Speed

Fast mode

Codex offers the ability to increase the speed of the model for increased credit consumption.

Fast mode is currently supported on GPT-5.4. When enabled, speed is increased by 1.5x and credits are consumed at a 2x rate.

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 GPT-5.4 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.