Codex Fast Mode vs Claude Fast Mode: What's Actually Different?
This article explores the differences between the fast modes offered by Codex and Claude, two popular large language models. While both provide faster response times, the underlying approaches are quite different.
Why it matters
Understanding the differences between Codex and Claude's fast modes is crucial for developers and users to choose the right option for their specific workflow and requirements.
Key Points
- 1Codex has two fast mode options: a 1.5x faster version of the GPT-5.4 model, and a separate ultra-fast Spark model running on Cerebras hardware
- 2Claude's fast mode simply prioritizes the same Opus 4.6 model at the infrastructure level, improving output speed by up to 2.5x
- 3The Codex Spark model is dramatically faster but uses a smaller, lower-capability model, while Claude retains the full Opus 4.6 model capabilities
Details
Codex's fast mode options include serving the same GPT-5.4 model about 1.5x faster, or running a separate smaller model called Spark on Cerebras' powerful Wafer-Scale Engine 3 hardware, which can generate over 1,000 tokens per second. In contrast, Claude's fast mode keeps the same Opus 4.6 model and speeds it up through infrastructure-level prioritization, improving output speed by up to 2.5x. The tradeoffs around price, speed, and intelligence retention are subtle, with the Codex Spark option providing dramatically faster speeds but potentially lower model capabilities, while Claude retains the full Opus 4.6 model at a 6x price premium. The article also discusses the technical details behind the Cerebras WSE-3 hardware and how Anthropic has optimized Claude's infrastructure for faster throughput.
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