To solve this, we built a real‑time access engine that counts usage. One of the layers in that engine is the ability to purchase credits. When users exceed their rate limits, credits let them keep using our products by spending down their credit balance.

Underneath this is a complex system that fuses limits, real‑time usage tracking, and credit balances in a single access model. This post covers why scaling Codex and Sora required rethinking access control, how a provably correct, real-time system blends rate limits and credits per request, and how that foundation now unlocks additional access for both products.

For Codex and Sora, neither was sufficient on its own. If we simply raised rate limits, we’d lose important demand-smoothing and fairness controls and run out of capacity to serve everyone. If we relied entirely on asynchronous usage billing, we’d introduce lag, overages, or reconciliation issues—exactly the kinds of problems users notice when they’re most engaged.

What we needed instead was a single hybrid system combining real-time limits with pay-as-you-go access: