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

source

Python Version

3.13

Operating System

macOS

Installation Method

git clone

Steps to Reproduce

1. Write a tool that returns an image
2. Provide that tool to a strands agent using OpenAI
3. Guide agent to use the tool
4. Exception!

@tool
def tool_with_image_return():
    print(test_image_path)
    with open(test_image_path, "rb") as image_file:
        encoded_image = image_file.read()

    return {
            "status": "success",
            "content": [
                {
                    "image": {
                        "format": "png",
                        "source": {"bytes": encoded_image},
                    }
                },
            ],
        }

agent = Agent(model=OpenAIModel(
    model_id="gpt-4o",
    client_args={
        "api_key": os.getenv("OPENAI_API_KEY"),
    },
), tools=[tool_with_image_return])
agent("Run the the tool and analyze the image")

Expected Behavior

It should work like the BedrockModel does:

Tool #1: tool_with_image_return
The image shows a solid bright yellow square. It's a simple geometric shape with a vibrant, intense yellow color that fills the entire frame. This is a basic color block or color swatch that displays a pure yellow hue without any patterns, textures, or other elements.

Yellow is often associated with:
- Happiness and optimism
- Energy and warmth
- Attention-grabbing (it's one of the most visible colors in the spectrum)
- Caution or warning in some contexts

This appears to be a simple color sample that might be used in design work, color theory demonstrations, or as a placeholder image.

Actual Behavior

It throws an exception:

strands.types.exceptions.EventLoopException: Error code: 400 - {'error': {'message': "Invalid 'messages[3]'. Image URLs are only allowed for messages with role 'user', but this message with role 'tool' contains an image URL.", 'type': 'invalid_request_error', 'param': 'messages[3]', 'code': 'invalid_value'}}

Additional Context

It seems like this is a limitation of the OpenAI api:

Returning image as tool output in Assistants API? - API - OpenAI Developer Community indicates:

Only “user” role messages can include images. You cannot push a user message onto a thread stack while a tool call is still open, and returning the tool call makes the AI write language about the results.

As does Returning image as result of function call to gpt-4-turbo - API / Bugs - OpenAI Developer Community:

Images are allowed in user, system and assistant messages.

Here’s the vision quick start guide:

This was discovered when the generate_image_stability was being added: strands-agents/tools#101 (comment)

Possible Solution

No response

Related Issues

No response