Client Requests (MRTR)

The Multi Round-Trip Request surface: how a tools/call or prompts/get handler asks the client for more input mid-call — elicitation, sampling, roots — without the server keeping any round-trip state between calls (your requestState travels via the client; the transport's ordinary protocol session is unaffected). The concept walkthrough is in Tools; this page is the API.

Answering input_required

function MCPInputRequired(AInputRequests: TJSONObject;
  const ARequestState: string = ''): TMCPToolResult;
function MCPPromptInputRequired(AInputRequests: TJSONObject;
  const ARequestState: string = ''): TMCPPromptResult;

AInputRequests maps your keys to request entries (built below); the client fulfils them and retries the original call with the responses. ARequestState is your opaque round-trip state, echoed verbatim by the client — treat it as attacker-controlled input on re-entry; integrity protection is your handler's job when the state influences authorization or business logic.

Request entry builders

One builder per request kind:

// Elicitation: a form the client renders...
function MCPElicitFormRequest(const AMessage: string;
  ARequestedSchema: TJSONObject): TJSONObject;
function MCPElicitFormRequest(const AMessage: string;
  constref ASchema: TMCPSchema): TJSONObject;

// ...or a URL the client sends the user to
function MCPElicitURLRequest(const AMessage, AUrl: string): TJSONObject;

// Sampling: ask the client's model for a completion
function MCPSamplingRequest(ASamplingParams: TJSONObject): TJSONObject;
function MCPSamplingTextRequest(const APrompt: string;
  AMaxTokens: Integer): TJSONObject;

// Roots: ask for the client's filesystem roots
function MCPRootsRequest: TJSONObject;

The form-request schema uses the same builders as tool schemas (ObjectSchema... or a raw TJSONObject). MCPSamplingTextRequest is the one-prompt convenience over MCPSamplingRequest's full params object.

Each kind is gated on the per-request client capabilities: requesting a kind the client did not declare answers -32021 instead of reaching the client.

Reading the responses on re-entry

On the retry, the responses are exposed on the request context:

function MCPInputResponse(const ACtx: TMCPRequestContext;
  const AKey: string): TJSONObject;
function MCPElicitationContent(const ACtx: TMCPRequestContext;
  const AKey: string): TJSONObject;

MCPInputResponse returns the raw response entry for your key (nil when this is not a retry or the key is absent) — the pattern for distinguishing round 1 from round 2. MCPElicitationContent goes one step further for elicitation entries, returning the accepted form content directly. The raw payload is also available as ACtx.InputResponses / ACtx.RequestState.

Era and capability notes

MRTR is modern-era (2026-07-28) only — a classic-handshake client calling an MRTR-answering tool or prompt receives a JSON-RPC error naming the limitation. Sampling and roots are deprecated in the final spec (SEP-2577) but deliberately carried; prefer elicitation where either would do. Spec grounding and interop evidence: Architecture.