Elicitation

Elicitation lets a tool ask the user for input mid-execution, via the client. The LLM doesn't see the question; the client surfaces it directly to the user. This turns one-shot tool calls into interactive flows — collecting confirmation, missing parameters, credentials (URL mode), etc.

Spec version: current through 2026-07-28 (elicitation is not among the v2 deprecations — sampling/roots/logging are). URL mode is the newer addition (originally 2025-06-18 had only form mode).

2026-07-28 changes the rules on HTTP. The current revision removes HTTP sessions and the server→client elicitation/create request, so ElicitAsync cannot be used on 2026-07-28 Streamable HTTP at all — a server with Stateless = false refuses that revision and serves clients via the legacy initialize fallback. The current-protocol way to ask mid-tool is the multi-round-trip pattern: throw InputRequiredException from the tool (building requests with InputRequest.ForElicitation(...)), and on the client's retried call read context.Params.InputResponses back as an ElicitResult. It works under both protocol revisions and both session modes, including fully stateless HTTP. The ElicitAsync guidance below applies to STDIO and down-level stateful HTTP — see the SDK elicitation docs for the full MRTR pattern.

Two modes

Mode	What it does	When to use
Form (in-band)	Server sends a JSON Schema; client renders a form; user submits values back through the same MCP channel.	Confirmations, missing parameters, structured choices.
URL (out-of-band)	Server sends a URL; client opens it in a browser; user completes the flow there; server checks state separately.	OAuth, payments, anything the MCP channel must not see.

Prerequisite for `ElicitAsync`: stateful transport (down-level revisions)

ElicitAsync requires the server to send a request to the client and wait for a response. That only works on:

STDIO (always).
Stateful HTTP (options.Stateless = false) — which, on 2.x, means down-level clients on an initialize-capable revision (see banner above).

In stateless HTTP — including all 2026-07-28 Streamable HTTP — ElicitAsync throws (InvalidOperationException): there's no transport channel back. Use the multi-round-trip InputRequiredException pattern instead.

Form mode — full example

using System.ComponentModel;
using ModelContextProtocol.Protocol;
using ModelContextProtocol.Server;

[McpServerToolType]
public class BookingTools
{
    [McpServerTool, Description("Books a meeting room. Asks the user for confirmation.")]
    public static async Task<string> BookRoom(
        IMcpServer server,
        [Description("Room name")] string room,
        [Description("Start time (ISO 8601)")] DateTime start,
        CancellationToken ct)
    {
        var elicit = await server.ElicitAsync(new ElicitRequestParams
        {
            Message = $"Confirm booking '{room}' at {start:HH:mm}?",
            RequestedSchema = new ElicitRequestParams.RequestSchema
            {
                Properties = new Dictionary<string, ElicitRequestParams.PrimitiveSchemaDefinition>
                {
                    ["confirm"] = new ElicitRequestParams.BooleanSchema
                    {
                        Description = "Confirm the booking",
                        Default = true
                    },
                    ["notes"] = new ElicitRequestParams.StringSchema
                    {
                        Description = "Optional notes for the booking"
                    }
                }
            }
        }, ct);

        if (elicit.Action != "accept")
            return "Booking cancelled by user.";

        var confirmed = elicit.Content?["confirm"].GetBoolean() ?? false;
        var notes     = elicit.Content?["notes"].GetString() ?? "";

        if (!confirmed)
            return "User declined to confirm.";

        // …perform the booking…
        return $"Booked '{room}' at {start:O}. Notes: {notes}";
    }
}

Schema primitive types

You can build a RequestedSchema from these:

Type	C# class	Notes
String	`StringSchema`	`Default`, `Description`. Add JSON-Schema validation at server side if you need it.
Number	`NumberSchema`	Use for ints and floats.
Boolean	`BooleanSchema`	Renders as a checkbox / toggle.
Single-select enum (untitled)	`UntitledSingleSelectEnumSchema`	List of values; client renders as dropdown/radio.
Single-select enum (titled)	`TitledSingleSelectEnumSchema`	Each value has a display title.
Multi-select enum	`UntitledMultiSelectEnumSchema` / `TitledMultiSelectEnumSchema`	Multi-select dropdown / checkbox group.

Each accepts Description and Default.

Response shape

ElicitResult:

Action — "accept", "reject", or "cancel". Always check this first.
Content — Dictionary<string, JsonElement>? with the user's submitted values. null if the user rejected/cancelled.

Always handle the non-accept paths:

if (elicit.Action == "cancel")
    return "User cancelled. No changes made.";
if (elicit.Action == "reject")
    return "User declined.";
// Action == "accept" → safe to read elicit.Content

URL mode — full example

URL mode is for flows where the user must complete something outside the MCP channel — typically OAuth.

[McpServerTool, Description("Connects the user's GitHub account.")]
public static async Task<string> ConnectGitHub(
    IMcpServer server,
    IOAuthService oauth,
    CancellationToken ct)
{
    var elicitationId = Guid.NewGuid().ToString();
    var authUrl = oauth.BuildAuthorizationUrl(state: elicitationId);

    var result = await server.ElicitAsync(new ElicitRequestParams
    {
        Mode = "url",
        ElicitationId = elicitationId,
        Url = authUrl,
        Message = "Please authorize access to GitHub in the browser window that just opened."
    }, ct);

    if (result.Action != "accept")
        return "Authorization cancelled.";

    // The user has come back. Look up the persisted token by elicitationId.
    var token = await oauth.GetTokenByStateAsync(elicitationId, ct);
    return token is not null ? "Connected." : "Authorization did not complete.";
}

`UrlElicitationRequiredException`

When a tool is blocked on auth (rather than walking the user through it), throw UrlElicitationRequiredException. The client surfaces the URL to the user and the call fails cleanly. Useful for retry-after-auth patterns:

if (!oauth.HasValidToken)
{
    var id = Guid.NewGuid().ToString();
    throw new UrlElicitationRequiredException(
        "Authorization required",
        new[]
        {
            new ElicitRequestParams
            {
                Mode = "url",
                ElicitationId = id,
                Url = oauth.BuildAuthorizationUrl(state: id),
                Message = "Sign in to continue."
            }
        });
}

When NOT to use elicitation

Trivial confirmations the LLM can ask in natural language. If you can phrase "Should I do X?" in your tool's docstring and let the LLM ask, that's lower friction than a modal form.
Branching that the LLM should reason about. Don't replace the LLM's judgment with a form — only elicit for things the LLM literally cannot decide (user secrets, real-time consent, picking from a list only the user knows).
Stateless deployments. Doesn't work — see prerequisite above.

Client capability check

Don't blindly call ElicitAsync. Check first:

if (server.ClientCapabilities?.Elicitation is null)
    return "This client doesn't support elicitation; please pass the value as an argument.";

var elicit = await server.ElicitAsync(...);

This degrades gracefully on older clients.

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