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3> For the complete documentation index, see [llms.txt](/llms.txt). Markdown versions of documentation pages are available by appending `.md` to the page URL.3> For the complete documentation index, see [llms.txt](/llms.txt). Markdown versions of documentation pages are available by appending `.md` to the page URL.
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5Ultrafast mode is the fastest service tier in the OpenAI API. It is broadly available for GPT-6 Astra and GPT-6.1 Sol, with [preview access](https://openai.com/index/previewing-ultrafast/) for GPT-5.6 Sol. Use it when speed justifies the higher cost.5Ultrafast mode is the fastest service tier in the OpenAI API. It is broadly available for GPT-6 Astra and GPT-6.1 Sol. Use it when speed justifies the higher cost.
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7We strongly recommend [WebSockets](https://developers.openai.com/api/docs/guides/websocket-mode), especially for agentic applications that make many tool calls in quick succession. Without a persistent connection, network overhead can reduce the latency gains.7We strongly recommend [WebSockets](https://developers.openai.com/api/docs/guides/websocket-mode), especially for agentic applications that make many tool calls in quick succession. Without a persistent connection, network overhead can reduce the latency gains.
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