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PHP MCP Client

PHP MCP Client is a PHP library for interacting with servers that implement the Model Context Protocol (MCP).

It provides a developer-friendly interface to connect to individual MCP servers using different transports (stdio, http+sse), manage the connection lifecycle, discover server capabilities (Tools, Resources, Prompts), and execute requests like calling tools or reading resources.

While utilizing asynchronous I/O internally via ReactPHP for robustness and handling features like server-sent events, the library offers both a straightforward synchronous (blocking) API for common use cases and an asynchronous (Promise-based) API for advanced control and concurrency.

This library aligns with the MCP specification's model where one client instance manages a stateful connection to one server.

Introduction to MCP

The Model Context Protocol (MCP) is an open standard designed to standardize how AI assistants and applications connect to external data sources, APIs, and tools (like codebases, databases, web browsers). It acts as a communication layer, enabling AI models (like Claude, or models integrated via frameworks like OpenAI's) to securely access and interact with context provided by different servers.

This client library allows your PHP application (acting as the "Host" in MCP terminology) to consume capabilities offered by one or more MCP servers.

Features

• Client-per-Server Model: Each Client instance manages a stateful connection to a single configured MCP server, aligning with the MCP specification.
• Fluent Configuration: Easy setup for each client instance using a Client::make()->with...() builder pattern.
• Dual API:
  • Synchronous Facade: Interact with the server using straightforward, blocking methods (e.g., $client->listTools(), $client->callTool(...)) for simple integration.
  • Asynchronous API: Access underlying Promise-based methods (e.g., $client->listToolsAsync(), $client->callToolAsync(...)) for concurrency and integration with async PHP applications.
• Multiple Transports: Built-in support for:
  • stdio: Communicating with server processes via Standard Input/Output.
  • http: Communicating with servers via HTTP POST and Server-Sent Events (SSE).
• Explicit Connection Lifecycle: Requires ->initialize() or ->initializeAsync() to connect and perform the handshake before making requests. Provides disconnect() / disconnectAsync().
• Tool/Resource/Prompt Interaction: Provides comprehensive methods (sync & async) to list available elements and execute requests like tools/call, resources/read, prompts/get.
• PSR Compliance: Integrates with standard PHP interfaces:
  • PSR-3 (LoggerInterface): Integrate your application's logger.
  • PSR-16 (SimpleCacheInterface): Optional caching for server definitions.
  • PSR-14 (EventDispatcherInterface): Optional handling of server-sent notifications via events (requires async handling).
• Robust Error Handling: Specific exceptions for different failure modes.
• Asynchronous Core: Utilizes ReactPHP internally for non-blocking I/O.

Requirements

• PHP >= 8.1
• Composer
• (For Stdio Transport): Ability to execute the server command.
• (For Http Transport): Network access to the MCP server URL.

Installation

Install the package via Composer:

The necessary ReactPHP dependencies (event-loop, promise, stream, child-process, http) should be installed automatically.

Quick Start: Simple Synchronous Usage (Stdio)

This example connects to a local filesystem server running via npx.

Configuration

Configuration involves setting up:

1. Client Identity: Your application's name and version, passed directly to the builder.
2. Client Capabilities: Features your client supports using ClientCapabilities.
3. Server Connection: Details for the single server this client instance will connect to, using ServerConfig.
4. (Optional) Dependencies: Logger, Cache, Event Dispatcher, Event Loop.

ClientCapabilities

Declares features your client supports. Use the static factory method.

ServerConfig

Defines how to connect to a single MCP server.

Loading Config from Array/JSON

You can easily parse configurations stored in arrays (e.g., from JSON files or framework config).

ClientBuilder

Use the builder to assemble the Client instance:

Usage

Once you have a configured Client instance for a specific server:

1. Initialize the Connection:

You must call initialize() or initializeAsync() before making requests.

2. Making Requests:

Use the client methods. They operate on the single connection established by initialize().

• Synchronous API (Recommended for simple scripts/frameworks):

  • Methods like listTools(), callTool(), readResource() block execution until a response is received or a timeout occurs.
  • They return the parsed result object (e.g., array<ToolDefinition>, CallToolResult) or throw an exception (TimeoutException, RequestException, ConnectionException, etc.).
• Asynchronous API (For async applications or concurrent requests):
  • Methods like listToolsAsync(), callToolAsync(), readResourceAsync() return a React\Promise\PromiseInterface.
  • You need to use promise methods (then, catch, finally) or React\Async\await (in a Fiber context) to handle the results.
  • Requires the event loop to be running.

3. Disconnecting:

Always disconnect when you are finished interacting with a server to release resources (especially for stdio transports).

Available Client Methods

The Client class provides methods for interacting with the connected MCP server. Most methods have both a synchronous (blocking) and an asynchronous (Promise-returning) variant.

Connection & Lifecycle:

• (Sync) initialize(): self Connects to the server and performs the MCP handshake. Blocks until ready or throws an exception. Returns the client instance.
• (Async) initializeAsync(): PromiseInterface<Client> Initiates connection and handshake asynchronously. Returns a promise resolving with the client instance when ready, or rejecting on failure.
• (Sync) disconnect(): void Closes the connection gracefully. Blocks until disconnection is complete or times out.
• (Async) disconnectAsync(): PromiseInterface<void> Initiates graceful disconnection asynchronously. Returns a promise resolving when disconnection is complete.
• getStatus(): ConnectionStatus Returns the current connection status enum (Disconnected, Connecting, Handshaking, Ready, Closing, Closed, Error).
• isReady(): bool Helper method, returns true if status is Ready.
• getServerName(): ?string Returns the name of the server (available after successful initialization).
• getServerVersion(): ?string Returns the version of the server (available after successful initialization).
• getNegotiatedCapabilities(): ?Capabilities Returns the capabilities negotiated with the server (available after successful initialization).
• getNegotiatedProtocolVersion(): ?string Returns the protocol version agreed upon with the server (available after successful initialization).

MCP Operations (Sync):

(These methods require the client to be initialized first and will block)

• ping(): void
• listTools(bool $useCache = true): array<ToolDefinition>
• listResources(bool $useCache = true): array<ResourceDefinition>
• listPrompts(bool $useCache = true): array<PromptDefinition>
• listResourceTemplates(bool $useCache = true): array<ResourceTemplateDefinition>
• callTool(string $toolName, array $arguments = []): CallToolResult
• readResource(string $uri): ReadResourceResult
• getPrompt(string $promptName, array $arguments = []): GetPromptResult
• subscribeResource(string $uri): void
• unsubscribeResource(string $uri): void
• setLogLevel(string $level): void

MCP Operations (Async):

(These methods require the client to be initialized first and return React\Promise\PromiseInterface)

• pingAsync(): PromiseInterface<void>
• listToolsAsync(): PromiseInterface<array<ToolDefinition>>
• listResourcesAsync(): PromiseInterface<array<ResourceDefinition>>
• listPromptsAsync(): PromiseInterface<array<PromptDefinition>>
• listResourceTemplatesAsync(): PromiseInterface<array<ResourceTemplateDefinition>>
• callToolAsync(string $toolName, array $arguments = []): PromiseInterface<CallToolResult>
• readResourceAsync(string $uri): PromiseInterface<ReadResourceResult>
• getPromptAsync(string $promptName, array $arguments = []): PromiseInterface<GetPromptResult>
• subscribeResourceAsync(string $uri): PromiseInterface<void>
• unsubscribeResourceAsync(string $uri): PromiseInterface<void>
• setLogLevelAsync(string $level): PromiseInterface<void>

Advanced:

• getLoop(): LoopInterface Access the underlying ReactPHP event loop instance.

Handling Server Notifications (Asynchronous Only)

MCP servers can send notifications (e.g., resources/didChange). To receive these:

1. Configure the client with a PSR-14 EventDispatcherInterface using ->withEventDispatcher(...).
2. Add listeners to your dispatcher for events like PhpMcp\Client\Event\ResourceChanged.
3. Use the asynchronous API (initializeAsync, potentially other *Async methods).
4. Run the event loop continuously ($client->getLoop()->run()). Notifications arrive via the underlying transport (usually SSE) only while the loop is active.

See examples/04-handling-notifications.php for a conceptual guide.

Error Handling

The client uses specific exceptions inheriting from PhpMcp\Client\Exception\McpClientException. Catching these allows for targeted error handling:

• ConfigurationException: Thrown during ClientBuilder::build() or ServerConfig::fromArray() if the provided configuration is invalid or missing required fields (e.g., missing command for stdio, invalid url for http).
• ConnectionException: Thrown by initialize() or initializeAsync() if the underlying transport connection fails (e.g., stdio process cannot start, TCP connection refused for HTTP, invalid initial response). Also thrown by request methods if called when the client is not in a Ready state or if the connection drops unexpectedly during an operation. Check $e->getPrevious() for lower-level transport or system errors.
• HandshakeException (Subclass of ConnectionException): Thrown specifically by initialize() or initializeAsync() if the MCP handshake phase fails after the transport connection is established (e.g., server returns an error to the initialize request, version mismatch, invalid capabilities received). May contain the server's JsonRpc\Error via getRequestException()->getRpcError().
• TransportException: Indicates a low-level error during communication after connection (e.g., failure to write to stdio stdin, SSE stream error, unexpected data format received from transport). Often wrapped by ConnectionException.
• TimeoutException: Thrown by synchronous methods (initialize, listTools, callTool, etc.) or rejects asynchronous promises if the server does not respond within the configured timeout for the ServerConfig. Access timeout value via $e->getTimeout().
• RequestException: Thrown by synchronous methods or rejects asynchronous promises when the MCP server successfully processed the request but returned a JSON-RPC error payload (e.g., method not found on server, invalid parameters for a tool, tool execution failed on server). Access the JsonRpc\Error object via $e->getRpcError() to get the code, message, and optional data from the server.
• UnsupportedCapabilityException: Thrown by methods like subscribeResource() or setLogLevel() if the connected server did not declare support for the required capability during the initial handshake.
• DefinitionException: Thrown if there's an error fetching, caching, or parsing server definitions (Tools, Resources, Prompts), often related to cache issues or invalid data structures.
• ProtocolException: Indicates a violation of the JSON-RPC 2.0 or MCP structure in messages received from the server (e.g., missing required fields, invalid types).

Always wrap client interactions in try...catch blocks to handle these potential failures gracefully.

Examples

See the examples/ directory for working code:

• 01-simple-stdio-sync.php: Demonstrates basic synchronous interaction with a stdio server.
• 02-simple-http-sync.php: Demonstrates basic synchronous interaction with an http+sse server.
• 03-multiple-servers-sync.php: Shows how to instantiate and use multiple Client objects for different servers within the same script (sequentially).
• 04-multiple-servers-async.php: Demonstrates asynchronous interaction with multiple servers using Promises for concurrency. Requires running the event loop.
• 05-openai-php-integration-sync: Full example integrating with openai-php for tool usage using the synchronous client API, including its own composer.json setup.

Testing

Contributing

Contributions are welcome! Please see CONTRIBUTING.md or open an issue/pull request.

License

The MIT License (MIT). See LICENSE.