OpenAI Launches Responses API and Agents SDK — Assistants API Sunset Announced
OpenAI ships its new core API with built-in web search, file search, and computer use tools. The Agents SDK provides a framework for multi-step AI workflows. Assistants API sunset planned for 2026.
Maya Johnson
OpenAI launched the Responses API on March 11, 2025, replacing the Assistants API as the company's primary interface for building AI applications. Alongside it shipped the Agents SDK and built-in tools for web search, file search, and computer use, according to OpenAI's changelog.
Responses API: The New Foundation
The Responses API is simpler and more powerful than the Assistants API it replaces. Where Assistants required managing threads, runs, and server-side state, Responses is a single endpoint that returns complete responses with built-in tool execution.
Built-in tools ship natively: web search (for current information), file search (for grounding in uploaded documents), and computer use (for operating software interfaces). These are server-side tools — OpenAI hosts the execution, so developers don't need their own infrastructure.
Agents SDK
The Agents SDK provides a framework for building multi-step AI agents — workflows where the model makes decisions, calls tools, and iterates toward a goal. It launched with Python support; TypeScript followed in June 2025.
This is OpenAI's answer to the growing ecosystem of agent frameworks. Rather than letting third parties define how agents are built, OpenAI is providing its own opinionated framework integrated with the Responses API.
Assistants API Sunset
The Assistants API, launched in late 2023, is planned for sunset in August 2026. OpenAI is giving developers roughly 18 months to migrate to the Responses API — a relatively generous timeline for a technology company.
The Assistants API had been criticized for complexity and reliability issues. The Responses API addresses both by simplifying the API surface and handling more execution server-side.
The Platform Strategy
The Responses API launch marked OpenAI's shift from model company to platform company. By building web search, file search, and computer use as hosted tools, OpenAI is capturing more of the application stack — and more of the revenue — that previously went to third-party providers.
Anthropic took a different approach with MCP, creating an open standard for tool integration rather than hosting tools server-side. Both strategies have merit: OpenAI's approach is simpler for developers; Anthropic's is more flexible.
Our Take
The Responses API is what the Assistants API should have been from day one. It's simpler, more reliable, and the built-in tools eliminate a huge amount of integration work. The Agents SDK is early but directionally right — standardizing agent patterns prevents fragmentation. The Assistants API sunset is overdue. The real competition now is between OpenAI's hosted tools approach and Anthropic's MCP standard for who defines how AI agents interact with the world.
FAQ
What is the Responses API? The Responses API is OpenAI's new core API for building AI applications, launched March 11, 2025. It replaces the Assistants API with a simpler interface and built-in tools for web search, file search, and computer use.
When does the Assistants API shut down? The Assistants API is planned for sunset in August 2026, giving developers approximately 18 months to migrate to the Responses API.
What is the OpenAI Agents SDK? The Agents SDK is a framework for building multi-step AI agents that can make decisions, use tools, and iterate toward goals. It launched with Python support and later added TypeScript.