Anthropic: https://support.claude.com/en/articles/8977456-how-do-i-pay-... - you can pre-pay and get a hard cutoff.
OpenAI: https://community.openai.com/t/how-to-set-billing-limits-and... - last time I looked OpenAI had a soft but not hard limit, I guess they fixed that last year.
I remember bugging them both about this last year, I need to update my mental model!
Deepseek has a prepaid model. (Pretty impressive, what fits into 10 Dollar)
> Billing data processing times can be delayed in AI Studio, up to around 10 minutes. You may experience overages beyond your project cap if billing data hasn't processed before more charges are accrued.
https://ai.google.dev/gemini-api/docs/billing#project-spend-...
That's a soft cap, not a hard cap