Enable servers to send sampling messages to clients (#101)

docs/client-communication.md

@@ -0,0 +1,101 @@
+# Client Communication
+
+MCP supports various ways a server can communicate back to a server on top of the main request-response flow.
+
+## Table of Contents
+
+- [ClientGateway](#client-gateway)
+- [Sampling](#sampling)
+- [Logging](#logging)
+- [Notification](#notification)
+- [Progress](#progress)
+
+## ClientGateway
+
+Every communication back to client is handled using the `Mcp\Server\ClientGateway` and its dedicated methods per
+operation. To use the `ClientGateway` in your code, there are two ways to do so:
+
+### 1. Method Argument Injection
+
+Every refernce of a MCP element, that translates to an actual method call, can just add an type-hinted argument for the
+`ClientGateway` and the SDK will take care to include the gateway in the arguments of the method call:
+
+```php
+use Mcp\Capability\Attribute\McpTool;
+use Mcp\Server\ClientGateway;
+
+class MyService
+{
+    #[McpTool('my_tool', 'My Tool Description')]
+    public function myTool(ClientGateway $client): string
+    {
+        $client->log(...);
+```
+
+### 2. Implementing `ClientAwareInterface`
+
+Whenever a service class of an MCP element implements the interface `Mcp\Server\ClientAwareInterface` the `setClient`
+method of that class will get called while handling the reference, and in combination with `Mcp\Server\ClientAwareTrait`
+this ends up with code like this:
+
+```php
+use Mcp\Capability\Attribute\McpTool;
+use Mcp\Server\ClientAwareInterface;
+use Mcp\Server\ClientAwareTrait;
+
+class MyService implements ClientAwareInterface
+{
+    use ClientAwareTrait;
+
+    #[McpTool('my_tool', 'My Tool Description')]
+    public function myTool(): string
+    {
+        $this->log(...);
+```
+
+## Sampling
+
+With [sampling](https://modelcontextprotocol.io/specification/2025-06-18/client/sampling) servers can request clients to
+execute "completions" or "generations" with a language model for them:
+
+```php
+$result = $clientGateway->sample('Roses are red, violets are', 350, 90, ['temperature' => 0.5]);
+```
+
+The `sample` method accepts four arguments:
+
+1. `message`, which is **required** and accepts a string, an instance of `Content` or an array of `SampleMessage` instances.
+2. `maxTokens`, which defaults to `1000`
+3. `timeout` in seconds, which defaults to `120`
+4. `options` which might include `system_prompt`, `preferences` for model choice, `includeContext`, `temperature`, `stopSequences` and `metadata`
+
+[Find more details to sampling payload in the specification.](https://modelcontextprotocol.io/specification/2025-06-18/client/sampling#protocol-messages)
+
+## Logging
+
+The [Logging](https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/logging) utility enables servers
+to send structured log messages as notifcation to clients:
+
+```php
+use Mcp\Schema\Enum\LoggingLevel;
+
+$clientGateway->log(LoggingLevel::Warning, 'The end is near.');
+```
+
+## Progress
+
+With a [Progress](https://modelcontextprotocol.io/specification/2025-06-18/basic/utilities/progress#progress)
+notification a server can update a client while an operation is ongoing:
+
+```php
+$clientGateway->progress(4.2, 10, 'Downloading needed images.');
+```
+
+## Notification
+
+Lastly, the server can push all kind of notifications, that implement the `Mcp\Schema\JsonRpc\Notification` interface
+to the client to:
+
+```php
+$clientGateway->notify($yourNotification);
+```

docs/examples.md

@@ -163,6 +163,16 @@ $server = Server::builder()
     ->setDiscovery(__DIR__, ['.'], [], $cache)
 ```
 
+### Client Communication
+
+**File**: `examples/stdio-client-communication/`
+
+**What it demostrates:**
+- Server initiated communcation back to the client
+- Logging, sampling, progress and notifications
+- Using `ClientGateway` in service class via `ClientAwareInterface` and corresponding trait
+- Using `ClientGateway` in tool method via method argument injection
+
 ## HTTP Examples
 
 ### Discovery User Profile