client.ts

/**
 * Asserts the `responseMode: 'json'` server answers a `tools/call` with a
 * `Content-Type: application/json` body (not `text/event-stream`) AND that the
 * regular `Client` works against it unchanged.
 *
 * HTTP-only, modern-only — `responseMode` shapes the 2026-07-28 per-request
 * HTTP path; there is no stdio equivalent and 2025-era traffic goes through
 * the stateless legacy fallback unaffected.
 */
import { check, parseExampleArgs } from '@mcp-examples/shared';
import { Client, isJsonContentType, StreamableHTTPClientTransport } from '@modelcontextprotocol/client';

const { url } = parseExampleArgs();

const client = new Client({ name: 'json-response-example-client', version: '1.0.0' }, { versionNegotiation: { mode: 'auto' } });

await client.connect(new StreamableHTTPClientTransport(new URL(url)));

// Low-level: a 2026-07-28 (envelope) request should come back as plain JSON —
// the JSON content-type assertion is the point of the story.
const probe = await fetch(url, {
    method: 'POST',
    headers: {
        'content-type': 'application/json',
        accept: 'application/json, text/event-stream',
        'mcp-protocol-version': '2026-07-28',
        'mcp-method': 'tools/list'
    },
    body: JSON.stringify({
        jsonrpc: '2.0',
        id: 1,
        method: 'tools/list',
        params: {
            _meta: {
                'io.modelcontextprotocol/protocolVersion': '2026-07-28',
                'io.modelcontextprotocol/clientInfo': { name: 'probe', version: '1.0.0' },
                'io.modelcontextprotocol/clientCapabilities': {}
            }
        }
    })
});
check.equal(isJsonContentType(probe.headers.get('content-type')), true);
check.equal(probe.status, 200);

// High-level: the regular Client works unchanged.
const result = await client.callTool({ name: 'greet', arguments: { name: 'json' } });
check.equal(result.content?.[0]?.type === 'text' ? result.content[0].text : '', 'Hello, json!');

await client.close();