What we learned building a personal AI assistant that lives in WhatsApp

Most people we wanted to help were never going to install another AI app. Parents, busy friends, people running a small business: they already live in WhatsApp. So we built Kubi, a personal AI assistant that lives there. You text it, send it a photo or a voice note, and it does the task.

This post is about the design decisions that turned out to matter more than the model choice.

There is no dashboard to fall back on, so everything has to work as a conversation:

The hard part was not any single feature. It was keeping replies short enough to read on a phone, and asking one clear question when something is ambiguous instead of guessing.

We use Meta's official Cloud API, not an unofficial bridge on a phone. That costs some flexibility, but it is the supported path, and unofficial bridges risk getting the number banned. It comes with rules you have to design around from day one:

Each user's agent runs in its own isolated environment (its own pod), with its own files and memory. One user's data is never in another user's context. This made some things harder (cold starts, scheduling), but it made the trust story simple to explain: your assistant is yours.

If an assistant can sign in to sites for you, the obvious risk is the password ending up in a prompt or a log. Our answer: credentials live in an encrypted vault, and a proxy fills them into the outgoing request. The agent can use a login without the secret ever entering the model's context.

Payments, deletions and changes to your accounts do not happen on the model's judgment. The agent stops and asks in the chat, and only continues after you approve. Routine reading and searching does not need approval; otherwise the assistant becomes a stream of "are you sure?" messages and nobody uses it.

New people don't fill a form. They message Maya, our onboarding assistant, on WhatsApp. She explains what Kubi does and puts them on the early-access list. We let people in in small batches so we can actually read the feedback.

Kubi works in English and Hebrew today, and it's free during early access. If you want to try it, message Maya on WhatsApp to request access: https://app.heykubi.ai/wa/3d618673 (or read more at https://heykubi.ai ).

We'd love to hear from people building similar things: what's the first task you'd hand to an assistant like this, and what would make you not trust it?