This piece assumes the GPT-5 dual-surface product map — it covers the Assistants → Responses migration deadline only. Full series in navigation below.
On March 11, 2025, OpenAI released the Responses API (/v1/responses) as the new primitive for agentic apps — combining Chat Completions–style simplicity with Assistants-style tool use. On August 26, 2025, OpenAI formally announced deprecation of the Assistants API beta, with a hard sunset of August 26, 2026. That deadline is a first-class product-stack fact, not a footnote.
What OpenAI says Responses is for
- A single call path for multi-step workflows across tools and model turns.
- Built-in / hosted tools that run server-side (web search, file search, computer use, and later additions such as code interpreter, image gen, and MCP called out in OpenAI’s Responses materials).
- Stateful multi-turn behavior suited to reasoning models — OpenAI argues Responses preserves reasoning context better than Chat Completions for agentic scoring.
- The recommended path for new projects; Chat Completions remains supported, but Responses is the directed future for agent features.
Confirmed Assistants sunset
- Deprecation notice: August 26, 2025 (OpenAI developer announcement).
- Sunset: August 26, 2026 — Assistants endpoints stop; OpenAI’s migration guidance points developers to Responses (and Conversations-style persistence where needed).
- Rationale OpenAI states: Assistants was an early agent interface before reasoning models; after Responses reached feature parity (including code interpreter and persistent conversations folded in), Assistants winds down.
Treat third-party migration cost estimates as secondary analysis. The confirmed spine is OpenAI’s own sunset date and destination API.
Where it sits in the stack
If ChatGPT vs API is the consumer/developer split, Responses is the developer default for agents. The Agents SDK sits above it when you want the SDK to own the loop; computer use and built-in tools are the action layer attached to Responses.
Our take
In our view, the Assistants sunset is OpenAI admitting that “agent product” and “chat completion product” needed one API grammar. Mid-2026 is the forced cutover window — the same season frontier model names keep changing above a stabilizing integration layer.
Series: OpenAI Product Stack · dual surface · next: Agents SDK.
