Implement OpenAI Responses API [1/N]

[WoosukKwon]

This PR implements the basic support for the OpenAI Responses API.

• Basic stateless functionalities with stream=False
• Reasoning parser support
• Structured outputs on normal outputs (not on reasoning part)
• Stateful APIs (with just a python dictionary)

TODOs:

• Tool/functional calling support
• Output token streaming
• Structural tag in reasoning
• Multimodal support

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[gemini-code-assist]

Summary of Changes

Hello @WoosukKwon, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new OpenAI-compatible Responses API endpoint to the vLLM server. It involves defining new request and response protocols, implementing the core serving logic for this API, and integrating it into the existing API server. The aim is to support more structured and advanced model interactions, potentially including features like tool use and reasoning extraction, beyond standard chat completions.

Highlights

• New API Endpoint: Introduced a new /v1/responses API endpoint, aligning with the OpenAI Responses API specification. This endpoint allows for more structured and potentially advanced interactions with the model.
• Protocol Definition: Defined new ResponsesRequest and ResponsesResponse Pydantic models in vllm/entrypoints/openai/protocol.py to handle the specific request and response formats for the new Responses API, including support for various parameters like background, include, input, instructions, tools, and reasoning.
• Core Serving Logic: Added a new OpenAIServingResponses class in vllm/entrypoints/openai/serving_responses.py (a new file) to encapsulate the business logic for the Responses API. This includes handling prompt preprocessing, scheduling requests with the engine, processing model outputs, and formatting the final response, including support for reasoning content extraction.
• Input Handling Enhancement: Modified vllm/entrypoints/chat_utils.py to recognize and process input_text content parts, ensuring compatibility and proper handling of text inputs within the new Responses API context.
• API Server Integration: Integrated the new Responses API service into vllm/entrypoints/openai/api_server.py by importing necessary components, adding a new FastAPI route for /v1/responses, and initializing the OpenAIServingResponses instance within the application state.

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[gemini-code-assist]

Code Review

This pull request introduces the OpenAI Responses API. The changes include adding a new endpoint, protocol definitions, and a serving class for this new API. The implementation looks like a good start, but there are a few issues to address. I've found a bug related to a duplicated argument, a correctness issue with guided decoding schema handling, and some areas for improvement in request object mutation and background task management. I've also pointed out a leftover debug print statement that should be removed.

[thomascaillier]

Hi ! It would be great to update the docs here about the Responses API support : https://docs.vllm.ai/en/latest/serving/openai_compatible_server.html 🙏