feat: allow adding system prompt to chat engine (#855)

Co-authored-by: Marcus Schiesser <mail@marcusschiesser.de>

packages/core/src/engines/chat/ContextChatEngine.ts

@@ -31,6 +31,7 @@ export class ContextChatEngine extends PromptMixin implements ChatEngine {
   chatModel: LLM;
   chatHistory: ChatHistory;
   contextGenerator: ContextGenerator;
+  systemPrompt?: string;
 
   constructor(init: {
     retriever: BaseRetriever;
@@ -38,9 +39,9 @@ export class ContextChatEngine extends PromptMixin implements ChatEngine {
     chatHistory?: ChatMessage[];
     contextSystemPrompt?: ContextSystemPrompt;
     nodePostprocessors?: BaseNodePostprocessor[];
+    systemPrompt?: string;
   }) {
     super();
-
     this.chatModel =
       init.chatModel ?? new OpenAI({ model: "gpt-3.5-turbo-16k" });
     this.chatHistory = getHistory(init?.chatHistory);
@@ -49,6 +50,7 @@ export class ContextChatEngine extends PromptMixin implements ChatEngine {
       contextSystemPrompt: init?.contextSystemPrompt,
       nodePostprocessors: init?.nodePostprocessors,
     });
+    this.systemPrompt = init.systemPrompt;
   }
 
   protected _getPromptModules(): Record<string, ContextGenerator> {
@@ -71,7 +73,6 @@ export class ContextChatEngine extends PromptMixin implements ChatEngine {
       message,
       chatHistory,
     );
-
     if (stream) {
       const stream = await this.chatModel.chat({
         messages: requestMessages.messages,
@@ -113,9 +114,16 @@ export class ContextChatEngine extends PromptMixin implements ChatEngine {
     });
     const textOnly = extractText(message);
     const context = await this.contextGenerator.generate(textOnly);
-    const messages = await chatHistory.requestMessages(
-      context ? [context.message] : undefined,
-    );
+    const systemMessage = this.prependSystemPrompt(context.message);
+    const messages = await chatHistory.requestMessages([systemMessage]);
     return { nodes: context.nodes, messages };
   }
+
+  private prependSystemPrompt(message: ChatMessage): ChatMessage {
+    if (!this.systemPrompt) return message;
+    return {
+      ...message,
+      content: this.systemPrompt.trim() + "\n" + message.content,
+    };
+  }
 }