Pith

The idea

Your Mac does the thinking.

Pith downloads an open-weight language model and runs it on the Mac with MLX. The iPhone app writes to that model. There is no account and no server of mine, and no text goes to a model provider. The Mac app also works as a local chat on its own.

The relay

Your own iCloud carries the conversation.

Conversations sync through the private CloudKit database of your Apple account. Message text, titles and replies are sealed with AES-GCM under a key that travels through iCloud Keychain. Record types, timestamps, job state, device names and installed model names stay readable as metadata. On the same Wi-Fi a direct encrypted link shows replies while they are written.

Limits

Slower over the internet, and only while the Mac is awake.

Through iCloud a reply starts after a few seconds and arrives in bursts. Apple publishes no latency figure, so the app measures the wait on your own devices and shows the median of the last 20 replies. iCloud cannot wake a sleeping Mac, and Pith has to be open there. Models range from about 1 to 17 GB on disk.

Status

Awaiting release.

Needs a Mac with Apple silicon and macOS 14 or later, an iPhone with iOS 17 or later, and one Apple account on both. Free, with no ads, purchases or analytics. 210 unit tests cover the seven packages; real CloudKit, real model runs and real devices are still being tested. The screenshots are the iPhone app with demo conversations. There is no App Store link until it is available.