Speed – Codex
Fast mode
Codex offers the ability to increase the speed of the model for increased credit consumption.
Fast mode increases supported model speed by 1.5x and consumes credits at a higher rate than Standard mode. It currently supports GPT-5.5 and GPT-5.4, consuming credits at 2.5x the Standard rate for GPT-5.5 and 2x the Standard rate for GPT-5.4.
Use /fast on, /fast off, or /fast status in the CLI to change or inspect
the current setting. You can also persist the default with service_tier = "fast" plus [features].fast_mode = true in config.toml. Fast mode is
available in the Codex IDE extension, Codex CLI, and the Codex app when you
sign in with ChatGPT. With an API key, Codex uses standard API pricing instead
and you can’t use Fast mode credits.
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Codex-Spark
GPT-5.3-Codex-Spark is a separate fast, less-capable Codex model optimized for near-instant, real-time coding iteration. Unlike fast mode, which speeds up a supported model at a higher credit rate, Codex-Spark is its own model choice and has its own usage limits.
During research preview Codex-Spark is only available for ChatGPT Pro subscribers.