feat: support stringable system prompt

src/Chain/InputProcessor/SystemPromptInputProcessor.php

@@ -15,11 +15,11 @@
 final readonly class SystemPromptInputProcessor implements InputProcessor
 {
     /**
-     * @param string                $systemPrompt the system prompt to prepend to the input messages
+     * @param \Stringable|string    $systemPrompt the system prompt to prepend to the input messages
      * @param ToolBoxInterface|null $toolBox      the tool box to be used to append the tool definitions to the system prompt
      */
     public function __construct(
-        private string $systemPrompt,
+        private \Stringable|string $systemPrompt,
         private ?ToolBoxInterface $toolBox = null,
         private LoggerInterface $logger = new NullLogger(),
     ) {
@@ -35,7 +35,7 @@ public function processInput(Input $input): void
             return;
         }
 
-        $message = $this->systemPrompt;
+        $message = (string) $this->systemPrompt;
 
         if ($this->toolBox instanceof ToolBoxInterface
             && [] !== $this->toolBox->getMap()

tests/Chain/InputProcessor/SystemPromptInputProcessorTest.php

@@ -147,4 +147,41 @@ public function execute(ToolCall $toolCall): mixed
             or not
             PROMPT, $messages[0]->content);
     }
+
+    #[Test]
+    public function withStringableSystemPrompt(): void
+    {
+        $processor = new SystemPromptInputProcessor(
+            new SystemPromptService(),
+            new class implements ToolBoxInterface {
+                public function getMap(): array
+                {
+                    return [
+                        new Metadata(ToolNoParams::class, 'tool_no_params', 'A tool without parameters', '__invoke', null),
+                    ];
+                }
+
+                public function execute(ToolCall $toolCall): mixed
+                {
+                    return null;
+                }
+            }
+        );
+
+        $input = new Input(new GPT(), new MessageBag(Message::ofUser('This is a user message')), []);
+        $processor->processInput($input);
+
+        $messages = $input->messages->getMessages();
+        self::assertCount(2, $messages);
+        self::assertInstanceOf(SystemMessage::class, $messages[0]);
+        self::assertInstanceOf(UserMessage::class, $messages[1]);
+        self::assertSame(<<<PROMPT
+            My dynamic system prompt.
+
+            # Available tools
+
+            ## tool_no_params
+            A tool without parameters
+            PROMPT, $messages[0]->content);
+    }
 }

tests/Chain/InputProcessor/SystemPromptService.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpLlm\LlmChain\Tests\Chain\InputProcessor;
+
+final class SystemPromptService implements \Stringable
+{
+    public function __toString(): string
+    {
+        return 'My dynamic system prompt.';
+    }
+}