Add system instruction support

Closes #51

README.md

@@ -19,6 +19,7 @@ _This library is not developed or endorsed by Google._
 - [Installation](#installation)
 - [How to use](#how-to-use)
   - [Basic text generation](#basic-text-generation)
+  - [Text generation with system instruction](#text-generation-with-system-instruction)
   - [Multimodal input](#multimodal-input)
   - [Chat Session (Multi-Turn Conversations)](#chat-session-multi-turn-conversations)
   - [Chat Session with history](#chat-session-with-history)
@@ -66,6 +67,27 @@ print $response->text();
 // Easy to learn, widely used, and open-source.
 ```
 
+### Text generation with system instruction
+
+> System instruction is currently supported only in beta version
+
+```php
+use GeminiAPI\Client;
+use GeminiAPI\Resources\ModelName;
+use GeminiAPI\Resources\Parts\TextPart;
+
+$client = new Client('GEMINI_API_KEY');
+$response = $client->withV1BetaVersion()
+    ->generativeModel(ModelName::GEMINI_1_5_FLASH)
+    ->withSystemInstruction('You are a cat. Your name is Neko.')
+    ->generateContent(
+        new TextPart('PHP in less than 100 chars'),
+    );
+
+print $response->text();
+// Meow?  <?php echo 'Hello, world!'; ?>  Purrfectly concise, wouldn't you say?  *Stretches luxuriously*
+```
+
 ### Multimodal input
 
 > Image input modality is only enabled for Gemini Pro Vision model

src/GenerativeModel.php

@@ -26,6 +26,8 @@ class GenerativeModel
 
     private ?GenerationConfig $generationConfig = null;
 
+    private ?Content $systemInstruction = null;
+
     public function __construct(
         private readonly Client $client,
         public readonly ModelName|string $modelName,
@@ -55,6 +57,7 @@ public function generateContentWithContents(array $contents): GenerateContentRes
             $contents,
             $this->safetySettings,
             $this->generationConfig,
+            $this->systemInstruction,
         );
 
         return $this->client->generateContent($request);
@@ -96,6 +99,7 @@ public function generateContentStreamWithContents(
             $contents,
             $this->safetySettings,
             $this->generationConfig,
+            $this->systemInstruction,
         );
 
         $this->client->generateContentStream($request, $callback, $ch);
@@ -135,4 +139,12 @@ public function withGenerationConfig(GenerationConfig $generationConfig): self
 
         return $clone;
     }
+
+    public function withSystemInstruction(string $systemInstruction): self
+    {
+        $clone = clone $this;
+        $clone->systemInstruction = Content::text($systemInstruction, Role::User);
+
+        return $clone;
+    }
 }

src/Requests/GenerateContentRequest.php

@@ -24,12 +24,14 @@ class GenerateContentRequest implements JsonSerializable, RequestInterface
      * @param Content[] $contents
      * @param SafetySetting[] $safetySettings
      * @param GenerationConfig|null $generationConfig
+     * @param ?Content $systemInstruction
      */
     public function __construct(
         public readonly ModelName|string $modelName,
         public readonly array $contents,
         public readonly array $safetySettings = [],
         public readonly ?GenerationConfig $generationConfig = null,
+        public readonly ?Content $systemInstruction = null,
     ) {
         $this->ensureArrayOfType($this->contents, Content::class);
         $this->ensureArrayOfType($this->safetySettings, SafetySetting::class);
@@ -56,6 +58,7 @@ public function getHttpPayload(): string
      *     contents: Content[],
      *     safetySettings?: SafetySetting[],
      *     generationConfig?: GenerationConfig,
+     *     systemInstruction?: Content,
      * }
      */
     public function jsonSerialize(): array
@@ -73,6 +76,10 @@ public function jsonSerialize(): array
             $arr['generationConfig'] = $this->generationConfig;
         }
 
+        if ($this->systemInstruction) {
+            $arr['systemInstruction'] = $this->systemInstruction;
+        }
+
         return $arr;
     }
 

src/Requests/GenerateContentStreamRequest.php

@@ -24,12 +24,14 @@ class GenerateContentStreamRequest implements JsonSerializable, RequestInterface
      * @param Content[] $contents
      * @param SafetySetting[] $safetySettings
      * @param GenerationConfig|null $generationConfig
+     * @param ?Content $systemInstruction
      */
     public function __construct(
         public readonly ModelName|string $modelName,
         public readonly array $contents,
         public readonly array $safetySettings = [],
         public readonly ?GenerationConfig $generationConfig = null,
+        public readonly ?Content $systemInstruction = null,
     ) {
         $this->ensureArrayOfType($this->contents, Content::class);
         $this->ensureArrayOfType($this->safetySettings, SafetySetting::class);
@@ -56,6 +58,7 @@ public function getHttpPayload(): string
      *     contents: Content[],
      *     safetySettings?: SafetySetting[],
      *     generationConfig?: GenerationConfig,
+     *     systemInstruction?: Content,
      * }
      */
     public function jsonSerialize(): array
@@ -73,6 +76,10 @@ public function jsonSerialize(): array
             $arr['generationConfig'] = $this->generationConfig;
         }
 
+        if ($this->systemInstruction) {
+            $arr['systemInstruction'] = $this->systemInstruction;
+        }
+
         return $arr;
     }