Build your first client

Build an MCP client — the program that launches a server, lists its tools, and calls them — against the weather server from Build your first server.

Connect to a server

In the weather project, add the client package — it ships separately from @modelcontextprotocol/server.

npm install @modelcontextprotocol/client

Create src/client.ts. A Client plus one transport is a complete MCP client.

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

const client = new Client({ name: 'my-first-client', version: '1.0.0' });

const transport = new StdioClientTransport({
    command: 'npx',
    args: ['tsx', 'src/index.ts']
});

await client.connect(transport);

connect() spawns npx tsx src/index.ts as a child process, speaks JSON-RPC over its stdin and stdout, and completes the initialize handshake. The client owns that process from here: it lives exactly as long as the transport.

TIP

Never start src/index.ts yourself — connect() does. A spawn npx ENOENT error here means command is not an executable on your PATH.

stdio is the transport local hosts use; Connect to a server covers HTTP for remote servers.

List the server's tools

listTools returns every tool the server registered, with the JSON Schema it derived for each one's arguments.

const { tools } = await client.listTools();
for (const tool of tools) {
    console.log(tool.name, '—', tool.description);
}

Run what you have so far — npx tsx src/client.ts from the project root. The first line is the server's banner, forwarded from the child's stderr; the second is your loop.

weather MCP server running on stdio
get-alerts — Get the active weather alerts for a US state

The script does not exit on its own — the client still owns a live server process. Stop it with Ctrl+C for now; Close the connection ends it properly.

Call a tool

callTool takes the tool's name and an arguments object that must satisfy its inputSchema.

const result = await client.callTool({ name: 'get-alerts', arguments: { state: 'CA' } });

for (const block of result.content) {
    if (block.type === 'text') console.log(block.text);
}

A tool result is a list of typed content blocks; get-alerts returns one text block. Its text is the live answer from the National Weather Service — one headline per active California alert, or No active alerts for CA. when there are none — so your output differs from anyone else's.

A handler that throws, and arguments the inputSchema rejects, come back in this same shape with isError: true set. A tool name the server never registered is a protocol-level failure, and that one does throw out of await callTool — Errors draws the line.

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Change the argument to { state: 'California' } and the SDK rejects it before the handler (and the network request inside it) ever runs:

Input validation error: Invalid arguments for tool get-alerts: state: Too big: expected string to have <=2 characters

The rejection is an ordinary isError: true result, so a model reads the message and retries with arguments that fit.

Add a resource and read it

The weather server registers no resources yet — a resource is data a client reads by URI, where a tool is an action it invokes. In src/index.ts, register one above the return server line.

server.registerResource('about', 'weather://about', { title: 'About this server', mimeType: 'text/plain' }, async uri => ({
    contents: [{ uri: uri.href, text: 'Alert data comes from the US National Weather Service.' }]
}));

The read handler returns contents — a list, because one read can return several text or binary parts. Resources covers templates, binary contents, and subscriptions.

Back in src/client.ts, list the resources and read the new one by its uri.

const { resources } = await client.listResources();
console.log(resources);

const { contents } = await client.readResource({ uri: 'weather://about' });
console.log(contents);

Run it again. After the lines you already have, the two new logs are:

[
  {
    name: 'about',
    title: 'About this server',
    uri: 'weather://about',
    mimeType: 'text/plain'
  }
]
[
  {
    uri: 'weather://about',
    text: 'Alert data comes from the US National Weather Service.'
  }
]

listResources advertises the metadata you registered; readResource returns the handler's contents unchanged.

Close the connection

End the file with close.

await client.close();

close() ends the spawned server's stdin and kills the process if it does not exit on its own. Run the finished script once more: it prints everything above and exits without Ctrl+C.

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In a client that can throw between connect and close, put close() in a finally block — otherwise a crash leaves the server process running.

Hand the tool list to a model

Nothing on this page calls a model. The handoff is listTools(): each entry's name, description, and inputSchema — plain JSON Schema — map one-to-one onto the tool definition every tool-calling LLM API takes. Send the conversation with that list; when the model returns a tool call, pass its name and arguments to callTool unchanged and append result.content as the tool result.

A host — an application with a model in it — runs that loop for you, through a client of its own. Plug into a real host registers the weather server in VS Code, Claude Code, and Cursor with no client code; examples/cli-client in the SDK repository is a complete, provider-neutral host built from the calls on this page.

Recap

• A Client plus one transport is a complete MCP client; connect() runs the initialize handshake.
• StdioClientTransport spawns and owns the server process — never start it yourself.
• listTools, callTool, listResources, and readResource are the client verbs; each returns a typed result.
• A failed handler or rejected arguments come back as an ordinary result with isError: true set.
• close() tears down the transport and the spawned process.
• A model consumes listTools() output unchanged: name, description, inputSchema.