What "local" means for an iPhone app
Most AI chat apps send each message to a data centre and send the reply back. A local app keeps that step on the phone. The model file lives in the app, and your iPhone's chip writes every reply. Nothing you type has to leave the device to get an answer, which is also why the app keeps working when the signal drops.
The facts that matter before you install
- Memory. The model needs an iPhone with 6 GB of RAM or more. Phones with 4 GB won't run it well, so check yours first.
- Download. About 2 GB, downloaded once over Wi-Fi or mobile data. After that, you don't need a connection to chat.
- Account. None. There's no sign-in and no API key.
- Price. You get 30 free messages a day. Pro is $6.99 a month, $29.99 a year, or $44.99 once.
Local AI apps are not all the same
Several apps let you run a general-purpose model on an iPhone. They're built for asking questions, summarising text or testing models, and they often let you choose the model yourself. Castmates is narrower on purpose. It ships one model, tuned for roleplay, and wraps it in 35 characters who stay in role and remember your story. If you want a model to draft emails or answer research questions, a general-purpose local app will suit you better. If you want to talk to a character, this is the one built for it.
What you give up
- Model choice. You can't swap in another model or load your own.
- Size. A 2 GB model on a phone is smaller than the cloud models behind most chatbots, so long, rich prose is weaker.
- Older iPhones. Phones with 4 GB of RAM are locked out.
- Battery. Generating replies works the phone harder than texting, so long sessions use more battery.
Who this is for
If you want to chat with a character on your phone, keep the chat private, and avoid making an account, this fits. If your phone is older than 6 GB of RAM, a browser-based setup or a cloud app is the realistic option for now. You can meet the cast before you install, and our private roleplay page explains what stays on the device.