Tool calling broken for Gemini with legacy agent

[Finndersen]

Example Code

from langchain_google_genai import ChatGoogleGenerativeAI
from langchain_core.prompts import ChatPromptTemplate
from langchain.agents.agent import AgentExecutor
from langchain.agents import create_tool_calling_agent

model = ChatGoogleGenerativeAI(model="gemini-2.0-flash-exp")
prompt = ChatPromptTemplate.from_messages(
        [
            ("system", PROMPT_TEMPLATE),
            ("placeholder", "{chat_history}"),
            ("human", "{input}"),
            ("placeholder", "{agent_scratchpad}"),
        ]
    )
tools = [my_tool, ...]

agent = create_tool_calling_agent(model, tools, prompt)
agent_executor = AgentExecutor(agent=agent, tools=tools, verbose=True)

response = agent_executor.invoke({"input": "call my_tool"})

Error Message and Stack Trace (if applicable)

langchain_google_genai.chat_models.ChatGoogleGenerativeAIError: Invalid argument provided to Gemini: 400 * GenerateContentRequest.contents[2].parts[0].function_response.name: Name cannot be empty.

Description

Tool calling with Gemini requires the tool name to be returned in the tool response, but ToolMessage.name is not being set when created by a legacy agent.

Instead, ToolMessage.additional_kwargs is set to: {"name": "my_tool"}, but this isn't used when building the message Parts to send to Gemini.

So this function needs to be updated to use name = ToolMessage.additional_kwargs["name"]

[haard7]

I am having the same error for multi agent environment where I have system prompt as well.

ChatGoogleGenerativeAI is not able to fullfil that, while chatVertexAI is capable enough sometime.

[dhairyakataria]

I am having the same error when trying to execute agent_executor using ChatGoogleGenerativeAI.

Does anyone has any solution for this?

[Finndersen]

Downgrading to langchain-google-genai==2.0.7 should fix it until a new version is released with the fix I made