OpenAI released "Why We Built the Responses API," explaining its design direction: Responses replaces single-turn dialogue with a stateful "reason-action-feedback" loop. This allows models to retain internal reasoning state across multiple turns and return multiple "items" (messages, function calls, structured outputs, etc.) simultaneously, adapting to multimodal interactions such as text, images, audio, and function calls. This API aims to provide native agent workflow support and auditable traces for reasoning models like GPT-5.
Officials also emphasized the advanced integration of managed tools (such as web search, image generation, file search, code interpreter, and MCP) to reduce round trips and lower costs through server-side execution. Internal data indicates that Responses offers better cache utilization and latency than Chat Completions, and cites a slight improvement in TAUBench scores. Historically, Responses has been available to developers since March 2025 and is positioned as the default path going forward. The Assistants API is entering a migration phase, and officials recommend a gradual transition.
Frequently Asked Questions
Q: What is the Responses API?
A: A unified, stateful, multimodal interface that supports preserving inference state, structured output, and managed tool calls for building agent workflows.
Q: What is the difference with Chat Completions/Assistants?
A: The former is more of a single-round message flow; Responses introduces the event sequence of "reasoning and action items" and natively integrates tools; the capabilities of Assistants are gradually incorporated into Responses.
Q: What hosting tools and capabilities are supported?
A: Web search, image generation, file search, code interpreter, MCP, etc. can be executed on the server side to reduce latency and integration complexity.
Q: Will Link of Trace (CoT) be exposed?
A: Not directly exposed; the interface retains and encrypts internal reasoning, and secure continuation context such as previous_response_id can be used.
Q: Will the Assistants API be shut down?
A: We are in the migration phase, and both media and official information point to a mid-term elimination path. We recommend that new projects give priority to Responses, and existing projects migrate according to the guidelines.
