Speed

Fast mode

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

For GPT-5.6, GPT-5.5, and GPT-5.4, Fast mode increases model speed by 1.5x. GPT-5.6 and GPT-5.5 consume credits at 2.5x the Standard rate; GPT-5.4 consumes credits at 2x the Standard rate.

GPT-6 Astra Fast mode consumes credits at 2.5x the Standard rate where available. See Models for model availability and Pricing for token rates.

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 ChatGPT desktop app, Codex CLI, and IDE extension when you sign in with ChatGPT. Fast mode is a ChatGPT credit feature. With an API key, Codex uses API token pricing instead, and ChatGPT credit multipliers don't apply. API Priority processing has its own billing rate; for GPT-5.6, it costs 2x the Standard API token rate.

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.