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Codex Blocks Fast Mode with an API Key on Desktop

• Andrew Denta

I ran a matrix of Codex configurations to find out why Fast mode works in the CLI but not in the Desktop app with an API key.

Ok, so you are Sonic. You gotta go fast.

A hand-drawn blue hedgehog running under the words “Gotta go fast.”

gotta go fast

You recently switched to using an API key in the Codex Desktop app. You notice that there is no Fast mode toggle anymore. This bothers you.

You start looking into this and things seem weird. When using an API key, it appears:

  • The fast mode toggle is no longer exposed in the UI.
  • Updating the Codex TOML config file changes nothing when using the Desktop application. You can’t force fast mode at all.
  • Fast mode with an API key works in the Codex CLI, but not in Desktop.

The fact you can use fast mode with an API key in the CLI makes me think this is a bug vs something the Codex team is intentionally hiding.

I built codex-tps to benchmark this. I then ran a matrix of configuration options and measured their TPS responses. All experiments were run with GPT-5.6-sol on high reasoning effort.

Codex effective output speed by configuration.

Configuration Median TPS Branch and logs
Stock Desktop + API key + service_tier=fast 54.7 branch · log
Patched Desktop + API key + forced Fast UI state 87.3 branch · log
Stock Desktop + API key + tier + feature flag 60.7 branch · log
Stock Desktop + API key + feature flag only 45.4 branch · log
Stock Desktop + API key + no Fast settings 59.3 branch · log
CLI + API key + Standard 57.5 branch · log
CLI + API key + Fast 90.2 branch · log
Desktop + ChatGPT account + Standard 35.3 branch · log
Desktop + ChatGPT account + Fast/Priority 45.6 branch · log

Observed Fast treatment effects.

As you can see, the stock Desktop app is indeed slower unless you manually go in and tweak the Codex Desktop code itself. If you dig into the logs, you can see that the Codex Desktop application is stripping out service_tier=fast and passing service_tier=None.

I was able to fix this by updating the Codex Desktop electron app. I’m unsure why this is happening, but anyone else who wants to fix it can review this repo and patch their own Codex Desktop application like I did: github.com/adenta/codex-fast-mode-patcher. This is dangerous. Don’t blindly let your agent do this.