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Default draft protocol support: sessionless + handshake-less (SEP-2575 + SEP-2567) (#1610)

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Stephen Halter 2026-06-23 21:24:22 -07:00 • committed by GitHub
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docs/concepts/mrtr/mrtr.md
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@ -9,7 +9,7 @@ uid: mrtr
<!-- mlc-disable-next-line -->
> [!WARNING]
> MRTR is part of the **`DRAFT-2026-v1`** revision of the MCP specification ([SEP-2322](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2322)). The wire format and API surface may change before the revision is ratified. See the [Experimental APIs](../../experimental.md) documentation for details on working with experimental APIs.
> MRTR is part of the **`2026-07-28`** revision of the MCP specification ([SEP-2322](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2322)). The wire format and API surface may change before the revision is ratified. See the [Experimental APIs](../../experimental.md) documentation for details on working with experimental APIs.
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.
@ -33,13 +33,12 @@ MRTR is useful when:
## Opting in
MRTR activates when both peers negotiate protocol revision **`DRAFT-2026-v1`** during `initialize`. The C# SDK opts in by listing `DRAFT-2026-v1` as a supported protocol version on the client; servers automatically accept it when offered. No experimental flags are required.
MRTR activates when both peers negotiate protocol revision **`2026-07-28`**. The C# SDK client prefers the draft revision by default — it probes with `server/discover` and falls back to a legacy `initialize` handshake only when the server doesn't support draft. Servers accept the draft automatically when a client offers it. No experimental flags are required; pinning `ProtocolVersion` to a legacy revision opts back out.
```csharp
// Client
// Client — the SDK prefers the 2026-07-28 draft (and therefore MRTR) by default.
var clientOptions = new McpClientOptions
{
ProtocolVersion = "DRAFT-2026-v1",
Handlers = new McpClientHandlers
{
ElicitationHandler = HandleElicitationAsync,
@ -48,7 +47,7 @@ var clientOptions = new McpClientOptions
};
```
Under `DRAFT-2026-v1`, 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 `DRAFT-2026-v1` Streamable HTTP server (which will be stateless-only in a future revision) 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 — that's how stdio servers keep working under draft today — but they throw `InvalidOperationException("X is not supported in stateless mode.")` on any stateless session, current or draft.
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 (which is stateless-only under the draft revision) 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 — that's how stdio servers keep working under draft today — but they throw `InvalidOperationException("X is not supported in stateless mode.")` on any stateless session, current or draft.
Under the current protocol revision (`2025-06-18` and earlier), `InputRequiredException` is still supported in stateful sessions via a backward-compatibility resolver — see [Compatibility](#compatibility) below.
@ -60,7 +59,7 @@ A tool participates in MRTR by throwing <xref:ModelContextProtocol.Protocol.Inpu
Tools should check <xref:ModelContextProtocol.Server.McpServer.IsMrtrSupported> before throwing `InputRequiredException`. It returns `true` when either:
- The negotiated protocol revision is `DRAFT-2026-v1` (MRTR is native), or
- The negotiated protocol revision is `2026-07-28` (MRTR is native), or
- The session is stateful under the current protocol (the SDK can resolve input requests via legacy JSON-RPC and retry the handler).
```csharp
@ -71,7 +70,7 @@ public static string MyTool(
{
if (!server.IsMrtrSupported)
{
return "This tool requires a client that negotiates DRAFT-2026-v1, "
return "This tool requires a client that negotiates 2026-07-28, "
+ "or a stateful current-protocol session.";
}
@ -258,7 +257,7 @@ When MRTR is not supported, you can provide domain-specific guidance:
if (!server.IsMrtrSupported)
{
return "This tool requires interactive input. To use it:\n"
+ "1. Connect with a client that negotiates MCP protocol revision DRAFT-2026-v1, or\n"
+ "1. Connect with a client that negotiates MCP protocol revision 2026-07-28, or\n"
+ "2. Use a stateful current-protocol session so the server can resolve the input requests for you.\n"
+ "\nStateless current-protocol sessions cannot resolve MRTR input requests.";
}
@ -270,22 +269,18 @@ The SDK supports `InputRequiredException` across two protocol revisions and two
| Negotiated protocol | Session mode | Behavior |
|---|---|---|
| `DRAFT-2026-v1` | Stateful | Native MRTR — `InputRequiredResult` is serialized directly to the wire. |
| `DRAFT-2026-v1` | Stateless | Native MRTR — `InputRequiredResult` is serialized directly to the wire. No server-side handler state needed. |
| `2026-07-28` | Stateful | Native MRTR — `InputRequiredResult` is serialized directly to the wire. |
| `2026-07-28` | Stateless | Native MRTR — `InputRequiredResult` is serialized directly to the wire. No server-side handler state needed. |
| Current (`2025-06-18` 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. |
| Current (`2025-06-18` 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. |
> [!NOTE]
> The backcompat resolver is intentionally limited to 10 retry rounds. Tools that need more rounds should require `DRAFT-2026-v1` (check `IsMrtrSupported`).
> The backcompat resolver is intentionally limited to 10 retry rounds. Tools that need more rounds should require `2026-07-28` (check `IsMrtrSupported`).
### Why `ElicitAsync` / `SampleAsync` / `RequestRootsAsync` throw on stateless servers
`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).
Under the current protocol revision (`2025-06-18` 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 `DRAFT-2026-v1`, the spec removes those request methods from Streamable HTTP entirely; the SDK still allows the legacy methods on draft 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.
Under the current protocol revision (`2025-06-18` 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 draft 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.
### Future direction
The `DRAFT-2026-v1` revision is moving toward a stateless-only model: `Mcp-Session-Id` is being removed, and Streamable HTTP servers will run statelessly by default under the draft revision. When that lands, the `Stateful` row for `DRAFT-2026-v1` in the compatibility matrix above collapses into the `Stateless` row (Streamable HTTP under draft becomes stateless-only), and `InputRequiredException` becomes uniformly required for non-stdio servers. The current-protocol resolver path will remain for backward compatibility with older clients and stateful servers.
This work is a follow-up to the present PR.
Because `2026-07-28` removes `Mcp-Session-Id` (SEP-2567) and the `initialize` handshake (SEP-2575), Streamable HTTP runs statelessly whenever a client speaks the draft. The `Stateful` row for `2026-07-28` in the compatibility matrix above therefore applies only to stdio — a server explicitly set to `Stateless = false` still serves draft requests sessionlessly and creates a legacy session only when an older client falls back to `initialize`.