[BUG] GeminiModel doesn't support the structured_output implementation

[dpolistwm]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.14.0

Python Version

3.12.3

Operating System

Ubuntu 24.04.2 LTS

Installation Method

pip

Steps to Reproduce

class IntencaoDoUsuarioSchema(BaseModel):
    intencaoDoUsuario: str
    output: str
    buscaBemSucedida: bool | None

gemini_model = GeminiModel(
            model_id=os.getenv("GEMINI_MODEL_ID"),
            params = {
                "max_output_tokens": 40000
            }
        )

agent = Agent(
                name="strands-ai-trek",
                model=gemini_model, 
                prompt="<<PROMPT HERE>>")

agent.structured_output(IntencaoDoUsuarioSchema, "Do you have my data?")

Expected Behavior

The response using the provided Pydantic schema

Actual Behavior

Error: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': "The GenerateContentRequest proto is invalid:\n * tools[0].tool_type: required one_of 'tool_type' must have one initialized field", 'status': 'INVALID_ARGUMENT'}}

Additional Context

No response

Possible Solution

No response

Related Issues

No response

[dbschmigelski]

Hi, this method has now been deprecated in favor of the more holistic approach described in https://strandsagents.com/latest/documentation/docs/user-guide/concepts/agents/structured-output/#basic-usage

result = agent(
    "John Smith is a 30 year-old software engineer",
    structured_output_model=PersonInfo
)

# Access the `structured_output` from the result
person_info: PersonInfo = result.structured_output

This approach should work with all providers. Please let us know if you see any problems with the new approach, thanks!

[dpolistwm]

I got:

ERROR | strands.event_loop.event_loop | cycle failed
Traceback (most recent call last):
File "/home/danielp/r-d-poc/strands-ai-trek/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 189, in event_loop_cycle
async for tool_event in tool_events:
File "/home/danielp/r-d-poc/strands-ai-trek/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 526, in _handle_tool_execution
async for event in events:
File "/home/danielp/r-d-poc/strands-ai-trek/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 277, in recurse_event_loop
async for event in events:
File "/home/danielp/r-d-poc/strands-ai-trek/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 237, in event_loop_cycle
async for typed_event in events:
File "/home/danielp/r-d-poc/strands-ai-trek/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 277, in recurse_event_loop
async for event in events:
File "/home/danielp/r-d-poc/strands-ai-trek/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 225, in event_loop_cycle
raise StructuredOutputException(
strands.types.exceptions.StructuredOutputException: The model failed to invoke the structured output tool even after it was forced.

I am using GeminiModel with a large list of tools and mcp servers.

[dbschmigelski]

Thanks for testing this! Will bring this up with the team

[afarntrog]

Hi @dpolistwm! can you confirm which gemini model you are using?

[dpolistwm]

Sure:
gemini-2.5-flash

[pshiko]

I believe this issue is not actually related to structured_output, but rather occurs when using Gemini via Vertex AI instead of Google AI Studio.
You should be able to work around it by providing some kind of dummy tool to the GeminiModel.

Also, note that structured_output itself is covered in an integration test — see this test case.
However, the behavior differs between Google AI Studio and Vertex AI, which is why this issue occurs.

I’ve already addressed this in my PR #1040, but it hasn’t been reviewed yet.

[dpolistwm]

I have actual tools being used by GeminiModel. In fact, around 20 :)