fix(models/litellm): allow proxy to bypass supports_response_schema in structured_output

[Ratish1]

Description

Previously LiteLLMModel.structured_output rejected models that litellm.utils.supports_response_schema reported as unsupported. That caused false failures when the user configured a LiteLLM proxy, because the proxy may support response_format or negotiate support differently.

This PR:

• Allows client_args["use_litellm_proxy"] (or use_proxy) to bypass the supports_response_schema() check.

• Continues to enforce supports_response_schema() for non-proxy usage.

• Calls litellm.acompletion(..., response_format=) and parses returned tool_calls content into the Pydantic model.

• Adds a unit test verifying the proxy-bypass behavior.

Key Changes

src/strands/models/litellm.py

• Update structured_output to skip the supports_response_schema guard when using a LiteLLM proxy.
• Defensive check for presence of model_id.

tests/strands/models/test_litellm.py

• Add test_structured_output_with_proxy_bypasses_support_check.

Related Issues

Closes #862

Documentation PR

N/A

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare
• I ran hatch fmt --formatter locally.
• I ran hatch fmt --linter and pre-commit run --all-files.
• I ran unit tests locally:

pytest tests/strands/models/test_litellm.py::test_structured_output → passed

pytest tests/strands/models/test_litellm.py::test_structured_output_with_proxy_bypasses_support_check → passed

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[dbschmigelski]

Hi, thanks for raising this. I think I want to go in a different direction though ( I'm working on this currently actually)

I believe we should never fail. We use the native response_format if we return True, else we switch to the tool approach we see in the Bedrock provider.

As for proxy usage, I will create an issue and a tracking issue for us regarding litellm. I think this is actually a bug in their side that the method does not propagate the proxy fields.

[Ratish1]

Hi, thanks for raising this. I think I want to go in a different direction though ( I'm working on this currently actually)

I believe we should never fail. We use the native response_format if we return True, else we switch to the tool approach we see in the Bedrock provider.

As for proxy usage, I will create an issue and a tracking issue for us regarding litellm. I think this is actually a bug in their side that the method does not propagate the proxy fields.

Thanks for letting me know. Let me know if you need any help with anything else.