Handle requests from the server

Declare what your client can do

Declare each capability in the Client constructor's options — a server only sends your client a request it declared a capability for, and the SDK enforces that on both sides.

import { Client } from '@modelcontextprotocol/client';

const client = new Client(
    { name: 'my-client', version: '1.0.0' },
    {
        capabilities: {
            sampling: {},
            elicitation: { form: {}, url: {} }
        }
    }
);

Every result quoted on this page comes from this client connected over an in-memory transport pair to a server whose tools elicit input and request sampling. Test a server shows that wiring; Elicitation and Sampling show the server side.

TIP

An empty elicitation: {} declares form mode only — url must be listed explicitly. getSupportedElicitationModes, exported from @modelcontextprotocol/client, turns any elicitation capability object into { supportsFormMode, supportsUrlMode }.

Handle an elicitation request

A tool that calls elicitInput sends your client an elicitation/create request. Branch on request.params.mode: 'url' carries a URL to open in the user's browser, and anything else is a form your client builds from request.params.requestedSchema.

client.setRequestHandler('elicitation/create', async request => {
    if (request.params.mode === 'url') {
        // Open request.params.url in the user's browser; answer when they finish.
        return { action: 'accept' };
    }
    // Render request.params.requestedSchema as a form; return what the user entered.
    return { action: 'accept', content: { city: 'Lisbon' } };
});

action is the user's decision: 'accept' carries the submitted content, 'decline' and 'cancel' carry nothing. Calling a tool that asks where to ship an order now round-trips through the form branch:

[ { type: 'text', text: 'Order placed: Travel mug ships to Lisbon.' } ]

TIP

Form requests sent before mode existed omit it entirely — branch on 'url' and treat everything else as a form, never on mode === 'form'.

Handle a sampling request

Deprecated — SEP-2577

Servers should call their LLM provider directly instead of sampling — see Sampling. Keep this handler to support servers that have not migrated yet.

A tool that calls requestSampling sends your client a sampling/createMessage request: a list of messages to run through a model your application controls.

client.setRequestHandler('sampling/createMessage', async request => {
    const lastMessage = request.params.messages.at(-1);
    console.log('Sampling request:', lastMessage?.content);

    // In production, run the messages through your model here.
    return {
        model: 'host-model',
        role: 'assistant',
        content: { type: 'text', text: 'One travel mug to Lisbon.' }
    };
});

Calling a tool that summarizes the order logs the prompt the server sent, and the tool result carries the handler's completion:

Sampling request: { type: 'text', text: 'Summarize this order: 1 Travel mug to Lisbon' }
[ { type: 'text', text: 'host-model: One travel mug to Lisbon.' } ]

Register each handler once

Register each handler once, on the Client you construct. The same handler answers a request the server pushes to your client and a request the SDK fulfils for you inside a callTool() round — your code never sees the difference.

INFO

Which of those two delivery paths a connection uses depends on its protocol version — see Protocol versions.

Cap or disable automatic fulfilment

When the SDK fulfils requests inside a call, the inputRequired option caps how many rounds it runs on your behalf.

const client = new Client(
    { name: 'my-client', version: '1.0.0' },
    {
        capabilities: { sampling: {}, elicitation: { form: {}, url: {} } },
        inputRequired: { maxRounds: 3 }
    }
);

Past maxRounds (default 10) the call rejects with an SdkError coded INPUT_REQUIRED_ROUNDS_EXCEEDED. Set autoFulfill: false to turn the loop off entirely: a call that needs input rejects on its first round instead, and the round trips are yours to drive.

Recap

• Declare a capability in the Client constructor or the server never sends that request.
• setRequestHandler('elicitation/create') branches on mode and returns the user's action, plus content on accept.
• setRequestHandler('sampling/createMessage') runs the messages through your model and returns { model, role, content }.
• Register each handler once; it answers the request however the connection delivers it.
• inputRequired caps the automatic interactive rounds; autoFulfill: false disables them.