feat(models): use tool for litellm structured_output when supports_response_schema=false

[dbschmigelski]

Description

In LiteLLMModel we currently attempt to perform structured_output using the model's native capabilities. This is checked using the supports_response_schema method.

This has two edge cases which are causing customer failures.

1. The first obvious one is that if a model does not support structured output we are failing hard. This means that if a customer changes the model id, code that previously worked will now fail with ValueError("Model does not support response_format").

2. The second is what appears to be a bug in LiteLLM. supports_response_schema does not appear to work with proxies. Looking at their code, they are not passing along proxy information, meaning there is no ability to perform a runtime check for proxies.

To fix this, when supports_response_schema returns false, we now fallback to the similar tool approach used in other model providers. A fix in LiteLLM will be explored but in the immediate term we need to unblock customers.

Related Issues

#862
#909

Type of Change

New feature

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

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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[codecov]

Codecov Report

❌ Patch coverage is 62.85714% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/models/litellm.py	62.85%	10 Missing and 3 partials ⚠️

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