Claude and OpenAI: why your subscription does not include API credits
What the chat subscriptions cover, what the developer platforms bill for, and which one to pick.
Two separate billing systems sit behind the same login. A Claude Pro or ChatGPT Plus subscription pays for the chat application. It does not carry credits to the developer platform where API keys are created. The error you see — Insufficient Credits while signed in to a paid plan — is the split working as designed.
Claude: the exception is Claude Code#
Claude Code is the part to check first. claude login authenticates the CLI through your chat account and draws on subscription usage rather than API billing. The rest of the ecosystem follows the stricter split: console.anthropic.com is the metered developer platform, and claude.ai is the chat app. To get an API key, create a developer account, buy usage credits under Billing, and generate the key under API Keys. No tier of the chat subscription carries API credits.
OpenAI: Codex is a naming collision#
OpenAI has the same subscription/platform split, but the sharper confusion is the word Codex. Codex is OpenAI's coding agent, shipped as a CLI, an IDE extension, a desktop app, and Codex Web. It is not the data-analysis tool inside ChatGPT, and it is not a model name for that sandbox. If you want Codex in your terminal or editor, you authenticate through a ChatGPT account. If you want to call models from your own script or Cursor, you need a platform account and an API key at platform.openai.com.
Choose by who carries the context cost#
Use the API when you are calling models from code. Per-token billing makes context spend explicit: every request resends the history you include, so a long conversation costs more than a short one. A subscription hides that cost inside the seat price, which makes it easy to ignore and impossible to optimise.
The chat app's cache advantage follows from the same design. It keeps the conversation on the server and reuses it across turns. An API caller has to reconstruct that behaviour deliberately; the platform does not cache your prompts unless you build it.
Use the subscription when you are talking to the interface, or when the tool authenticates through it — Claude Code and Codex both do this. Use the API when the work must run unattended, bill per call, or be redistributed. Subscription auth is per-seat and cannot be delegated to a service.
The split does not mark an overcharge. One product sells a seat; the other sells use. The mistake is paying for the seat and assuming the next automation is covered.