Thought process language not adapting to input language in gemini-2.5-flash-preview-05-20

[rick-bm]

Describe the bug

When using the gemini-2.5-flash-preview-05-20 model with the reasoning feature enabled, the language of the thought process output appears fixed. It consistently outputs in English, even when my input question is in another language (e.g., Japanese). I expect the reasoning steps and final response to automatically match the input language.

To Reproduce

root_agent = Agent(
    model='gemini-2.5-flash-preview-05-20',
    name='root_agent',
    instruction="""answer user questions and output both reasoning steps and results in the language of the user's query""",
    planner=BuiltInPlanner(
        thinking_config=types.ThinkingConfig(
            include_thoughts=True,
        ),
    ),
)

query: 面白い映画をおすすめしてください
thougt: Okay, here's what I'm thinking. The user wants recommendations for "interesting movies" (面白い映画), which, honestly, is incredibly broad. My immediate thought is, "Interesting to whom?" That word covers everything!...

Expected behavior
The reasoning steps are also expected to be output in Japanese

Question

1. Is there a configuration option I might have missed to control the thought process language?
2. Could you provide guidance on how to modify this behavior?

Desktop (please complete the following information):

• OS: MacOS
• Python version(python -V): 3.13
• ADK version(pip show google-adk): v1.2.1

Additional context
Add any other context about the problem here.

[seanzhou1023]

Thank you for the questions :) However, controlling the language of thinking is not supported by the model. ADK cannot control that.