[Frontend] Enable generic structured_outputs for responses API

[alecsolder]

Purpose

The current ResponsesAPI implementation only supports setting an output text format using json_schema, however for more complicated use cases like grammars, regexes, choices, etc, you need to be able to pass in the full structured_outputs object

Test Plan

vllm serve openai/gpt-oss-20b --enforce-eager --max-model-len=65536 \
--tool-call-parser=openai --enable-auto-tool-choice --reasoning-parser=openai_gptoss

curl -X POST http://localhost:8000/v1/responses \
    -H "Content-Type: application/json" \
    -d '{
      "model": "openai/gpt-oss-20b",
      "input": "Pick a color",
      "structured_outputs": {
        "choice": ["red", "green", "blue"]
      }
    }'

Test Result

Final output message:

{"id":"msg_946aeff87d4ed2e2","content":[{"annotations":[],"text":"green","type":"output_text","logprobs":null}],"

Full response, showing it still respects only enabling it after reasoning

{"id":"resp_be3abebb226eafdf","created_at":1770136083,"incomplete_details":null,"instructions":null,"metadata":null,"model":"openai/gpt-oss-20b","object":"response","output":[{"id":"rs_9664daf6b147c565","summary":[],"type":"reasoning","content":[{"text":"User says: \"Pick a color\". They want a color. Probably answer with a color name. We can also give maybe a suggestion like \"sky blue\" or just pick a random color; maybe include an RGB hex code. Probably pick one: like \"emerald green\". Let's pick \"emerald green (#50C878)\".","type":"reasoning_text"}],"encrypted_content":null,"status":null},{"id":"msg_946aeff87d4ed2e2","content":[{"annotations":[],"text":"green","type":"output_text","logprobs":null}],"role":"assistant","status":"completed","type":"message"}],"parallel_tool_calls":true,"temperature":1.0,"tool_choice":"auto","tools":[],"top_p":1.0,"background":false,"max_output_tokens":65468,"max_tool_calls":null,"previous_response_id":null,"prompt":null,"reasoning":null,"service_tier":"auto","status":"completed","text":null,"top_logprobs":null,"truncation":"disabled","usage":{"input_tokens":68,"input_tokens_details":{"cached_tokens":64,"input_tokens_per_turn":[68],"cached_tokens_per_turn":[64]},"output_tokens":79,"output_tokens_details":{"reasoning_tokens":69,"tool_output_tokens":0,"output_tokens_per_turn":[79],"tool_output_tokens_per_turn":[0]},"total_tokens":147},"user":null,"input_messages":null,"output_messages":null}%

[gemini-code-assist]

Code Review

This pull request enables generic structured_outputs for the responses API, which is a great enhancement. The implementation is straightforward and includes relevant tests. I've found one area for improvement regarding the conflict detection logic to make it more consistent and robust. My feedback is detailed in the review comment.

[yeqcharlotte]

how does users access these from http endpoints? openai responses support passing structured output using text.format

[chaunceyjiang]

+1

[alecsolder]

You can just set it directly for http, there is an example in the PR description

curl -X POST http://localhost:8000/v1/responses \
    -H "Content-Type: application/json" \
    -d '{
      "model": "openai/gpt-oss-20b",
      "input": "Pick a color",
      "structured_outputs": {
        "choice": ["red", "green", "blue"]
      }
    }'

If we wanted to put it on text.format, we would have to implement our own new class which can differentiate the OpenAI ResponseFormatTextConfig type from the structured output type, which has already had annoying changes in the past.

IMO I think I prefer keeping the two fields separate because it would allow us to more clearly differentiate "the code needed to provide a complete Responses API implementation" from "extra features on top of responses API for vLLM specifically". Keeping it as the StructuredOutputsParams type would also mean that it is reusable across the different provider APIs longer term, it would be nice to be able to set the same thing for Anthropic apis and Openai apis to guide model behavior in a way that isn't explicitly tied to API functionality.

[daniel-salib]

LGTM!