How to get reasoning_content with ChatVertexAI ?

[chris-lee-lb]

As title, currently I use following setup.

llm = ChatVertexAI(
            model_name="gemini-2.5-flash",
            project=settings.gcp.project,
            location=settings.gcp.location,
            temperature=0,
            max_output_tokens=65535,
            thinking_budget=-1,
            streaming=True,
        )

async for event in llm.astream_events(messages):
        logger.info(event)

But i can not see any reasoning_content in any events.

[chris-lee-lb]

Update (self-resolved):

I found out that:
1. I needed to upgrade langchain-google-vertexai to v2.0.27
2. I had to add the argument include_thoughts=True

After doing so, the model started to return thought reasoning content.
However, unlike other LangChain chat models (e.g., OpenAI, xAI), the reasoning content is not placed under additional_kwargs["reasoning_content"].

Instead:
• It appears inside AIMessageChunk.content as a list of objects, e.g. {"type": "thinking", "thinking": "..."}
• As a result, AIMessageChunk.content is now a list, not a string as it used to be

Just noting this difference in case others run into the same issue or expect the same structure as other models.