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Client Communication

MCP supports various ways a server can communicate back to a server on top of the main request-response flow.

Table of Contents

ClientGateway

Every communication back to client is handled using the Mcp\Server\ClientGateway and its dedicated methods per operation. To use the ClientGateway in your code, there are two ways to do so:

1. Method Argument Injection

Every refernce of a MCP element, that translates to an actual method call, can just add an type-hinted argument for the ClientGateway and the SDK will take care to include the gateway in the arguments of the method call:

use Mcp\Capability\Attribute\McpTool;
use Mcp\Server\ClientGateway;

class MyService
{
    #[McpTool('my_tool', 'My Tool Description')]
    public function myTool(ClientGateway $client): string
    {
        $client->log(...);

2. Implementing ClientAwareInterface

Whenever a service class of an MCP element implements the interface Mcp\Server\ClientAwareInterface the setClient method of that class will get called while handling the reference, and in combination with Mcp\Server\ClientAwareTrait this ends up with code like this:

use Mcp\Capability\Attribute\McpTool;
use Mcp\Server\ClientAwareInterface;
use Mcp\Server\ClientAwareTrait;

class MyService implements ClientAwareInterface
{
    use ClientAwareTrait;

    #[McpTool('my_tool', 'My Tool Description')]
    public function myTool(): string
    {
        $this->log(...);

Sampling

With sampling servers can request clients to execute "completions" or "generations" with a language model for them:

$result = $clientGateway->sample('Roses are red, violets are', 350, 90, ['temperature' => 0.5]);

The sample method accepts four arguments:

  1. message, which is required and accepts a string, an instance of Content or an array of SampleMessage instances.
  2. maxTokens, which defaults to 1000
  3. timeout in seconds, which defaults to 120
  4. options which might include system_prompt, preferences for model choice, includeContext, temperature, stopSequences and metadata

Find more details to sampling payload in the specification.

Logging

The Logging utility enables servers to send structured log messages as notifcation to clients:

use Mcp\Schema\Enum\LoggingLevel;

$clientGateway->log(LoggingLevel::Warning, 'The end is near.');

Progress

With a Progress notification a server can update a client while an operation is ongoing:

$clientGateway->progress(4.2, 10, 'Downloading needed images.');

Notification

Lastly, the server can push all kind of notifications, that implement the Mcp\Schema\JsonRpc\Notification interface to the client to:

$clientGateway->notify($yourNotification);