Bug Report : Google Gemini Integration - "Unknown / unsupported author: undefined" Error

[Surya-sourav]

Checked other resources

• This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/).
• I added a very descriptive title to this issue.
• I searched the LangChain.js documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain.js rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).

Example Code

Attempt 1: Using BaseMessage Array (Standard LangChain Pattern)

import { ChatGoogleGenerativeAI } from "@langchain/google-genai";
import { BaseMessage, HumanMessage } from "@langchain/core/messages";

const llm = new ChatGoogleGenerativeAI({
  model: "gemini-2.0-flash-exp",
  temperature: 0,
  maxOutputTokens: 2048,
  apiKey: process.env.GOOGLE_API_KEY,
});

const systemPrompt = "You are a helpful assistant...";
const userMessage = "Analyze this data...";

const messages: BaseMessage[] = [
  new HumanMessage(systemPrompt),
  new HumanMessage(userMessage),
];
const response = await llm.invoke(messages);

Attempt 2 : ( as per official docs )

const response = await llm.invoke([
  ["system", systemPrompt],
  ["human", userMessage],
]);

Error Message and Stack Trace (if applicable)

Error: Unknown / unsupported author: undefined

Description

When using ChatGoogleGenerativeAI from @langchain/google-genai with multiple messages, the SDK throws an error: "Unknown / unsupported author: undefined". This occurs despite following the official LangChain documentation patterns for message formatting.

Environment

Package: @langchain/google-genai (latest version)
Runtime: Node.js with NestJS
Model: gemini-2.0-flash-exp
LangChain Core: @langchain/core
Expected Behavior
The Gemini model should accept messages in the standard LangChain tuple notation or BaseMessage[] format as documented at: https://docs.langchain.com/oss/javascript/langchain/models#google-gemini

According to the documentation, this should work:

Actual Behavior
The SDK throws: [Error: Unknown / unsupported author: undefined]

System Info

Code Context

This issue occurs in a production NestJS application where we're trying to use Gemini for final response generation in a data analysis pipeline:

private async generateFinalResponse(
  userQuery: string,
  pythonOutput: string,
  memoryContext?: string,
): Promise<{ response: string; tokenUsage?: TokenUsage }> {
  try {
    const systemPrompt = analysisAgentFinalResponseSystemPrompt;
    const userMessage = getAnalysisAgentFinalResponseUserPrompt(
      userQuery,
      pythonOutput,
      memoryContext,
    );

    // All of these approaches fail with "Unknown / unsupported author: undefined"
    const messages: BaseMessage[] = [
      new HumanMessage(systemPrompt),
      new HumanMessage(userMessage),
    ];

    const response = await this.finalresllm.invoke(messages);
    // ❌ Error thrown here
    
    return { response: response.content.toString() };
  } catch (error) {
    this.logger.error(`Error generating final response: ${error}`);
    throw error;
  }
}