server-client-communication.md

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
• Sampling
• Logging
• Notification
• Progress

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, you need to use method argument injection for RequestContext.

Every reference of a MCP element, that translates to an actual method call, can just add an type-hinted argument for the RequestContext 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\RequestContext;

class MyService
{
    #[McpTool(name: 'my_tool', description: 'My Tool Description')]
    public function myTool(RequestContext $context): string
    {
        $context->getClientGateway()->log(...);

The same object also carries the protocol revision negotiated for the current request, which is useful when a feature is only available from a certain revision on:

use Mcp\Schema\Enum\ProtocolVersion;

if ($context->getProtocolVersion()->isAtLeast(ProtocolVersion::V2026_07_28)) {
    // e.g. a bare list is only valid as `structuredContent` from this revision on
}

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 SamplingMessage instances.
2. maxTokens, which defaults to 1000
3. timeout in seconds, which defaults to 120
4. options which might include systemPrompt, preferences for model choice, includeContext, temperature, stopSequences, metadata, tools, and toolChoice

Both tools/toolChoice and includeContext are gated on what the client advertised, so check before sending:

if ($clientGateway->supportsSamplingTools()) {
    $result = $clientGateway->sample($messages, options: ['tools' => $tools]);
}

A server must not send tools or toolChoice to a client that did not advertise sampling.tools. The includeContext values other than none are soft-deprecated and should only be sent when the client advertises sampling.context — supportsSamplingContext() reports that one.

Tool loops

When the model wants to call a tool, the result comes back with stopReason: 'toolUse' and one or more ToolUseContent blocks. Execute them, then send a follow-up request with the assistant's message and a user message carrying a matching ToolResultContent for every ToolUseContent:

$messages[] = new SamplingMessage(Role::Assistant, $result->content);
$messages[] = new SamplingMessage(Role::User, [new ToolResultContent($toolUse->id, [new TextContent($output)])]);

The specification is strict about the shape of that exchange: tool results may not be mixed with other content in a message, and every tool use must be answered before the conversation continues. sample() checks these rules before sending and throws an InvalidArgumentException rather than letting the client reject the request with -32602. Use $result->getContentBlocks() to iterate the response regardless of whether it holds one block or a list.

Find more details to sampling payload in the specification.

Logging

The Logging utility enables servers to send structured log messages as notification 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);