[BUG] SageMaker payload additional_args being ignored

[athewsey]

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

1.10.0

Python Version

3.13.3

Operating System

macOS 15.7

Installation Method

pip

Steps to Reproduce

1. Create a Strands SageMakerAIModel with a dict of additional_args in the payload config, for example:

model = SageMakerAIModel(
    endpoint_config={
        "endpoint_name": "{your-endpoint}",
    },
    payload_config={
        "additional_args": {
            "logprobs": True,
        },
    }
)

2. Invoke the model (either direct or via an agent) and check the generated payload (or just call the model.format_request(...) method to check it out)

Expected Behavior

The extra parameters provided in payload_config.additional_args should be added to the Body of every request.

Actual Behavior

The additional_args are ignored because:

1. This key of payload_config is (correctly) skipped over when directly adding config keys to the payload here... but
2. Nothing after that actually unpacks the contents of additional_args to the payload as expected. Only the endpoint_config.additional args are addressed (here) which are different because those are actual boto3 parameters, not components within the payload.

Additional Context

As far as I can tell (since some docs are missing), it seems like the intention in the current implementation was for:

• SageMakerAIEndpointConfig.additional_args to allow inserting additional top-level SageMaker boto3 invoke_endpoint(...) or invoke_endpoint_with_response_stream(...) arguments (presumably for features like EnableExplanations, TargetContainerName`, or etc)
• SageMakerAIPayloadSchema.additional_args to allow inserting additional fields to the actual body of your requests to the model (like requesting logprobs, setting non-standard hyperparameters, or etc).

Possible Solution

Assuming the above interpretation is correct, I think we just need to add a step in format_request() to update the payload with the self.payload_config["additional_args"], where they're present?

Related Issues

Found during investigation of #982