feat: add instructions in server initialization result

src/Configuration.php

@@ -26,6 +26,7 @@ class Configuration
      * @param  CacheInterface|null  $cache  Optional PSR-16 Cache instance for registry/state.
      * @param  ContainerInterface  $container  PSR-11 DI Container for resolving handlers/dependencies.
      * @param  int  $paginationLimit  Maximum number of items to return for list methods.
+     * @param  string|null  $instructions  Instructions describing how to use the server and its features.
      */
     public function __construct(
         public readonly Implementation $serverInfo,
@@ -35,5 +36,6 @@ public function __construct(
         public readonly ?CacheInterface $cache,
         public readonly ContainerInterface $container,
         public readonly int $paginationLimit = 50,
+        public readonly ?string $instructions = null,
     ) {}
 }

src/Dispatcher.php

@@ -132,8 +132,9 @@ public function handleInitialize(InitializeRequest $request, SessionInterface $s
 
         $serverInfo = $this->configuration->serverInfo;
         $capabilities = $this->configuration->capabilities;
+        $instructions = $this->configuration->instructions;
 
-        return new InitializeResult($protocolVersion, $capabilities, $serverInfo);
+        return new InitializeResult($protocolVersion, $capabilities, $serverInfo, $instructions);
     }
 
     public function handlePing(PingRequest $request): EmptyResult

src/ServerBuilder.php

@@ -52,6 +52,8 @@ final class ServerBuilder
 
     private ?int $paginationLimit = 50;
 
+    private ?string $instructions = null;
+
     /** @var array<
      *     array{handler: array|string,
      *     name: string|null,
@@ -120,6 +122,20 @@ public function withPaginationLimit(int $paginationLimit): self
         return $this;
     }
 
+    /**
+     * Configures the instructions describing how to use the server and its features. 
+     * 
+     * This can be used by clients to improve the LLM's understanding of available tools, resources,
+     * etc. It can be thought of like a "hint" to the model. For example, this information MAY 
+     * be added to the system prompt.
+     */
+    public function withInstructions(string $instructions): self
+    {
+        $this->instructions = $instructions;
+
+        return $this;
+    }
+
     /**
      * Provides a PSR-3 logger instance. Defaults to NullLogger.
      */
@@ -257,7 +273,8 @@ public function build(): Server
             loop: $loop,
             cache: $cache,
             container: $container,
-            paginationLimit: $this->paginationLimit ?? 50
+            paginationLimit: $this->paginationLimit ?? 50,
+            instructions: $this->instructions,
         );
 
         $sessionHandler = $this->createSessionHandler();