php-sdk/docs/client.md

# Client

The MCP Client SDK provides a synchronous, framework-agnostic API for communicating with MCP servers from PHP applications.
It handles connection management, request/response correlation, server-initiated requests (sampling), and real-time notifications.

## Table of Contents

- [Overview](#overview)
- [Client Builder](#client-builder)
- [Transports](#transports)
- [Connecting to Servers](#connecting-to-servers)
- [Server Information](#server-information)
- [Working with Tools](#working-with-tools)
- [Working with Resources](#working-with-resources)
- [Working with Prompts](#working-with-prompts)
- [Server-Initiated Communication](#server-initiated-communication)
- [Error Handling](#error-handling)
- [Complete Example](#complete-example)

## Overview

The client follows a builder pattern for configuration and provides a synchronous API for all operations:

```php
use Mcp\Client;
use Mcp\Client\Transport\StdioTransport;

// Build and configure the client
$client = Client::builder()
    ->setClientInfo('My Client', '1.0.0')
    ->setInitTimeout(30)
    ->setRequestTimeout(120)
    ->build();

// Create a transport
$transport = new StdioTransport(
    command: 'php',
    args: ['/path/to/server.php'],
);

// Connect and use the server
$client->connect($transport);
$tools = $client->listTools();
$client->disconnect();
```

## Client Builder

The `Client\Builder` provides fluent configuration of client instances.

### Basic Configuration

```php
use Mcp\Client;

$client = Client::builder()
    ->setClientInfo('My Application', '1.0.0', 'Description of my client')
    ->setInitTimeout(30)      // Seconds to wait for initialization
    ->setRequestTimeout(120)  // Seconds to wait for request responses
    ->setMaxRetries(3)        // Retries for failed connections
    ->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:

```php
$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 reported to servers during initialization:

```php
$client = Client::builder()
    ->setClientInfo(
        name: 'AI Assistant Client',
        version: '2.1.0',
        description: 'Client for automated AI workflows'
    )
    ->build();
```

### Protocol Version

Specify the MCP protocol version to offer during the handshake (defaults to the latest):

```php
use Mcp\Schema\Enum\ProtocolVersion;

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

This 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](https://modelcontextprotocol.io/specification/draft/basic/versioning#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. Use `$client->getProtocolVersion()` after connecting to read what was actually negotiated.

Modern revisions such as `2026-07-28` replaced `initialize` with per-request metadata, so they cannot be offered here.
Configuring one still opens the handshake with `ProtocolVersion::latestHandshake()`, and the client logs a warning
saying so.

See [Protocol Version Negotiation](server-builder.md#protocol-version-negotiation) for the server side of the exchange.

### Capabilities

Declare client capabilities to enable server features:

```php
use Mcp\Schema\ClientCapabilities;

$client = Client::builder()
    ->setCapabilities(new ClientCapabilities(
        sampling: true,  // Enable LLM sampling requests from server
        roots: true,     // Enable filesystem root listing
    ))
    ->build();
```

### Notification Handlers

Register handlers for server-initiated notifications:

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

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

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

### Request Handlers

Register handlers for server-initiated requests (e.g., sampling):

```php
use Mcp\Client\Handler\Request\SamplingRequestHandler;
use Mcp\Client\Handler\Request\SamplingCallbackInterface;
use Mcp\Schema\Request\CreateSamplingMessageRequest;
use Mcp\Schema\Result\CreateSamplingMessageResult;

$samplingCallback = new class implements SamplingCallbackInterface {
    public function __invoke(CreateSamplingMessageRequest $request): CreateSamplingMessageResult
    {
        // Perform LLM sampling and return result
    }
};

$client = Client::builder()
    ->addRequestHandler(new SamplingRequestHandler($samplingCallback))
    ->build();
```

### Logger

Configure PSR-3 logging for debugging:

```php
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();
```

## Transports

Transports handle the communication layer between client and server.

### STDIO Transport

Spawns a server process and communicates via standard input/output:

```php
use Mcp\Client\Transport\StdioTransport;

$transport = new StdioTransport(
    command: 'php',
    args: ['/path/to/server.php'],
    cwd: '/working/directory',     // Optional working directory
    env: ['KEY' => 'value'],       // Optional environment variables
);
```

**Parameters:**
- `command` (string): The command to execute
- `args` (array): Command arguments
- `cwd` (string|null): Working directory for the process
- `env` (array|null): Environment variables
- `logger` (LoggerInterface|null): Optional PSR-3 logger

### HTTP Transport

Communicates with remote MCP servers over HTTP:

```php
use Mcp\Client\Transport\HttpTransport;

$transport = new HttpTransport(
    endpoint: 'http://localhost:8000',
    headers: ['Authorization' => 'Bearer token'],
);
```

**Parameters:**
- `endpoint` (string): The MCP server URL
- `headers` (array): Additional HTTP headers
- `httpClient` (ClientInterface|null): PSR-18 HTTP client (auto-discovered)
- `requestFactory` (RequestFactoryInterface|null): PSR-17 request factory (auto-discovered)
- `streamFactory` (StreamFactoryInterface|null): PSR-17 stream factory (auto-discovered)
- `logger` (LoggerInterface|null): Optional PSR-3 logger

**PSR-18 Auto-Discovery:**

The transport automatically discovers PSR-18 HTTP clients from:
- `php-http/guzzle7-adapter`
- `php-http/curl-client`
- `symfony/http-client`
- And other PSR-18 compatible implementations

```bash
# Install any PSR-18 client - discovery works automatically
composer require php-http/guzzle7-adapter
```


## Connecting to Servers

### Establishing Connection

```php
$client->connect($transport);
```

The `connect()` method performs the MCP initialization handshake:
1. Opens the transport connection
2. Sends InitializeRequest with client capabilities
3. Waits for InitializeResult from server
4. Sends InitializedNotification

> [!IMPORTANT]
> Always wrap connection in try/catch to handle `ConnectionException` for failed connections.

### Checking Connection State

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

### Disconnecting

```php
$client->disconnect();
```

Always disconnect when finished to clean up resources:

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

## Server Information

After successful connection, retrieve server metadata:

```php
// Get server implementation info
$serverInfo = $client->getServerInfo();
echo "Server: {$serverInfo->name} v{$serverInfo->version}\n";

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

## Working with Tools

### Listing Tools

```php
$toolsResult = $client->listTools();

foreach ($toolsResult->tools as $tool) {
    echo "- {$tool->name}: {$tool->description}\n";
}

// Handle pagination
if ($toolsResult->nextCursor) {
    $moreTools = $client->listTools($toolsResult->nextCursor);
}
```

### Calling Tools

```php
$result = $client->callTool(
    name: 'calculate',
    arguments: ['a' => 5, 'b' => 3, 'operation' => 'add'],
);

// Access results
foreach ($result->content as $content) {
    if ($content instanceof TextContent) {
        echo $content->text;
    }
}
```

### Progress Notifications

Hook into tool execution progress (if server supports it):

```php
$result = $client->callTool(
    name: 'long_running_task',
    arguments: ['data' => 'large_dataset'],
    onProgress: static function (float $progress, ?float $total, ?string $message) {
        $percent = $total > 0 ? round(($progress / $total) * 100) : 0;
        echo "Progress: {$percent}% - {$message}\n";
    }
);
```

> [!NOTE]
> Progress notifications are only received if the server sends them. The callback will not be invoked if the server doesn't support or send progress updates.

## Working with Resources

### Listing Resources

```php
$resourcesResult = $client->listResources();

foreach ($resourcesResult->resources as $resource) {
    echo "- {$resource->uri}: {$resource->name}\n";
}
```

### Listing Resource Templates

```php
$templatesResult = $client->listResourceTemplates();

foreach ($templatesResult->resourceTemplates as $template) {
    echo "- {$template->uriTemplate}: {$template->name}\n";
}
```

### Reading Resources

```php
$resourceResult = $client->readResource('config://app/settings');

foreach ($resourceResult->contents as $content) {
    if ($content instanceof TextResourceContents) {
        echo "Text: {$content->text}\n";
    } elseif ($content instanceof BlobResourceContents) {
        echo "Binary data (base64): {$content->blob}\n";
    }
}
```

Resources also support progress notifications:

```php
$result = $client->readResource(
    uri: 'file://large-file.bin',
    onProgress: static function (float $progress, ?float $total, ?string $message) {
        echo "Reading: {$progress}/{$total} bytes\n";
    }
);
```

## Working with Prompts

### Listing Prompts

```php
$promptsResult = $client->listPrompts();

foreach ($promptsResult->prompts as $prompt) {
    echo "- {$prompt->name}: {$prompt->description}\n";
}
```

### Getting Prompts

```php
$promptResult = $client->getPrompt(
    name: 'code_review',
    arguments: ['language' => 'php', 'code' => '...'],
);

foreach ($promptResult->messages as $message) {
    echo "{$message->role->value}: {$message->content->text}\n";
}
```

Prompts also support progress notifications:

```php
$result = $client->getPrompt(
    name: 'generate_report',
    arguments: ['topic' => 'quarterly_analysis'],
    onProgress: static function (float $progress, ?float $total, ?string $message) {
        echo "Generating: {$message}\n";
    }
);
```

### Requesting Completions

Request auto-completion suggestions for prompt or resource arguments:

```php
use Mcp\Schema\PromptReference;

$completionResult = $client->complete(
    ref: new PromptReference('code_review'),
    argument: ['name' => 'language', 'value' => 'ph'],
);

foreach ($completionResult->values as $value) {
    echo "Suggestion: {$value}\n";
}
```

## Server-Initiated Communication

The client can receive requests and notifications from the server when configured with appropriate handlers.

### Logging Notifications

> **Deprecated** since protocol revision `2026-07-28` ([SEP-2577](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2577)), earliest removal `2027-07-28`. Logging keeps working until then; new integrations should log to stderr (stdio) or use OpenTelemetry instead.

Receive structured log messages from the server:

```php
use Mcp\Client\Handler\Notification\LoggingNotificationHandler;
use Mcp\Schema\Notification\LoggingMessageNotification;
use Mcp\Schema\Enum\LoggingLevel;

$loggingHandler = new LoggingNotificationHandler(
    static function (LoggingMessageNotification $notification) {
        // Route to your application's logging system
        $level = $notification->level;
        $message = $notification->data;
        
        match ($level) {
            LoggingLevel::Debug => logger()->debug($message),
            LoggingLevel::Info => logger()->info($message),
            LoggingLevel::Warning => logger()->warning($message),
            LoggingLevel::Error => logger()->error($message),
            default => logger()->info($message),
        };
    }
);

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

// Set minimum log level (optional)
$client->setLoggingLevel(LoggingLevel::Info);
```

### Sampling (LLM Requests)

> **Deprecated** since protocol revision `2026-07-28` ([SEP-2577](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2577)), earliest removal `2027-07-28`. Sampling keeps working until then; new integrations should call an LLM provider's API directly instead.

Handle server requests for LLM completions:

```php
use Mcp\Client\Handler\Request\SamplingRequestHandler;
use Mcp\Client\Handler\Request\SamplingCallbackInterface;
use Mcp\Exception\SamplingException;
use Mcp\Schema\ClientCapabilities;
use Mcp\Schema\Request\CreateSamplingMessageRequest;
use Mcp\Schema\Result\CreateSamplingMessageResult;
use Mcp\Schema\Content\TextContent;
use Mcp\Schema\Enum\Role;

class LlmSamplingCallback implements SamplingCallbackInterface
{
    public function __invoke(CreateSamplingMessageRequest $request): CreateSamplingMessageResult
    {
        try {
            // Call your LLM provider
            $response = $this->llmClient->complete(
                messages: $request->messages,
                maxTokens: $request->maxTokens,
                temperature: $request->temperature ?? 0.7,
            );
            
            return new CreateSamplingMessageResult(
                role: Role::Assistant,
                content: new TextContent($response->text),
                model: $response->model,
                stopReason: $response->stopReason,
            );
        } catch (\Throwable $e) {
            // Throw SamplingException to surface error to server
            throw new SamplingException(
                "LLM sampling failed: {$e->getMessage()}",
                (int) $e->getCode(),
                $e
            );
        }
    }
}

$client = Client::builder()
    ->setCapabilities(new ClientCapabilities(sampling: true))
    ->addRequestHandler(new SamplingRequestHandler(new LlmSamplingCallback))
    ->build();
```

#### Sampling with Tools

Clients that support tool-enabled sampling should advertise that capability and forward the request's `tools` and
`toolChoice` fields to their LLM provider. A provider response that requests tools can be returned as one or more
`ToolUseContent` blocks:

```php
use Mcp\Schema\ClientCapabilities;
use Mcp\Schema\Content\ToolUseContent;
use Mcp\Schema\Enum\Role;
use Mcp\Schema\Result\CreateSamplingMessageResult;

$client = Client::builder()
    ->setCapabilities(new ClientCapabilities(
        sampling: true,
        samplingContext: true,
        samplingTools: true,
    ))
    ->addRequestHandler(new SamplingRequestHandler($samplingCallback))
    ->build();

// Inside the sampling callback, after invoking the LLM provider:
return new CreateSamplingMessageResult(
    role: Role::Assistant,
    content: array_map(
        static fn ($call) => new ToolUseContent($call->id, $call->name, $call->input),
        $providerResponse->toolCalls,
    ),
    model: $providerResponse->model,
    stopReason: 'toolUse',
);
```

The server executes the requested tools and sends their results in a later sampling request as `ToolResultContent`
blocks in a user message. The client should pass those blocks back to the LLM provider to continue the sampling loop.

> [!IMPORTANT]
> **Error Handling in Sampling Callbacks:**
> 
> When implementing sampling callbacks, error handling is critical:
> 
> - **Throw `SamplingException`** to forward specific error messages to the server
> - **Any other exception** will be logged but return a generic error to the server
> 
> This distinction allows you to control what error information the server receives:
> 
> ```php
> // Good: Server receives "Rate limit exceeded" message
> throw new SamplingException('Rate limit exceeded. Retry after 60 seconds.');
> 
> // Bad: Server receives generic "Error while sampling LLM" message
> throw new \RuntimeException('Rate limit exceeded');
> ```

### Elicitation (User Input Requests)

Handle server requests to elicit additional information from the user during tool
execution. The server sends an `elicitation/create` request describing the fields it
needs; your callback presents them to the user and returns an `ElicitResult` with one of
three actions — accept (with the collected content), decline, or cancel:

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

class ConsoleElicitationCallback implements ElicitationCallbackInterface
{
    public function __invoke(ElicitRequest $request): ElicitResult
    {
        echo $request->message.\PHP_EOL;

        // Present $request->requestedSchema->properties to the user and collect input.
        $content = [];
        foreach ($request->requestedSchema->properties as $name => $definition) {
            $answer = readline($definition->title.': ');

            if (false === $answer) {
                // No input available — let the server know the user cancelled.
                return new ElicitResult(ElicitAction::Cancel);
            }

            $content[$name] = $answer;
        }

        return new ElicitResult(ElicitAction::Accept, $content);
    }
}

$client = Client::builder()
    ->setCapabilities(new ClientCapabilities(elicitation: true))
    ->addRequestHandler(new ElicitationRequestHandler(new ConsoleElicitationCallback))
    ->build();
```

Return `new ElicitResult(ElicitAction::Decline)` when the user refuses to provide the
information, and `new ElicitResult(ElicitAction::Cancel)` when they dismiss the request.
Only the `Accept` action carries content.

> [!IMPORTANT]
> **Error Handling in Elicitation Callbacks:**
>
> - **Throw `ElicitationException`** to forward a specific error message to the server
> - **Any other exception** is logged but returns a generic error to the server
>
> ```php
> // Good: Server receives "No interactive console available" message
> throw new ElicitationException('No interactive console available');
>
> // Bad: Server receives generic "Error while processing elicitation" message
> throw new \RuntimeException('No interactive console available');
> ```

See `examples/client/stdio_elicitation.php` for a runnable example against the
elicitation demo server.

### Roots

> **Deprecated** since protocol revision `2026-07-28` ([SEP-2577](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2577)), earliest removal `2027-07-28`. Roots keep working until then; new integrations should pass directories or files through tool arguments, resource URIs or server configuration instead.

Roots let the client expose a list of `file://` "workspace folders" that the server
is allowed to operate on. Advertise the `roots` capability and register a handler
that answers server `roots/list` requests:

```php
use Mcp\Client\Handler\Request\ListRootsRequestHandler;
use Mcp\Client\Handler\Request\RootsCallbackInterface;
use Mcp\Schema\ClientCapabilities;
use Mcp\Schema\Request\ListRootsRequest;
use Mcp\Schema\Result\ListRootsResult;
use Mcp\Schema\Root;

class WorkspaceRootsCallback implements RootsCallbackInterface
{
    public function __invoke(ListRootsRequest $request): ListRootsResult
    {
        return new ListRootsResult([
            new Root('file:///home/user/projects/app', 'Application'),
            new Root('file:///home/user/projects/library', 'Library'),
        ]);
    }
}

$client = Client::builder()
    ->setCapabilities(new ClientCapabilities(roots: true, rootsListChanged: true))
    ->addRequestHandler(new ListRootsRequestHandler(new WorkspaceRootsCallback))
    ->build();
```

When the client's roots change, notify the server so it can request the updated
list via `roots/list`. This requires advertising the `roots.listChanged`
capability (`rootsListChanged: true` above); otherwise `sendRootsListChanged()`
throws a `RuntimeException`. On a client that is not connected it throws a
`ConnectionException`:

```php
$client->sendRootsListChanged();
```

See `examples/client/stdio_roots.php` for a runnable example: it calls the
`inspect_workspace_roots` tool of the client-communication demo server, which
answers by issuing the `roots/list` request back to the client.

## Error Handling

The client throws exceptions for various error conditions:

### ConnectionException

Thrown when connection or initialization fails:

```php
use Mcp\Exception\ConnectionException;

try {
    $client->connect($transport);
} catch (ConnectionException $e) {
    echo "Failed to connect: {$e->getMessage()}\n";
}
```

### RequestException

Thrown when a request returns an error response:

```php
use Mcp\Exception\RequestException;

try {
    $result = $client->callTool('unknown_tool', []);
} catch (RequestException $e) {
    echo "Request failed: {$e->getMessage()}\n";
    echo "Error code: {$e->getCode()}\n";
}
```

## Complete Example

Here's a comprehensive example demonstrating client usage:

```php
<?php

use Mcp\Client;
use Mcp\Client\Handler\Notification\LoggingNotificationHandler;
use Mcp\Client\Handler\Request\SamplingCallbackInterface;
use Mcp\Client\Handler\Request\SamplingRequestHandler;
use Mcp\Client\Transport\StdioTransport;
use Mcp\Exception\SamplingException;
use Mcp\Schema\ClientCapabilities;
use Mcp\Schema\Content\TextContent;
use Mcp\Schema\Enum\LoggingLevel;
use Mcp\Schema\Enum\Role;
use Mcp\Schema\Notification\LoggingMessageNotification;
use Mcp\Schema\Request\CreateSamplingMessageRequest;
use Mcp\Schema\Result\CreateSamplingMessageResult;

// Configure logging notification handler
$loggingHandler = new LoggingNotificationHandler(
    static function (LoggingMessageNotification $notification) {
        echo "[LOG {$notification->level->value}] {$notification->data}\n";
    }
);

// Configure sampling callback
$samplingCallback = new class implements SamplingCallbackInterface {
    public function __invoke(CreateSamplingMessageRequest $request): CreateSamplingMessageResult
    {
        echo "[SAMPLING] Processing request (max {$request->maxTokens} tokens)\n";
        
        try {
            // Integration with your LLM provider
            $response = "This is a mock LLM response for: " . 
                json_encode($request->messages);
            
            return new CreateSamplingMessageResult(
                role: Role::Assistant,
                content: new TextContent($response),
                model: 'mock-llm',
                stopReason: 'endTurn',
            );
        } catch (\Throwable $e) {
            throw new SamplingException(
                "Sampling failed: {$e->getMessage()}",
                0,
                $e
            );
        }
    }
};

// Build client
$client = Client::builder()
    ->setClientInfo('Example Client', '1.0.0')
    ->setInitTimeout(30)
    ->setRequestTimeout(120)
    ->setCapabilities(new ClientCapabilities(sampling: true))
    ->addNotificationHandler($loggingHandler)
    ->addRequestHandler(new SamplingRequestHandler($samplingCallback))
    ->build();

// Create transport
$transport = new StdioTransport(
    command: 'php',
    args: [__DIR__ . '/server.php'],
);

// Connect and use server
try {
    echo "Connecting to server...\n";
    $client->connect($transport);
    
    // Get server info
    $serverInfo = $client->getServerInfo();
    echo "Connected to: {$serverInfo->name} v{$serverInfo->version}\n\n";
    
    // List capabilities
    echo "Available tools:\n";
    $tools = $client->listTools();
    foreach ($tools->tools as $tool) {
        echo "  - {$tool->name}\n";
    }
    
    echo "\nAvailable resources:\n";
    $resources = $client->listResources();
    foreach ($resources->resources as $resource) {
        echo "  - {$resource->uri}\n";
    }
    
    // Set logging level
    $client->setLoggingLevel(LoggingLevel::Debug);
    
    // Call tool with progress
    echo "\nCalling tool with progress...\n";
    $result = $client->callTool(
        name: 'process_data',
        arguments: ['dataset' => 'large_file.csv'],
        onProgress: static function (float $progress, ?float $total, ?string $message) {
            $percent = $total > 0 ? round(($progress / $total) * 100) : 0;
            echo "  Progress: {$percent}% - {$message}\n";
        }
    );
    
    echo "\nResult:\n";
    foreach ($result->content as $content) {
        if ($content instanceof TextContent) {
            echo $content->text . "\n";
        }
    }
    
} catch (\Throwable $e) {
    echo "Error: {$e->getMessage()}\n";
    echo $e->getTraceAsString() . "\n";
} finally {
    $client->disconnect();
    echo "\nDisconnected.\n";
}
```