The Assistants API endpoints stop on 26 August
Announced a year ago, due in nine days. Every integration still on it has to move to MCP for external tool connections — and a protocol has outlived the product that competed with it.
OpenAI announced the Assistants API sunset on 26 August 2025, with endpoints stopping on 26 August 2026. That deadline is nine days away, and integrations relying on it need to adopt MCP for external tool connections.
The date is the practical story: a year of notice is generous, and a year of notice is also exactly how long it takes for a deprecation to be forgotten. Anything still calling those endpoints stops working next week, and the failure will present as a broken integration rather than a deprecation notice.
The structural story is larger. The Assistants API was a vendor's attempt to own the layer above the model — threads, tools, state — as proprietary surface. MCP is an open protocol for the same job. The proprietary surface is being switched off while the open protocol is the thing every client implements.
That outcome was not obvious two years ago, and it repeats a familiar sequence: a vendor ships an integrated abstraction, an open protocol standardises the same capability, the integrated version's advantage erodes because customers will not rebuild per vendor. Tooling standardises even where models do not.
The consequence for anyone building agents is that the integration layer is now a commodity and the differentiation moved elsewhere — into what the model does with the tools, and into how the agent's authority is bounded. One government has now written that boundary into law.
First AI Movers — MCP and the Assistants API Sunset: Migrate by August 2026 → · Codersera — AI Coding Agents 2026: Claude Code, Cursor, Muse Code →