feat: add OpenAI Responses API model implementation

[notgitika]

Description

This PR implements OpenAI Responses API as a separate model provider supporting streaming, structured output and tool calling.

Note: this commit does not include the extra capabilities that the Responses API supports such as the built-in tools and the stateful conversation runs.

Related Issues

#253

Documentation PR

TODO; coming next: will be adding a section for the Responses API within the existing openai model page

Type of Change

New feature

Testing

Added a unit test file similar to the existing test_openai.py model provider. Reusing the integ tests in the same file test_model_openai.py using pytest.parameterize with OpenAIResponses model so that I could test that the funcitonality is the same between the two models.

I ran everything in the CONTRIBUTING.md file.

hatch run integ-test
================== 81 passed, 68 skipped, 49 warnings in 106.56s (0:01:46) ==========

hatch run test-integ tests_integ/models/test_model_openai.py -v

============= 18 passed, 2 skipped in 13.44s =============

pre-commit run --all-files

• I ran hatch run prepare --> yes, all tests pass

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

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[dbschmigelski]

reach out to @zastrowm , but I believe we can make this more readable by now returning the typed StreamEvents rather than the dictionaries

[notgitika]

I was looking into this briefly but did not implement this. I will talk to him about this and maybe we can implement this in the next iteration.

[fsajjad]

Hey Team, this is one of the feature my customer request and concerned as blocker from adopting Strands Agent. They mentioned Strands currently only support deprecated chatcompletion API and not responses API. When can we expect this to be merged? and will this also enable streaming structured output?

[dbschmigelski]

A major version bump is proposed in #1370.

Adding a comment here that we would need to consider when the user has v1 installed compared to v2.

For example

import pydantic
from packaging import version

# Detect the version once at the module level
PYDANTIC_V2 = version.parse(pydantic.VERSION) >= version.parse("2.0.0")

if PYDANTIC_V2:
    from pydantic import ConfigDict
    def get_model_fields(model):
        return model.model_fields
else:
    def get_model_fields(model):
        return model.__fields__

[codecov]

Codecov Report

❌ Patch coverage is 90.74074% with 25 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/models/openai_responses.py	90.74%	4 Missing and 21 partials ⚠️

📢 Thoughts on this report? Let us know!

[dbschmigelski]

Here we are only ever emitting one delta per tool? Is this correct? Wouldn't we expect multiple?

[notgitika]

this is intentional. the repsonses API streams chunks via response.function_call_arguments.delta which we accumulate, then use the final arguments from response.function_call_arguments.done as a complete json. added this as a small comment there as well.

please lmk if this is not an accepted design and if I should explore something else.

[notowen333]

Some refactoring is required to make this maintainable. Otherwise LGTM