[Client] Feat: Implement MCP client component (#192)

* feat(client): add complete MCP client SDK with STDIO and HTTP transports

Add Client, Builder, ClientProtocol, ClientSession, and transport
implementations for communicating with MCP servers. Supports tools, resources, prompts, and real-time progress/logging notifications.

* refactor(examples): reorganize examples into server/ and client/ folders

* feat: add SamplingRequestHandler for handling server sampling requests

* feat: add setLoggingLevel method to control server logging verbosity

* refactor: make progress token dependent on request ID

* docs: Update server IP in usage instructions and add notes on sampling server requirements.

* feat: add complete() method for completion/complete requests

* refactor: properly parse and type initialize result

* refactor: standardize method names

* fix: revert changes to server related components

* fix: revert changes to server related components

* refactor: Restructure client transports and handlers into dedicated namespaces.

* fix: cs and php-stan errors fixes(partial)

* feat: Refactor request handlers to return `Response` or `Error` objects directly, adding a `SamplingException` and error logging for sampling requests.

* feat: Replace `ClientTransportInterface` with `TransportInterface` and move response result deserialization to client methods.

* refactor: remove redundant timeout check for intialize on transports

The processFiber that executes per tick while fiber is suspended already handles timeouts while waiting

* feat: broaden symfony/http-client dependency to support Symfony 5.4 to 8.0

* Update examples/client/README.md

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

* Update examples/client/README.md

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

* refactor: remove outgoing message queue from session and simplify transport send method to directly dispatch messages.

* refactor: Replace ClientSession with ClientState for clearer intent on runtime state management

* refactor: rename connectAndInitialize to connect for consistency

* chore: cs fix

* chore: use SamplingCallbackInterface instead of a closure for sampling request handler

---------

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

src/Client/Builder.php

@@ -0,0 +1,170 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client;
+
+use Mcp\Client;
+use Mcp\Client\Handler\Notification\NotificationHandlerInterface;
+use Mcp\Client\Handler\Request\RequestHandlerInterface;
+use Mcp\Schema\ClientCapabilities;
+use Mcp\Schema\Enum\ProtocolVersion;
+use Mcp\Schema\Implementation;
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
+
+/**
+ * Fluent builder for creating Client instances.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+final class Builder
+{
+    private string $name = 'mcp-php-client';
+    private string $version = '1.0.0';
+    private ?string $description = null;
+    private ?ProtocolVersion $protocolVersion = null;
+    private ?ClientCapabilities $capabilities = null;
+    private int $initTimeout = 30;
+    private int $requestTimeout = 120;
+    private int $maxRetries = 3;
+    private ?LoggerInterface $logger = null;
+
+    /** @var NotificationHandlerInterface[] */
+    private array $notificationHandlers = [];
+
+    /** @var RequestHandlerInterface<mixed>[] */
+    private array $requestHandlers = [];
+
+    /**
+     * Set the client name and version.
+     */
+    public function setClientInfo(string $name, string $version, ?string $description = null): self
+    {
+        $this->name = $name;
+        $this->version = $version;
+        $this->description = $description;
+
+        return $this;
+    }
+
+    /**
+     * Set the protocol version to use.
+     */
+    public function setProtocolVersion(ProtocolVersion $protocolVersion): self
+    {
+        $this->protocolVersion = $protocolVersion;
+
+        return $this;
+    }
+
+    /**
+     * Set client capabilities.
+     */
+    public function setCapabilities(ClientCapabilities $capabilities): self
+    {
+        $this->capabilities = $capabilities;
+
+        return $this;
+    }
+
+    /**
+     * Set initialization timeout in seconds.
+     */
+    public function setInitTimeout(int $seconds): self
+    {
+        $this->initTimeout = $seconds;
+
+        return $this;
+    }
+
+    /**
+     * Set request timeout in seconds.
+     */
+    public function setRequestTimeout(int $seconds): self
+    {
+        $this->requestTimeout = $seconds;
+
+        return $this;
+    }
+
+    /**
+     * Set maximum retry attempts for failed connections.
+     */
+    public function setMaxRetries(int $retries): self
+    {
+        $this->maxRetries = $retries;
+
+        return $this;
+    }
+
+    /**
+     * Set the logger.
+     */
+    public function setLogger(LoggerInterface $logger): self
+    {
+        $this->logger = $logger;
+
+        return $this;
+    }
+
+    /**
+     * Add a notification handler for server notifications.
+     */
+    public function addNotificationHandler(NotificationHandlerInterface $handler): self
+    {
+        $this->notificationHandlers[] = $handler;
+
+        return $this;
+    }
+
+    /**
+     * Add a request handler for server requests (e.g., sampling).
+     *
+     * @param RequestHandlerInterface<mixed> $handler
+     */
+    public function addRequestHandler(RequestHandlerInterface $handler): self
+    {
+        $this->requestHandlers[] = $handler;
+
+        return $this;
+    }
+
+    /**
+     * Build the client instance.
+     */
+    public function build(): Client
+    {
+        $logger = $this->logger ?? new NullLogger();
+
+        $clientInfo = new Implementation(
+            $this->name,
+            $this->version,
+            $this->description,
+        );
+
+        $config = new Configuration(
+            clientInfo: $clientInfo,
+            capabilities: $this->capabilities ?? new ClientCapabilities(),
+            protocolVersion: $this->protocolVersion ?? ProtocolVersion::V2025_06_18,
+            initTimeout: $this->initTimeout,
+            requestTimeout: $this->requestTimeout,
+            maxRetries: $this->maxRetries,
+        );
+
+        $protocol = new Protocol(
+            requestHandlers: $this->requestHandlers,
+            notificationHandlers: $this->notificationHandlers,
+            logger: $logger,
+        );
+
+        return new Client($protocol, $config, $logger);
+    }
+}

src/Client/Configuration.php

@@ -0,0 +1,34 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client;
+
+use Mcp\Schema\ClientCapabilities;
+use Mcp\Schema\Enum\ProtocolVersion;
+use Mcp\Schema\Implementation;
+
+/**
+ * Client configuration holder.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+class Configuration
+{
+    public function __construct(
+        public readonly Implementation $clientInfo,
+        public readonly ClientCapabilities $capabilities,
+        public readonly ProtocolVersion $protocolVersion = ProtocolVersion::V2025_06_18,
+        public readonly int $initTimeout = 30,
+        public readonly int $requestTimeout = 120,
+        public readonly int $maxRetries = 3,
+    ) {
+    }
+}

src/Client/Handler/Notification/LoggingNotificationHandler.php

@@ -0,0 +1,43 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Handler\Notification;
+
+use Mcp\Schema\JsonRpc\Notification;
+use Mcp\Schema\Notification\LoggingMessageNotification;
+
+/**
+ * Handler for logging message notifications from the server.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+class LoggingNotificationHandler implements NotificationHandlerInterface
+{
+    /**
+     * @param callable(LoggingMessageNotification): void $callback
+     */
+    public function __construct(
+        private readonly mixed $callback,
+    ) {
+    }
+
+    public function supports(Notification $notification): bool
+    {
+        return $notification instanceof LoggingMessageNotification;
+    }
+
+    public function handle(Notification $notification): void
+    {
+        \assert($notification instanceof LoggingMessageNotification);
+
+        ($this->callback)($notification);
+    }
+}

src/Client/Handler/Notification/NotificationHandlerInterface.php

@@ -0,0 +1,32 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Handler\Notification;
+
+use Mcp\Schema\JsonRpc\Notification;
+
+/**
+ * Interface for handling notifications from the server.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+interface NotificationHandlerInterface
+{
+    /**
+     * Check if this handler supports the given notification.
+     */
+    public function supports(Notification $notification): bool;
+
+    /**
+     * Handle the notification.
+     */
+    public function handle(Notification $notification): void;
+}

src/Client/Handler/Notification/ProgressNotificationHandler.php

@@ -0,0 +1,52 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Handler\Notification;
+
+use Mcp\Client\State\ClientStateInterface;
+use Mcp\Schema\JsonRpc\Notification;
+use Mcp\Schema\Notification\ProgressNotification;
+
+/**
+ * Internal handler for progress notifications.
+ *
+ * Writes progress data to state for transport to consume and execute callbacks.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ *
+ * @internal
+ */
+class ProgressNotificationHandler implements NotificationHandlerInterface
+{
+    public function __construct(
+        private readonly ClientStateInterface $state,
+    ) {
+    }
+
+    public function supports(Notification $notification): bool
+    {
+        return $notification instanceof ProgressNotification;
+    }
+
+    public function handle(Notification $notification): void
+    {
+        if (!$notification instanceof ProgressNotification) {
+            return;
+        }
+
+        $this->state->storeProgress(
+            (string) $notification->progressToken,
+            $notification->progress,
+            $notification->total,
+            $notification->message,
+        );
+    }
+}

src/Client/Handler/Request/RequestHandlerInterface.php

@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Handler\Request;
+
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Request;
+use Mcp\Schema\JsonRpc\Response;
+
+/**
+ * Interface for handling requests from the server.
+ *
+ * @template TResult
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+interface RequestHandlerInterface
+{
+    /**
+     * Check if this handler supports the given request.
+     */
+    public function supports(Request $request): bool;
+
+    /**
+     * Handle the request and return a response or error.
+     *
+     * @return Response<TResult>|Error
+     */
+    public function handle(Request $request): Response|Error;
+}

src/Client/Handler/Request/SamplingCallbackInterface.php

@@ -0,0 +1,25 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Handler\Request;
+
+use Mcp\Schema\Request\CreateSamplingMessageRequest;
+use Mcp\Schema\Result\CreateSamplingMessageResult;
+
+/**
+ * Contract for callbacks used by SamplingRequestHandler.
+ *
+ * Implementations perform the actual LLM sampling when requested by the server.
+ */
+interface SamplingCallbackInterface
+{
+    public function __invoke(CreateSamplingMessageRequest $request): CreateSamplingMessageResult;
+}

src/Client/Handler/Request/SamplingRequestHandler.php

@@ -0,0 +1,67 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Handler\Request;
+
+use Mcp\Exception\SamplingException;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Request;
+use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\Request\CreateSamplingMessageRequest;
+use Mcp\Schema\Result\CreateSamplingMessageResult;
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
+
+/**
+ * Handler for sampling requests from the server.
+ *
+ * The MCP server may request the client to sample an LLM during tool execution.
+ * This handler wraps a user-provided callback that performs the actual LLM call.
+ *
+ * @implements RequestHandlerInterface<CreateSamplingMessageResult>
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+class SamplingRequestHandler implements RequestHandlerInterface
+{
+    public function __construct(
+        private readonly SamplingCallbackInterface $callback,
+        private readonly LoggerInterface $logger = new NullLogger(),
+    ) {
+    }
+
+    public function supports(Request $request): bool
+    {
+        return $request instanceof CreateSamplingMessageRequest;
+    }
+
+    /**
+     * @return Response<CreateSamplingMessageResult>|Error
+     */
+    public function handle(Request $request): Response|Error
+    {
+        \assert($request instanceof CreateSamplingMessageRequest);
+
+        try {
+            $result = $this->callback->__invoke($request);
+
+            return new Response($request->getId(), $result);
+        } catch (SamplingException $e) {
+            $this->logger->error('Sampling failed: '.$e->getMessage());
+
+            return Error::forInternalError($e->getMessage(), $request->getId());
+        } catch (\Throwable $e) {
+            $this->logger->error('Unexpected error during sampling', ['exception' => $e]);
+
+            return Error::forInternalError('Error while sampling LLM', $request->getId());
+        }
+    }
+}

src/Client/Protocol.php

@@ -0,0 +1,318 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client;
+
+use Mcp\Client\Handler\Notification\NotificationHandlerInterface;
+use Mcp\Client\Handler\Notification\ProgressNotificationHandler;
+use Mcp\Client\Handler\Request\RequestHandlerInterface;
+use Mcp\Client\State\ClientState;
+use Mcp\Client\State\ClientStateInterface;
+use Mcp\Client\Transport\TransportInterface;
+use Mcp\JsonRpc\MessageFactory;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Notification;
+use Mcp\Schema\JsonRpc\Request;
+use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\Notification\InitializedNotification;
+use Mcp\Schema\Request\InitializeRequest;
+use Mcp\Schema\Result\InitializeResult;
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
+
+/**
+ * Client protocol handler for MCP communication.
+ *
+ * Handles message routing, request/response correlation, and the initialization handshake.
+ * All blocking operations are delegated to the transport.
+ *
+ * @phpstan-import-type FiberSuspend from TransportInterface
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+class Protocol
+{
+    private ?TransportInterface $transport = null;
+    private ClientStateInterface $state;
+    private MessageFactory $messageFactory;
+    private LoggerInterface $logger;
+
+    /** @var NotificationHandlerInterface[] */
+    private array $notificationHandlers;
+
+    /**
+     * @param RequestHandlerInterface<mixed>[] $requestHandlers
+     * @param NotificationHandlerInterface[]   $notificationHandlers
+     */
+    public function __construct(
+        private readonly array $requestHandlers = [],
+        array $notificationHandlers = [],
+        ?MessageFactory $messageFactory = null,
+        ?LoggerInterface $logger = null,
+    ) {
+        $this->state = new ClientState();
+        $this->messageFactory = $messageFactory ?? MessageFactory::make();
+        $this->logger = $logger ?? new NullLogger();
+
+        $this->notificationHandlers = [
+            new ProgressNotificationHandler($this->state),
+            ...$notificationHandlers,
+        ];
+    }
+
+    /**
+     * Connect this protocol to a transport.
+     *
+     * Sets up message handling callbacks.
+     *
+     * @param TransportInterface $transport The transport to connect
+     * @param Configuration      $config    The client configuration for initialization
+     */
+    public function connect(TransportInterface $transport, Configuration $config): void
+    {
+        $this->transport = $transport;
+        $transport->setState($this->state);
+        $transport->onInitialize(fn () => $this->initialize($config));
+        $transport->onMessage($this->processMessage(...));
+        $transport->onError(fn (\Throwable $e) => $this->logger->error('Transport error', ['exception' => $e]));
+
+        $this->logger->info('Protocol connected to transport', ['transport' => $transport::class]);
+    }
+
+    /**
+     * Perform the MCP initialization handshake.
+     *
+     * Sends InitializeRequest and waits for response, then sends InitializedNotification.
+     *
+     * @param Configuration $config The client configuration
+     *
+     * @return Response<array<string, mixed>>|Error
+     */
+    public function initialize(Configuration $config): Response|Error
+    {
+        $request = new InitializeRequest(
+            $config->protocolVersion->value,
+            $config->capabilities,
+            $config->clientInfo,
+        );
+
+        $response = $this->request($request, $config->initTimeout);
+
+        if ($response instanceof Response) {
+            $initResult = InitializeResult::fromArray($response->result);
+            $this->state->setServerInfo($initResult->serverInfo);
+            $this->state->setInstructions($initResult->instructions);
+            $this->state->setInitialized(true);
+
+            $this->sendNotification(new InitializedNotification());
+
+            $this->logger->info('Initialization complete', [
+                'server' => $initResult->serverInfo->name,
+            ]);
+        }
+
+        return $response;
+    }
+
+    /**
+     * Send a request to the server and wait for response.
+     *
+     * If a response is immediately available (sync HTTP), returns it.
+     * Otherwise, suspends the Fiber and waits for the transport to resume it.
+     *
+     * @param Request $request      The request to send
+     * @param int     $timeout      The timeout in seconds
+     * @param bool    $withProgress Whether to attach a progress token to the request
+     *
+     * @return Response<array<string, mixed>>|Error
+     */
+    public function request(Request $request, int $timeout, bool $withProgress = false): Response|Error
+    {
+        $requestId = $this->state->nextRequestId();
+        $request = $request->withId($requestId);
+
+        if ($withProgress) {
+            $progressToken = "prog-{$requestId}";
+            $request = $request->withMeta(['progressToken' => $progressToken]);
+        }
+
+        $this->state->addPendingRequest($requestId, $timeout);
+        $this->sendRequest($request);
+
+        $immediate = $this->state->consumeResponse($requestId);
+        if (null !== $immediate) {
+            $this->logger->debug('Received immediate response', ['id' => $requestId]);
+
+            return $immediate;
+        }
+
+        $this->logger->debug('Suspending fiber for response', ['id' => $requestId]);
+
+        return \Fiber::suspend([
+            'type' => 'await_response',
+            'request_id' => $requestId,
+            'timeout' => $timeout,
+        ]);
+    }
+
+    /**
+     * Send a request to the server.
+     */
+    private function sendRequest(Request $request): void
+    {
+        $this->logger->debug('Sending request', [
+            'id' => $request->getId(),
+            'method' => $request::getMethod(),
+        ]);
+
+        $encoded = json_encode($request, \JSON_THROW_ON_ERROR);
+        $this->transport?->send($encoded);
+    }
+
+    /**
+     * Send a notification to the server (fire and forget).
+     */
+    public function sendNotification(Notification $notification): void
+    {
+        $this->logger->debug('Sending notification', ['method' => $notification::getMethod()]);
+
+        $encoded = json_encode($notification, \JSON_THROW_ON_ERROR);
+        $this->transport?->send($encoded);
+    }
+
+    /**
+     * Send a response back to the server (for server-initiated requests).
+     *
+     * @param Response<mixed>|Error $response
+     */
+    private function sendResponse(Response|Error $response): void
+    {
+        $this->logger->debug('Sending response', ['id' => $response->getId()]);
+
+        $encoded = json_encode($response, \JSON_THROW_ON_ERROR);
+        $this->transport?->send($encoded);
+    }
+
+    /**
+     * Process an incoming message from the server.
+     *
+     * Routes to appropriate handler based on message type.
+     */
+    public function processMessage(string $input): void
+    {
+        $this->logger->debug('Received message', ['input' => $input]);
+
+        try {
+            $messages = $this->messageFactory->create($input);
+        } catch (\JsonException $e) {
+            $this->logger->warning('Failed to parse message', ['exception' => $e]);
+
+            return;
+        }
+
+        foreach ($messages as $message) {
+            if ($message instanceof Response || $message instanceof Error) {
+                $this->handleResponse($message);
+            } elseif ($message instanceof Request) {
+                $this->handleRequest($message);
+            } elseif ($message instanceof Notification) {
+                $this->handleNotification($message);
+            }
+        }
+    }
+
+    /**
+     * Handle a response from the server.
+     *
+     * This stores it in session. The transport will pick it up and resume the Fiber.
+     *
+     * @param Response<mixed>|Error $response
+     */
+    private function handleResponse(Response|Error $response): void
+    {
+        $requestId = $response->getId();
+
+        $this->logger->debug('Handling response', ['id' => $requestId]);
+
+        $this->state->storeResponse($requestId, $response->jsonSerialize());
+    }
+
+    /**
+     * Handle a request from the server (e.g., sampling request).
+     */
+    private function handleRequest(Request $request): void
+    {
+        $method = $request::getMethod();
+
+        $this->logger->debug('Received server request', [
+            'method' => $method,
+            'id' => $request->getId(),
+        ]);
+
+        foreach ($this->requestHandlers as $handler) {
+            if ($handler->supports($request)) {
+                try {
+                    $response = $handler->handle($request);
+                } catch (\Throwable $e) {
+                    $this->logger->error('Unexpected error while handling request', [
+                        'method' => $method,
+                        'exception' => $e,
+                    ]);
+
+                    $response = Error::forInternalError(
+                        \sprintf('Unexpected error while handling "%s" request', $method),
+                        $request->getId()
+                    );
+                }
+
+                $this->sendResponse($response);
+
+                return;
+            }
+        }
+
+        $error = Error::forMethodNotFound(
+            \sprintf('Client does not handle "%s" requests.', $method),
+            $request->getId()
+        );
+
+        $this->sendResponse($error);
+    }
+
+    /**
+     * Handle a notification from the server.
+     */
+    private function handleNotification(Notification $notification): void
+    {
+        $method = $notification::getMethod();
+
+        $this->logger->debug('Received server notification', [
+            'method' => $method,
+        ]);
+
+        foreach ($this->notificationHandlers as $handler) {
+            if ($handler->supports($notification)) {
+                try {
+                    $handler->handle($notification);
+                } catch (\Throwable $e) {
+                    $this->logger->warning('Notification handler failed', ['exception' => $e]);
+                }
+
+                return;
+            }
+        }
+    }
+
+    public function getState(): ClientStateInterface
+    {
+        return $this->state;
+    }
+}

src/Client/State/ClientState.php

@@ -0,0 +1,136 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\State;
+
+use Mcp\Schema\Implementation;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Response;
+
+/**
+ * In-memory client state implementation.
+ *
+ * Stores ephemeral runtime state for the client's connection to a server.
+ * This includes pending requests, responses, progress updates, and
+ * negotiated parameters from initialization.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+class ClientState implements ClientStateInterface
+{
+    private int $requestIdCounter = 1;
+    private bool $initialized = false;
+    private ?Implementation $serverInfo = null;
+    private ?string $instructions = null;
+
+    /** @var array<int|string, array{request_id: int|string, timestamp: int, timeout: int}> */
+    private array $pendingRequests = [];
+
+    /** @var array<int|string, array<string, mixed>> */
+    private array $responses = [];
+
+    /** @var array<int, array{token: string, progress: float, total: ?float, message: ?string}> */
+    private array $progressUpdates = [];
+
+    public function nextRequestId(): int
+    {
+        return $this->requestIdCounter++;
+    }
+
+    public function addPendingRequest(int|string $requestId, int $timeout): void
+    {
+        $this->pendingRequests[$requestId] = [
+            'request_id' => $requestId,
+            'timestamp' => time(),
+            'timeout' => $timeout,
+        ];
+    }
+
+    public function removePendingRequest(int|string $requestId): void
+    {
+        unset($this->pendingRequests[$requestId]);
+    }
+
+    public function getPendingRequests(): array
+    {
+        return $this->pendingRequests;
+    }
+
+    public function storeResponse(int|string $requestId, array $responseData): void
+    {
+        $this->responses[$requestId] = $responseData;
+    }
+
+    public function consumeResponse(int|string $requestId): Response|Error|null
+    {
+        if (!isset($this->responses[$requestId])) {
+            return null;
+        }
+
+        $data = $this->responses[$requestId];
+        unset($this->responses[$requestId]);
+        $this->removePendingRequest($requestId);
+
+        if (isset($data['error'])) {
+            return Error::fromArray($data);
+        }
+
+        return Response::fromArray($data);
+    }
+
+    public function setInitialized(bool $initialized): void
+    {
+        $this->initialized = $initialized;
+    }
+
+    public function isInitialized(): bool
+    {
+        return $this->initialized;
+    }
+
+    public function setServerInfo(Implementation $serverInfo): void
+    {
+        $this->serverInfo = $serverInfo;
+    }
+
+    public function getServerInfo(): ?Implementation
+    {
+        return $this->serverInfo;
+    }
+
+    public function setInstructions(?string $instructions): void
+    {
+        $this->instructions = $instructions;
+    }
+
+    public function getInstructions(): ?string
+    {
+        return $this->instructions;
+    }
+
+    public function storeProgress(string $token, float $progress, ?float $total, ?string $message): void
+    {
+        $this->progressUpdates[] = [
+            'token' => $token,
+            'progress' => $progress,
+            'total' => $total,
+            'message' => $message,
+        ];
+    }
+
+    public function consumeProgressUpdates(): array
+    {
+        $updates = $this->progressUpdates;
+        $this->progressUpdates = [];
+
+        return $updates;
+    }
+}

src/Client/State/ClientStateInterface.php

@@ -0,0 +1,115 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\State;
+
+use Mcp\Schema\Implementation;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Response;
+
+/**
+ * Interface for client state management.
+ *
+ * Tracks pending requests, stores responses, and manages runtime state
+ * for the client's connection to a server. This is ephemeral state that
+ * exists only for the lifetime of the connection.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+interface ClientStateInterface
+{
+    /**
+     * Get the next request ID for outgoing requests.
+     */
+    public function nextRequestId(): int;
+
+    /**
+     * Add a pending request to track.
+     *
+     * @param int|string $requestId The request ID
+     * @param int        $timeout   Timeout in seconds
+     */
+    public function addPendingRequest(int|string $requestId, int $timeout): void;
+
+    /**
+     * Remove a pending request.
+     */
+    public function removePendingRequest(int|string $requestId): void;
+
+    /**
+     * Get all pending requests.
+     *
+     * @return array<int|string, array{request_id: int|string, timestamp: int, timeout: int}>
+     */
+    public function getPendingRequests(): array;
+
+    /**
+     * Store a received response.
+     *
+     * @param int|string           $requestId    The request ID
+     * @param array<string, mixed> $responseData The raw response data
+     */
+    public function storeResponse(int|string $requestId, array $responseData): void;
+
+    /**
+     * Check and consume a response for a request ID.
+     *
+     * @return Response<array<string, mixed>>|Error|null
+     */
+    public function consumeResponse(int|string $requestId): Response|Error|null;
+
+    /**
+     * Set initialization state.
+     */
+    public function setInitialized(bool $initialized): void;
+
+    /**
+     * Check if connection is initialized.
+     */
+    public function isInitialized(): bool;
+
+    /**
+     * Store the server info from initialization.
+     */
+    public function setServerInfo(Implementation $serverInfo): void;
+
+    /**
+     * Get the server info from initialization.
+     */
+    public function getServerInfo(): ?Implementation;
+
+    /**
+     * Store the server instructions from initialization.
+     */
+    public function setInstructions(?string $instructions): void;
+
+    /**
+     * Get the server instructions from initialization.
+     */
+    public function getInstructions(): ?string;
+
+    /**
+     * Store progress data received from a notification.
+     *
+     * @param string      $token    The progress token
+     * @param float       $progress Current progress value
+     * @param float|null  $total    Total progress value (if known)
+     * @param string|null $message  Progress message
+     */
+    public function storeProgress(string $token, float $progress, ?float $total, ?string $message): void;
+
+    /**
+     * Consume all pending progress updates.
+     *
+     * @return array<int, array{token: string, progress: float, total: ?float, message: ?string}>
+     */
+    public function consumeProgressUpdates(): array;
+}

src/Client/Transport/BaseTransport.php

@@ -0,0 +1,126 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Transport;
+
+use Mcp\Client\State\ClientStateInterface;
+use Mcp\Exception\RuntimeException;
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
+
+/**
+ * Base implementation for client transports.
+ *
+ * Provides callback management and common utilities.
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+abstract class BaseTransport implements TransportInterface
+{
+    /** @var callable(): mixed|null */
+    protected $initializeCallback;
+
+    /** @var callable(string): void|null */
+    protected $messageCallback;
+
+    /** @var callable(\Throwable): void|null */
+    protected $errorCallback;
+
+    /** @var callable(string): void|null */
+    protected $closeCallback;
+
+    protected ?ClientStateInterface $state = null;
+    protected LoggerInterface $logger;
+
+    public function __construct(?LoggerInterface $logger = null)
+    {
+        $this->logger = $logger ?? new NullLogger();
+    }
+
+    public function onInitialize(callable $listener): void
+    {
+        $this->initializeCallback = $listener;
+    }
+
+    public function onMessage(callable $listener): void
+    {
+        $this->messageCallback = $listener;
+    }
+
+    public function onError(callable $listener): void
+    {
+        $this->errorCallback = $listener;
+    }
+
+    public function onClose(callable $listener): void
+    {
+        $this->closeCallback = $listener;
+    }
+
+    public function setState(ClientStateInterface $state): void
+    {
+        $this->state = $state;
+    }
+
+    /**
+     * Perform initialization via the registered callback.
+     *
+     * @return mixed The result from the initialization callback
+     *
+     * @throws RuntimeException If no initialize listener is registered
+     */
+    protected function handleInitialize(): mixed
+    {
+        if (!\is_callable($this->initializeCallback)) {
+            throw new RuntimeException('No initialize listener registered');
+        }
+
+        return ($this->initializeCallback)();
+    }
+
+    /**
+     * Handle an incoming message from the server.
+     */
+    protected function handleMessage(string $message): void
+    {
+        if (\is_callable($this->messageCallback)) {
+            try {
+                ($this->messageCallback)($message);
+            } catch (\Throwable $e) {
+                $this->handleError($e);
+            }
+        }
+    }
+
+    /**
+     * Handle a transport error.
+     */
+    protected function handleError(\Throwable $error): void
+    {
+        $this->logger->error('Transport error', ['exception' => $error]);
+
+        if (\is_callable($this->errorCallback)) {
+            ($this->errorCallback)($error);
+        }
+    }
+
+    /**
+     * Handle connection close.
+     */
+    protected function handleClose(string $reason): void
+    {
+        $this->logger->info('Transport closed', ['reason' => $reason]);
+
+        if (\is_callable($this->closeCallback)) {
+            ($this->closeCallback)($reason);
+        }
+    }
+}

src/Client/Transport/HttpTransport.php

@@ -0,0 +1,298 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Transport;
+
+use Http\Discovery\Psr17FactoryDiscovery;
+use Http\Discovery\Psr18ClientDiscovery;
+use Mcp\Exception\ConnectionException;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Response;
+use Psr\Http\Client\ClientInterface;
+use Psr\Http\Message\RequestFactoryInterface;
+use Psr\Http\Message\StreamFactoryInterface;
+use Psr\Http\Message\StreamInterface;
+use Psr\Log\LoggerInterface;
+
+/**
+ * HTTP-based client transport using PSR-18 HTTP client.
+ *
+ * PSR-18 HTTP clients are auto-discovered if not provided.
+ *
+ * @phpstan-import-type McpFiber from TransportInterface
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+class HttpTransport extends BaseTransport
+{
+    private ClientInterface $httpClient;
+    private RequestFactoryInterface $requestFactory;
+    private StreamFactoryInterface $streamFactory;
+
+    private ?string $sessionId = null;
+
+    /** @var McpFiber|null */
+    private ?\Fiber $activeFiber = null;
+
+    /** @var (callable(float, ?float, ?string): void)|null */
+    private $activeProgressCallback;
+
+    /** @var StreamInterface|null Active SSE stream being read */
+    private ?StreamInterface $activeStream = null;
+
+    /** @var string Buffer for incomplete SSE data */
+    private string $sseBuffer = '';
+
+    /**
+     * @param string                       $endpoint       The MCP server endpoint URL
+     * @param array<string, string>        $headers        Additional headers to send
+     * @param ClientInterface|null         $httpClient     PSR-18 HTTP client (auto-discovered if null)
+     * @param RequestFactoryInterface|null $requestFactory PSR-17 request factory (auto-discovered if null)
+     * @param StreamFactoryInterface|null  $streamFactory  PSR-17 stream factory (auto-discovered if null)
+     */
+    public function __construct(
+        private readonly string $endpoint,
+        private readonly array $headers = [],
+        ?ClientInterface $httpClient = null,
+        ?RequestFactoryInterface $requestFactory = null,
+        ?StreamFactoryInterface $streamFactory = null,
+        ?LoggerInterface $logger = null,
+    ) {
+        parent::__construct($logger);
+
+        $this->httpClient = $httpClient ?? Psr18ClientDiscovery::find();
+        $this->requestFactory = $requestFactory ?? Psr17FactoryDiscovery::findRequestFactory();
+        $this->streamFactory = $streamFactory ?? Psr17FactoryDiscovery::findStreamFactory();
+    }
+
+    public function connect(): void
+    {
+        $this->activeFiber = new \Fiber(fn () => $this->handleInitialize());
+
+        $this->activeFiber->start();
+
+        while (!$this->activeFiber->isTerminated()) {
+            $this->tick();
+        }
+
+        $result = $this->activeFiber->getReturn();
+        $this->activeFiber = null;
+
+        if ($result instanceof Error) {
+            throw new ConnectionException('Initialization failed: '.$result->message);
+        }
+
+        $this->logger->info('HTTP client connected and initialized', ['endpoint' => $this->endpoint]);
+    }
+
+    public function send(string $data): void
+    {
+        $request = $this->requestFactory->createRequest('POST', $this->endpoint)
+            ->withHeader('Content-Type', 'application/json')
+            ->withHeader('Accept', 'application/json, text/event-stream')
+            ->withBody($this->streamFactory->createStream($data));
+
+        if (null !== $this->sessionId) {
+            $request = $request->withHeader('Mcp-Session-Id', $this->sessionId);
+        }
+
+        foreach ($this->headers as $name => $value) {
+            $request = $request->withHeader($name, $value);
+        }
+
+        $this->logger->debug('Sending HTTP request', ['data' => $data]);
+
+        try {
+            $response = $this->httpClient->sendRequest($request);
+        } catch (\Throwable $e) {
+            $this->handleError($e);
+            throw new ConnectionException('HTTP request failed: '.$e->getMessage(), 0, $e);
+        }
+
+        if ($response->hasHeader('Mcp-Session-Id')) {
+            $this->sessionId = $response->getHeaderLine('Mcp-Session-Id');
+            $this->logger->debug('Received session ID', ['session_id' => $this->sessionId]);
+        }
+
+        $contentType = $response->getHeaderLine('Content-Type');
+
+        if (str_contains($contentType, 'text/event-stream')) {
+            $this->activeStream = $response->getBody();
+            $this->sseBuffer = '';
+        } elseif (str_contains($contentType, 'application/json')) {
+            $body = $response->getBody()->getContents();
+            if (!empty($body)) {
+                $this->handleMessage($body);
+            }
+        }
+    }
+
+    /**
+     * @param McpFiber                                                                $fiber
+     * @param (callable(float $progress, ?float $total, ?string $message): void)|null $onProgress
+     */
+    public function runRequest(\Fiber $fiber, ?callable $onProgress = null): Response|Error
+    {
+        $this->activeFiber = $fiber;
+        $this->activeProgressCallback = $onProgress;
+        $fiber->start();
+
+        while (!$fiber->isTerminated()) {
+            $this->tick();
+        }
+
+        $this->activeFiber = null;
+        $this->activeProgressCallback = null;
+        $this->activeStream = null;
+
+        return $fiber->getReturn();
+    }
+
+    public function close(): void
+    {
+        if (null !== $this->sessionId) {
+            try {
+                $request = $this->requestFactory->createRequest('DELETE', $this->endpoint)
+                    ->withHeader('Mcp-Session-Id', $this->sessionId);
+
+                foreach ($this->headers as $name => $value) {
+                    $request = $request->withHeader($name, $value);
+                }
+
+                $this->httpClient->sendRequest($request);
+                $this->logger->info('Session closed', ['session_id' => $this->sessionId]);
+            } catch (\Throwable $e) {
+                $this->logger->warning('Failed to close session', ['error' => $e->getMessage()]);
+            }
+        }
+
+        $this->sessionId = null;
+        $this->activeStream = null;
+        $this->handleClose('Transport closed');
+    }
+
+    private function tick(): void
+    {
+        $this->processSSEStream();
+        $this->processProgress();
+        $this->processFiber();
+
+        usleep(1000); // 1ms
+    }
+
+    /**
+     * Read SSE data incrementally from active stream.
+     */
+    private function processSSEStream(): void
+    {
+        if (null === $this->activeStream) {
+            return;
+        }
+
+        if (!$this->activeStream->eof()) {
+            $chunk = $this->activeStream->read(4096);
+            if ('' !== $chunk) {
+                $this->sseBuffer .= $chunk;
+            }
+        }
+
+        while (false !== ($pos = strpos($this->sseBuffer, "\n\n"))) {
+            $event = substr($this->sseBuffer, 0, $pos);
+            $this->sseBuffer = substr($this->sseBuffer, $pos + 2);
+
+            if (!empty(trim($event))) {
+                $this->processSSEEvent($event);
+            }
+        }
+
+        if ($this->activeStream->eof() && empty($this->sseBuffer)) {
+            $this->activeStream = null;
+        }
+    }
+
+    /**
+     * Parse a single SSE event and handle the message.
+     */
+    private function processSSEEvent(string $event): void
+    {
+        $data = '';
+
+        foreach (explode("\n", $event) as $line) {
+            if (str_starts_with($line, 'data:')) {
+                $data .= trim(substr($line, 5));
+            }
+        }
+
+        if (!empty($data)) {
+            $this->handleMessage($data);
+        }
+    }
+
+    /**
+     * Process pending progress updates from session and execute callback.
+     */
+    private function processProgress(): void
+    {
+        if (null === $this->activeProgressCallback || null === $this->state) {
+            return;
+        }
+
+        $updates = $this->state->consumeProgressUpdates();
+
+        foreach ($updates as $update) {
+            try {
+                ($this->activeProgressCallback)(
+                    $update['progress'],
+                    $update['total'],
+                    $update['message'],
+                );
+            } catch (\Throwable $e) {
+                $this->logger->warning('Progress callback failed', ['exception' => $e]);
+            }
+        }
+    }
+
+    private function processFiber(): void
+    {
+        if (null === $this->activeFiber || !$this->activeFiber->isSuspended()) {
+            return;
+        }
+
+        if (null === $this->state) {
+            return;
+        }
+
+        $pendingRequests = $this->state->getPendingRequests();
+
+        foreach ($pendingRequests as $pending) {
+            $requestId = $pending['request_id'];
+            $timestamp = $pending['timestamp'];
+            $timeout = $pending['timeout'];
+
+            $response = $this->state->consumeResponse($requestId);
+
+            if (null !== $response) {
+                $this->logger->debug('Resuming fiber with response', ['request_id' => $requestId]);
+                $this->activeFiber->resume($response);
+
+                return;
+            }
+
+            if (time() - $timestamp >= $timeout) {
+                $this->logger->warning('Request timed out', ['request_id' => $requestId]);
+                $error = Error::forInternalError('Request timed out', $requestId);
+                $this->activeFiber->resume($error);
+
+                return;
+            }
+        }
+    }
+}

src/Client/Transport/StdioTransport.php

@@ -0,0 +1,289 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Transport;
+
+use Mcp\Exception\ConnectionException;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Response;
+use Psr\Log\LoggerInterface;
+
+/**
+ * Client transport that spawns a child process and communicates via stdio.
+ *
+ * This transport handles all blocking operations:
+ * - Spawning the server process
+ * - Reading from stdout in a polling loop
+ * - Writing to stdin
+ * - Managing Fibers waiting for responses
+ *
+ * @phpstan-import-type McpFiber from TransportInterface
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+class StdioTransport extends BaseTransport
+{
+    /** @var resource|null */
+    private $process;
+
+    /** @var resource|null */
+    private $stdin;
+
+    /** @var resource|null */
+    private $stdout;
+
+    /** @var resource|null */
+    private $stderr;
+
+    private string $inputBuffer = '';
+
+    /** @var McpFiber|null */
+    private ?\Fiber $activeFiber = null;
+
+    /** @var (callable(float, ?float, ?string): void)|null */
+    private $activeProgressCallback;
+
+    /**
+     * @param string                     $command The command to run
+     * @param array<int, string>         $args    Command arguments
+     * @param string|null                $cwd     Working directory
+     * @param array<string, string>|null $env     Environment variables
+     */
+    public function __construct(
+        private readonly string $command,
+        private readonly array $args = [],
+        private readonly ?string $cwd = null,
+        private readonly ?array $env = null,
+        ?LoggerInterface $logger = null,
+    ) {
+        parent::__construct($logger);
+    }
+
+    public function connect(): void
+    {
+        $this->spawnProcess();
+
+        $this->activeFiber = new \Fiber(fn () => $this->handleInitialize());
+
+        $this->activeFiber->start();
+
+        while (!$this->activeFiber->isTerminated()) {
+            $this->tick();
+        }
+
+        $result = $this->activeFiber->getReturn();
+        $this->activeFiber = null;
+
+        if ($result instanceof Error) {
+            $this->close();
+            throw new ConnectionException('Initialization failed: '.$result->message);
+        }
+
+        $this->logger->info('Client connected and initialized');
+    }
+
+    public function send(string $data): void
+    {
+        if (null === $this->stdin || !\is_resource($this->stdin)) {
+            throw new ConnectionException('Process stdin not available');
+        }
+
+        fwrite($this->stdin, $data."\n");
+        fflush($this->stdin);
+
+        $this->logger->debug('Sent message to server', ['data' => $data]);
+    }
+
+    /**
+     * @param McpFiber                                                                $fiber
+     * @param (callable(float $progress, ?float $total, ?string $message): void)|null $onProgress
+     */
+    public function runRequest(\Fiber $fiber, ?callable $onProgress = null): Response|Error
+    {
+        $this->activeFiber = $fiber;
+        $this->activeProgressCallback = $onProgress;
+        $fiber->start();
+
+        while (!$fiber->isTerminated()) {
+            $this->tick();
+        }
+
+        $this->activeFiber = null;
+        $this->activeProgressCallback = null;
+
+        return $fiber->getReturn();
+    }
+
+    public function close(): void
+    {
+        if (\is_resource($this->stdin)) {
+            fclose($this->stdin);
+            $this->stdin = null;
+        }
+        if (\is_resource($this->stdout)) {
+            fclose($this->stdout);
+            $this->stdout = null;
+        }
+        if (\is_resource($this->stderr)) {
+            fclose($this->stderr);
+            $this->stderr = null;
+        }
+        if (\is_resource($this->process)) {
+            proc_terminate($this->process, 15); // SIGTERM
+            proc_close($this->process);
+            $this->process = null;
+        }
+
+        $this->handleClose('Transport closed');
+    }
+
+    private function spawnProcess(): void
+    {
+        $descriptors = [
+            0 => ['pipe', 'r'], // stdin
+            1 => ['pipe', 'w'], // stdout
+            2 => ['pipe', 'w'], // stderr
+        ];
+
+        $cmd = escapeshellcmd($this->command);
+        foreach ($this->args as $arg) {
+            $cmd .= ' '.escapeshellarg($arg);
+        }
+
+        $this->process = proc_open(
+            $cmd,
+            $descriptors,
+            $pipes,
+            $this->cwd,
+            $this->env
+        );
+
+        if (!\is_resource($this->process)) {
+            throw new ConnectionException('Failed to start process: '.$cmd);
+        }
+
+        $this->stdin = $pipes[0];
+        $this->stdout = $pipes[1];
+        $this->stderr = $pipes[2];
+
+        // Set non-blocking mode for reading
+        stream_set_blocking($this->stdout, false);
+        stream_set_blocking($this->stderr, false);
+
+        $this->logger->info('Started MCP server process', ['command' => $cmd]);
+    }
+
+    private function tick(): void
+    {
+        $this->processInput();
+        $this->processProgress();
+        $this->processFiber();
+        $this->processStderr();
+
+        usleep(1000); // 1ms
+    }
+
+    /**
+     * Process pending progress updates from session and execute callback.
+     */
+    private function processProgress(): void
+    {
+        if (null === $this->activeProgressCallback || null === $this->state) {
+            return;
+        }
+
+        $updates = $this->state->consumeProgressUpdates();
+
+        foreach ($updates as $update) {
+            try {
+                ($this->activeProgressCallback)(
+                    $update['progress'],
+                    $update['total'],
+                    $update['message'],
+                );
+            } catch (\Throwable $e) {
+                $this->logger->warning('Progress callback failed', ['exception' => $e]);
+            }
+        }
+    }
+
+    private function processInput(): void
+    {
+        if (null === $this->stdout || !\is_resource($this->stdout)) {
+            return;
+        }
+
+        $data = fread($this->stdout, 8192);
+        if (false !== $data && '' !== $data) {
+            $this->inputBuffer .= $data;
+        }
+
+        while (false !== ($pos = strpos($this->inputBuffer, "\n"))) {
+            $line = substr($this->inputBuffer, 0, $pos);
+            $this->inputBuffer = substr($this->inputBuffer, $pos + 1);
+
+            $trimmed = trim($line);
+            if (!empty($trimmed)) {
+                $this->handleMessage($trimmed);
+            }
+        }
+    }
+
+    private function processFiber(): void
+    {
+        if (null === $this->activeFiber || !$this->activeFiber->isSuspended()) {
+            return;
+        }
+
+        if (null === $this->state) {
+            return;
+        }
+
+        $pendingRequests = $this->state->getPendingRequests();
+
+        foreach ($pendingRequests as $pending) {
+            $requestId = $pending['request_id'];
+            $timestamp = $pending['timestamp'];
+            $timeout = $pending['timeout'];
+
+            // Check if response arrived
+            $response = $this->state->consumeResponse($requestId);
+
+            if (null !== $response) {
+                $this->logger->debug('Resuming fiber with response', ['request_id' => $requestId]);
+                $this->activeFiber->resume($response);
+
+                return;
+            }
+
+            // Check timeout
+            if (time() - $timestamp >= $timeout) {
+                $this->logger->warning('Request timed out', ['request_id' => $requestId]);
+                $error = Error::forInternalError('Request timed out', $requestId);
+                $this->activeFiber->resume($error);
+
+                return;
+            }
+        }
+    }
+
+    private function processStderr(): void
+    {
+        if (null === $this->stderr || !\is_resource($this->stderr)) {
+            return;
+        }
+
+        $stderr = fread($this->stderr, 8192);
+        if (false !== $stderr && '' !== $stderr) {
+            $this->logger->debug('Server stderr', ['output' => trim($stderr)]);
+        }
+    }
+}

src/Client/Transport/TransportInterface.php

@@ -0,0 +1,107 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Transport;
+
+use Mcp\Client\State\ClientStateInterface;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Response;
+
+/**
+ * Interface for client transports that communicate with MCP servers.
+ *
+ * The transport owns its execution loop and manages all blocking operations.
+ * The client delegates completely to the transport for I/O.
+ *
+ * @phpstan-type FiberReturn (Response<mixed>|Error)
+ * @phpstan-type FiberResume (Response<mixed>|Error)
+ * @phpstan-type FiberSuspend array{type: 'await_response', request_id: int, timeout: int}
+ * @phpstan-type McpFiber \Fiber<null, FiberResume, FiberReturn, FiberSuspend>
+ *
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+interface TransportInterface
+{
+    /**
+     * Connect to the MCP server and perform initialization handshake.
+     *
+     * This method blocks until:
+     * - Initialization completes successfully
+     * - Connection fails (throws ConnectionException)
+     *
+     * @throws \Mcp\Exception\ConnectionException
+     */
+    public function connect(): void;
+
+    /**
+     * Send a message to the server immediately.
+     *
+     * @param string $data JSON-encoded message
+     */
+    public function send(string $data): void;
+
+    /**
+     * Run a request fiber to completion.
+     *
+     * The transport starts the fiber, runs its internal loop, and resumes
+     * the fiber when a response arrives or timeout occurs.
+     *
+     * During the loop, the transport checks session for progress data and
+     * executes the callback if provided.
+     *
+     * @param McpFiber                                                                $fiber      The fiber to execute
+     * @param (callable(float $progress, ?float $total, ?string $message): void)|null $onProgress
+     *                                                                                            Optional callback for progress updates
+     *
+     * @return Response<array<string, mixed>>|Error The response or error
+     */
+    public function runRequest(\Fiber $fiber, ?callable $onProgress = null): Response|Error;
+
+    /**
+     * Close the transport and clean up resources.
+     */
+    public function close(): void;
+
+    /**
+     * Register callback for initialization handshake.
+     *
+     * The callback should return a Fiber that performs the initialization.
+     *
+     * @param callable(): mixed $callback
+     */
+    public function onInitialize(callable $callback): void;
+
+    /**
+     * Register callback for incoming messages from server.
+     *
+     * @param callable(string $message): void $callback
+     */
+    public function onMessage(callable $callback): void;
+
+    /**
+     * Register callback for transport errors.
+     *
+     * @param callable(\Throwable $error): void $callback
+     */
+    public function onError(callable $callback): void;
+
+    /**
+     * Register callback for when connection closes.
+     *
+     * @param callable(string $reason): void $callback
+     */
+    public function onClose(callable $callback): void;
+
+    /**
+     * Set the client state for runtime state management.
+     */
+    public function setState(ClientStateInterface $state): void;
+}