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Connecting to a server

A client is configured once through its builder, then connected to a transport. When you connect, the client and the server agree on a protocol version. After that, the server's capabilities are known and you can use its tools, resources and prompts.

Client Builder

The Client\Builder provides fluent configuration of client instances.

Basic Configuration

use Mcp\Client;

$client = Client::builder()
    ->setClientInfo('My Application', '1.0.0', 'Description of my client')
    ->setInitTimeout(30)      // seconds to wait while connecting (default: 30)
    ->setRequestTimeout(120)  // seconds to wait for each response (default: 120)
    ->setMaxRetries(3)        // retries for a failed connection (default: 3)
    ->build();

Connection Retries

setMaxRetries() controls how often connect() retries a failed connection. It counts retries rather than attempts, so the default of 3 means one initial attempt plus up to three retries — four in total — before the ConnectionException of the last attempt is rethrown:

$client = Client::builder()
    ->setMaxRetries(0)  // Fail on the first failed attempt
    ->build();

Between two attempts the transport is closed, so a retry never reuses a half-established connection: a StdioTransport spawns a fresh server process and an HttpTransport discards the session ID of the failed attempt. Each retry is preceded by a short, linearly growing delay (100ms, 200ms, 300ms, …).

Only the connection handshake is retried. Individual requests such as callTool() are always sent once — retrying them is unsafe as tool calls are not necessarily idempotent.

Client Information

Set the client's identity that is reported to the server:

$client = Client::builder()
    ->setClientInfo(
        name: 'AI Assistant Client',
        version: '2.1.0',
        description: 'Client for automated AI workflows',
        // optional name to display in user interfaces
        title: 'AI Assistant',
    )
    ->build();

Protocol Version

A client speaks both protocol eras by default. It prefers 2026-07-28, probes for it with server/discover when connecting, and falls back to the initialize handshake on 2025-11-25 when the server turns out not to speak it. Use $client->getProtocolVersion() after connecting to read what the connection settled on.

use Mcp\Schema\Enum\ProtocolVersion;

// Fall back to an older handshake revision instead of 2025-11-25…
$client = Client::builder()
    ->setFallbackProtocolVersion(ProtocolVersion::V2025_06_18)
    ->build();

// …or not at all: connect() then throws a ConnectionException
// when the server doesn't speak 2026-07-28
$client = Client::builder()
    ->setFallbackProtocolVersion(null)
    ->build();

Passing a handshake revision to setProtocolVersion() skips the probe and opens with initialize, the way a client from before the modern era would:

$client = Client::builder()
    ->setProtocolVersion(ProtocolVersion::V2025_11_25)
    ->build();

The handshake is an offer, not a demand. A server that does not support the requested revision counter-offers one it does, as described in the specification's protocol version negotiation section. The client accepts any counter-offer it knows about and continues on that revision; a counter-offer the SDK cannot speak fails the handshake with a ConnectionException rather than continuing on a revision neither side agreed on.

See Protocol versions for how the probe is read, and the handshake era for the server side of the exchange.

Capabilities

Declare client capabilities to enable server features:

use Mcp\Schema\ClientCapabilities;

$client = Client::builder()
    ->setCapabilities(new ClientCapabilities(
        elicitation: true, // Let the server ask the user for input
    ))
    ->build();

ClientCapabilities also takes sub-capabilities. Pass them as named arguments:

new ClientCapabilities(
    sampling: true,
    samplingContext: true,  // the server may ask to include context in sampling
    samplingTools: true,    // the server may pass tools in sampling requests
    elicitationForm: true,  // form mode, implied by elicitation: true alone
    elicitationUrl: true,   // url mode, the user continues in a browser
);

Form mode is implied only when you declare no mode. To support both modes, pass both flags.

To declare a protocol extension (e.g. MCP Apps), call enableExtension() on the builder. See Protocol extensions.

Notification Handlers

Register handlers for server-initiated notifications:

use Mcp\Client\Handler\Notification\LoggingNotificationHandler;
use Mcp\Schema\Notification\LoggingMessageNotification;

$loggingHandler = new LoggingNotificationHandler(
    static function (LoggingMessageNotification $notification) {
        $message = \is_string($notification->data) ? $notification->data : json_encode($notification->data);
        echo "[{$notification->level->value}] {$message}\n";
    }
);

$client = Client::builder()
    ->addNotificationHandler($loggingHandler)
    ->build();

!!! note LoggingNotificationHandler is deprecated, together with MCP logging. See Logging notifications.

Request Handlers

Register handlers for server-initiated requests (e.g., elicitation). The same handlers answer a multi round-trip input_required result on a modern revision, where the server returns its ask instead of sending a request:

use Mcp\Client\Handler\Request\ElicitationCallbackInterface;
use Mcp\Client\Handler\Request\ElicitationRequestHandler;
use Mcp\Schema\ClientCapabilities;
use Mcp\Schema\Enum\ElicitAction;
use Mcp\Schema\Request\ElicitRequest;
use Mcp\Schema\Result\ElicitResult;

$elicitationCallback = new class implements ElicitationCallbackInterface {
    public function __invoke(ElicitRequest $request): ElicitResult
    {
        // Ask the user for the requested input and return their answer.
        // Without a user interface, decline the request:
        return new ElicitResult(ElicitAction::Decline);
    }
};

$client = Client::builder()
    // the server only sends elicitation requests if the client declares the capability
    ->setCapabilities(new ClientCapabilities(elicitation: true))
    ->addRequestHandler(new ElicitationRequestHandler($elicitationCallback))
    ->build();

Logger

Configure PSR-3 logging for debugging:

use Monolog\Logger;
use Monolog\Handler\StreamHandler;

$logger = new Logger('mcp-client');
$logger->pushHandler(new StreamHandler('client.log', Logger::DEBUG));

$client = Client::builder()
    ->setLogger($logger)
    ->build();

Connecting to Servers

Establishing Connection

$client->connect($transport);

By default, the connect() method opens the transport and sends a server/discover request. If the server speaks revision 2026-07-28, the connection is ready. If it doesn't, the client falls back to the initialize handshake and offers 2025-11-25, or the revision set with setFallbackProtocolVersion(). A server process that exits fails the connection instead.

With setFallbackProtocolVersion(null), connect() fails instead. See How the client settles on an era.

A client pinned to a handshake revision with setProtocolVersion() skips server/discover and starts with the initialize handshake. See Protocol Version.

!!! warning Always wrap connection in try/catch to handle ConnectionException for failed connections.

Checking Connection State

if ($client->isConnected()) {
    // Client is connected and initialized
}

Disconnecting

$client->disconnect();

Always disconnect when finished to clean up resources:

try {
    $client->connect($transport);
    // ... use the client ...
} finally {
    $client->disconnect();
}

Server Information

After successful connection, retrieve server metadata:

// Get server implementation info
// (null on 2026-07-28 when the server/discover result has no serverInfo)
$serverInfo = $client->getServerInfo();
echo "Server: {$serverInfo?->name} v{$serverInfo?->version}\n";

// Get the capabilities the server declared
if ($client->getServerCapabilities()?->tools) {
    $tools = $client->listTools();
}

// Get server instructions
$instructions = $client->getInstructions();
if ($instructions) {
    echo "Instructions: {$instructions}\n";
}