTP servers accept `2026-07-28` automatically when a client offers it; HTTP servers configured with `Stateless = false` refuse that revision with `UnsupportedProtocolVersion` so dual-path clients can fall back to a session-capable revision. No experimental flags are required; pinning `ProtocolVersion` to an initialize-capable revision opts back out.
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```csharp
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// Client — the SDK prefers 2026-07-28 (and therefore MRTR) by default.
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var clientOptions = new McpClientOptions
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{
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Handlers = new McpClientHandlers
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{
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ElicitationHandler = HandleElicitationAsync,
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SamplingHandler = HandleSamplingAsync,
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}
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};
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```
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Under `2026-07-28`, MRTR is the recommended way to obtain client input from a server handler. The spec removes the legacy server-to-client `elicitation/create`, `sampling/createMessage`, and `roots/list` request methods, so any code that needs to work on a `2026-07-28` Streamable HTTP server (where Streamable HTTP no longer supports sessions) must use `InputRequiredException` rather than <xref:ModelContextProtocol.Server.McpServer.ElicitAsync*>, <xref:ModelContextProtocol.Server.McpServer.SampleAsync*>, or <xref:ModelContextProtocol.Server.McpServer.RequestRootsAsync*>. The legacy methods still work on stateful sessions — including `2026-07-28` stdio sessions — but they throw `InvalidOperationException("X is not supported in stateless mode.")` on every stateless session.
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Under `2025-11-25` and earlier, `InputRequiredException` is still supported in stateful sessions via a backward-compatibility resolver — see the [Compatibility](#compatibility) section.
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## Authoring an MRTR tool
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A tool participates in MRTR by throwing <xref:ModelContextProtocol.Protocol.InputRequiredException> with an <xref:ModelContextProtocol.Protocol.InputRequiredResult> describing what it needs. On retry, the client's responses arrive on the request parameters and the tool inspects them to decide what to do next.
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### Checking MRTR support
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Tools should check <xref:ModelContextProtocol.Server.McpServer.IsMrtrSupported> before throwing `InputRequiredException`. The property returns `true` when either:
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- The negotiated protocol revision is `2026-07-28` (MRTR is native), or
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- The session is stateful under protocol revision `2025-11-25` (the SDK can resolve input requests via legacy JSON-RPC and retry the handler).
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```csharp
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[McpServerTool, Description("A tool that uses MRTR")]
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public static string MyTool(
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McpServer server,
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RequestContext<CallToolRequestParams> context)
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{
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if (!server.IsMrtrSupported)
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{
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return "This tool requires a client that negotiates 2026-07-28, "
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+ "or a stateful session using protocol revision 2025-11-25.";
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}
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// ... MRTR logic
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}
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```
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### Returning an incomplete result
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Throw <xref:ModelContextProtocol.Protocol.InputRequiredException> to return an incomplete result. The exception carries an <xref:ModelContextProtocol.Protocol.InputRequiredResult> containing `inputRequests` and/or `requestState`:
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```csharp
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[McpServerTool, Description("Tool managing its own MRTR flow")]
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public static string AnswerTool(
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McpServer server,
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RequestContext<CallToolRequestParams> context,
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[Description("The user's question")] string question)
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{
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var requestState = context.Params!.RequestState;
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var inputResponses = context.Params!.InputResponses;
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// On retry, process the client's responses
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if (requestState is not null && inputResponses is not null)
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{
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var elicitResult = inputResponses["user_answer"].Deserialize(InputResponse.ElicitResultJsonTypeInfo);
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return $"You answered: {elicitResult?.Content?.FirstOrDefault().Value}";
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}
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if (!server.IsMrtrSupported)
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{
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return "MRTR is not supported by this client.";
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}
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// First call — request user input
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throw new InputRequiredException(
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inputRequests: new Dictionary<string, InputRequest>
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{
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["user_answer"] = InputRequest.ForElicitation(new ElicitRequestParams
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{
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Message = $"Please answer: {question}",
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RequestedSchema = new()
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{
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Properties = new Dictionary<string, ElicitRequestParams.PrimitiveSchemaDefinition>
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{
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["answer"] = new ElicitRequestParams.StringSchema
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{
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Description = "Your answer"
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}
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}
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}
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})
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},
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requestState: "awaiting-answer");
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}
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```
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### Accessing retry data
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When the client retries a tool call, the retry data is available on the request parameters:
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- <xref:ModelContextProtocol.Protocol.RequestParams.InputResponses> — a dictionary of client responses keyed by the same keys used in `inputRequests`.
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- <xref:ModelContextProtocol.Protocol.RequestParams.RequestState> — the opaque state string echoed back by the client.
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Use <xref:ModelContextProtocol.Protocol.InputResponse.Deserialize*> with the `JsonTypeInfo<T>` matching the response type. The expected type follows from the matching <xref:ModelContextProtocol.Protocol.InputRequest.Method> in the original `inputRequests` map — there is no on-the-wire discriminator.
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| Input | Deserialize call |
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|-------------|-----------------------------------------------------------------------|
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| Elicitation | `response.Deserialize(InputResponse.ElicitResultJsonTypeInfo)` |
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| Sampling | `response.Deserialize(InputResponse.CreateMessageResultJsonTypeInfo)` |
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| Roots list | `response.Deserialize(InputResponse.ListRootsResultJsonTypeInfo)` |
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### Load shedding with requestState-only responses
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A server can return a `requestState`-only incomplete result (without any `inputRequests`) to defer processing. This is useful for load shedding or breaking up long-running work across multiple requests:
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```csharp
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[McpServerTool, Description("Tool that defers work using requestState")]
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public static string DeferredTool(
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McpServer server,
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RequestContext<CallToolRequestParams> context)
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{
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var requestState = context.Params!.RequestState;
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if (requestState is not null)
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{
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// Resume deferred work
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var state = JsonSerializer.Deserialize<MyState>(
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Convert.FromBase64String(requestState));
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return $"Completed step {state!.Step}";
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}
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if (!server.IsMrtrSupported)
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{
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return "MRTR is not supported by this client.";
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}
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// Defer work to a later retry
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var initialState = new MyState { Step = 1 };
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throw new InputRequiredException(
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requestState: Convert.ToBase64String(
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JsonSerializer.SerializeToUtf8Bytes(initialState)));
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}
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```
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The client automatically retries `requestState`-only incomplete results, echoing the state back without needing to resolve any input requests.
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### Multiple round trips
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A tool can perform multiple rounds of interaction by throwing `InputRequiredException` multiple times across retries. Use `requestState` to track which round you're on:
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```csharp
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[McpServerTool, Description("Multi-step wizard")]
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public static string WizardTool(
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McpServer server,
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RequestContext<CallToolRequestParams> context)
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{
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var requestState = context.Params!.RequestState;
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var inputResponses = context.Params!.InputResponses;
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if (requestState == "step-2" && inputResponses is not null)
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{
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var name = inputResponses["name"].Deserialize(InputResponse.ElicitResultJsonTypeInfo)?.Content?.FirstOrDefault().Value;
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var age = inputResponses["age"].Deserialize(InputResponse.ElicitResultJsonTypeInfo)?.Content?.FirstOrDefault().Value;
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return $"Welcome, {name}! You are {age} years old.";
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}
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if (requestState == "step-1" && inputResponses is not null)
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{
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var name = inputResponses["name"].Deserialize(InputResponse.ElicitResultJsonTypeInfo)?.Content?.FirstOrDefault().Value;
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// Second round — ask for age
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throw new InputRequiredException(
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inputRequests: new Dictionary<string, InputRequest>
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{
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["age"] = InputRequest.ForElicitation(new ElicitRequestParams
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{
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Message = $"Hi {name}! How old are you?",
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RequestedSchema = new()
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{
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Properties = new Dictionary<string, ElicitRequestParams.PrimitiveSchemaDefinition>
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{
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["age"] = new ElicitRequestParams.NumberSchema
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{
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Description = "Your age"
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}
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}
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}
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})
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},
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requestState: "step-2");
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}
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if (!server.IsMrtrSupported)
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{
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return "MRTR is not supported. Please use a compatible client.";
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}
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// First round — ask for name
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throw new InputRequiredException(
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inputRequests: new Dictionary<string, InputRequest>
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{
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["name"] = InputRequest.ForElicitation(new ElicitRequestParams
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{
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Message = "What's your name?",
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RequestedSchema = new()
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{
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Properties = new Dictionary<string, ElicitRequestParams.PrimitiveSchemaDefinition>
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{
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["name"] = new ElicitRequestParams.StringSchema
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{
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Description = "Your name"
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}
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}
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}
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})
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},
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requestState: "step-1");
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}
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```
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### Providing custom error messages
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When MRTR is not supported, you can provide domain-specific guidance:
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```csharp
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if (!server.IsMrtrSupported)
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{
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return "This tool requires interactive input. To use it:\n"
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+ "1. Connect with a client that negotiates MCP protocol revision 2026-07-28, or\n"
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+ "2. Use a stateful session using protocol revision 2025-11-25 so the server can resolve the input requests for you.\n"
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+ "\nStateless sessions using protocol revision 2025-11-25 and earlier cannot resolve MRTR input requests.";
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}
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```
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## Compatibility
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The SDK supports `InputRequiredException` across two protocol revisions and two session modes:
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| Negotiated protocol | Session mode | Behavior |
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|----------------------------------|--------------|-----------------------------------------------------------------------------------------------------------------------------------------------------|
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| `2026-07-28` | Stateful | Native MRTR — `InputRequiredResult` is serialized directly to the wire. |
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| `2026-07-28` | Stateless | Native MRTR — `InputRequiredResult` is serialized directly to the wire. No server-side handler state needed. |
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| `2025-11-25` and earlier | Stateful | Backward-compatibility resolver — the SDK sends standard `elicitation/create` / `sampling/createMessage` / `roots/list` JSON-RPC requests to the client, collects the responses, and retries the handler with `inputResponses` populated. Up to 10 retry rounds. |
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| `2025-11-25` and earlier | Stateless | **Not supported** — `InputRequiredException` raises an `McpException`. The client doesn't speak MRTR, and the server can't resolve input requests via JSON-RPC without a persistent session. |
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> [!NOTE]
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> The backcompat resolver is intentionally limited to 10 retry rounds. Tools that need more rounds should require `2026-07-28` (check `IsMrtrSupported`).
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### Why `ElicitAsync` / `SampleAsync` / `RequestRootsAsync` throw on stateless servers
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`ElicitAsync` / `SampleAsync` / `RequestRootsAsync` issue a JSON-RPC request to the client and wait for the response on the same session. Stateless servers don't have a persistent session to wait on, so the SDK fails fast with `InvalidOperationException("X is not supported in stateless mode.")` (the check is `McpServer.ClientCapabilities is null`, which is the SDK's proxy for stateless).
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Under `2025-11-25` and earlier, stdio and stateful Streamable HTTP keep `ClientCapabilities` populated, so the legacy methods work normally and remain the recommended way to do one-shot client interactions. Under `2026-07-28`, the spec removes those request methods from Streamable HTTP entirely; the SDK still allows the legacy methods on `2026-07-28` stdio sessions because stdio is implicitly single-process / stateful and the client handler is wired up regardless of negotiated revision. `InputRequiredException` is the way to write tools that work on every supported configuration.
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Because `2026-07-28` removes `Mcp-Session-Id` (SEP-2567) and the `initialize` handshake (SEP-2575), Streamable HTTP can serve that revision only through the stateless path. The `Stateful` row for `2026-07-28` in the compatibility matrix above therefore applies to stdio and other non-HTTP stateful sessions; an HTTP server explicitly set to `Stateless = false` refuses `2026-07-28` with `UnsupportedProtocolVersion` and creates a session only when an older client falls back to `initialize`.
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