[Reasoning Engine] LangchainAgent.stream_query attempts to import from deprecated langchain.load path

[ygicp]

Environment details

• OS type and version: Ubuntu 24.04.4 LTS
• Python version: 3.12.10
• pip version: 25.2
• google-cloud-aiplatform version: Latest from main

Steps to reproduce

1. Initialize a LangchainAgent or LanggraphAgent from the agent engine or reasoning engine templates.
2. Call the query or stream_query method.
3. Observe that it fails with an ImportError because it attempts to import from langchain.load instead of langchain_core.load.

Code example

from vertexai.agent_engines.templates.langchain import LangchainAgent

# Initialize agent
agent = LangchainAgent(model="gemini-1.5-flash-001")
agent.set_up()

# Both query and stream_query are affected
result = agent.query(input="Hello")

for chunk in agent.stream_query(input="Hello"):
    print(chunk)

Stack trace

Traceback (most recent call last):
  File "example.py", line 8, in <module>
    for chunk in agent.stream_query(input="Hello"):
  File ".../vertexai/agent_engines/templates/langchain.py", line 665, in stream_query
    from langchain.load import dump as langchain_load_dump
ImportError: cannot import name 'dump' from 'langchain.load'

Affected files

The same deprecated import (from langchain.load import dump) exists in both query() and stream_query() across all 4 template modules:

• vertexai/agent_engines/templates/langchain.py
• vertexai/agent_engines/templates/langgraph.py
• vertexai/preview/reasoning_engines/templates/langchain.py
• vertexai/preview/reasoning_engines/templates/langgraph.py

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!