An AI personal trainer over text is a coach that lives in your Messages app. You text a phone number the way you would text a friend ("hey, I want to get in shape", "bench 185x5x3", a photo of lunch), and it replies with a plan, logs what you did, and checks in on you every day. There is nothing to install; it works over iMessage on iPhone and RCS or SMS on Android.
This article walks through what a day of it looks like with Bay, what happens behind the replies, and where the evidence for text-based coaching stands.
Why text, and not an app
Fitness apps fail at a predictable point: the day you stop opening them. A text thread does not have that failure mode. It is already open, it is already where your attention is, and a message from your coach arrives in the same place as a message from your sister. The cost of logging a meal drops to one sentence; the cost of being reminded drops to zero.
The research backs the format with the usual caveats. A meta-analysis of 59 text-message physical activity programs found participants took measurably more steps than controls (a small-to-medium effect), with larger effects when messages were tailored and interactive. A 2024 review of one-way, non-tailored messages found almost no effect, and concluded that two-way messaging is what matters. Broadcast reminders do little. A conversation does.
Day one: setup in five questions
Your first message gets an immediate reply: who Bay is, the three things it does best, and the first setup question. There are five: your name, your goal, your body (sex, birth year, height, weight), how active you are, and your timezone. You can answer in any order, or in passing ("I'm 34, 5'10, about 190 and I want to drop 15"). When the profile is complete, Bay computes calorie and protein targets and tells you what to try next.
If you skip setup and just send a photo of your breakfast, that works too. Bay logs it and picks setup back up later, one question at a time.
Asking for a plan
You: can you build me a 3 day plan, dumbbells only at home, 45 minutes
Bay: on it. three full-body days, dumbbells only, about 45 minutes each, every muscle twice a week. here's the week, with why it's built this way. 👍 to start it
Bay designs a program rather than improvising one: three full-body sessions (that is the split for three days), every major muscle hit twice a week, weekly set counts in the band for your experience, rep ranges for your goal, and only exercises you can do with dumbbells. It explains why it is built that way and sends a link to the full week. A 👍 or "yes" starts it.
If you mention a bad shoulder at any point, the lifts that load it come out and the plan says which slot was left empty and why.
A training day, start to finish
Morning. Before you train, a message with today's session and a concrete weight for each lift, based on what you did last time. If you have told Bay you train at 6 am, the plan arrives before that. On session days, a short heads-up lands a couple of hours before your usual training time.
At the gym. You log as you go, in shorthand:
You: goblet squat 50x10x3, db bench 55s 8,8,7, rows 60 3x10
Bay: logged. squats and rows hit every rep, so both go up 5 lb next time. bench stays at 55 until you get all three sets of 8
Bay files each set, tells you which targets you hit, and flags any personal records. The progression is a rule, not a vibe: hit every prescribed rep and the weight goes up next time (5 lb on dumbbells); miss, repeat the weight; miss twice, take 10 percent off. Bodyweight movements progress by reps. That is progressive overload with the bookkeeping done for you.
Meals. Breakfast is a sentence; lunch is a photo; dinner at a restaurant is a voice note. Bay replies with the estimate (calories and protein first) and your running total against today's target. Packaged foods and chain restaurant orders are matched to published nutrition data rather than guessed. A 👍 confirms; "make that a large" corrects. Details in how to track calories by texting.
Afternoon. If nothing has been logged by the time you usually eat lunch, one nudge, phrased about lunch. At most one a day, and "skip the nudges this weekend" turns it off.
Evening. A recap: how the day went against your targets, what you trained, and, when there is one, a real milestone (a new estimated one-rep max, a streak, a plate on the bar) as a card. How to read those numbers over weeks is covered in how to track fitness progress.
The week and the month
Every Sunday evening, Bay reviews the last two weeks: logged intake against the trend in your weigh-ins. If your weight is not moving the way your goal says it should, it proposes a bounded change to your calorie target (never more than 150 calories at a time, protein unchanged) and you tap to accept. Ask "am I on track?" any day for the same analysis.
Non-beginners run five-week training blocks with a lighter deload week built in as week five. Mention an injury, an allergy, a wedding in June or that you keep your gym sessions short, and Bay remembers it and plans around it. Ask "what do you know about me?" to see the list; "forget that" removes an item.
Everything is also on a private web dashboard (text "dashboard" for a sign-in link): history, charts, the plan, weigh-ins, and CSV export.
What it is not
Bay cannot see you lift, so it will not correct your technique; learn the lifts from a person or good video first. Its food numbers are estimates grounded in data, not measurements. It is a software assistant, not a doctor, dietitian or certified trainer, and it says so. And it is text: if you want a screen full of charts while you train, an app is a better fit.
Reminders and triggers
Two features people use more than they expect. "Remind me to go to the gym at 7 tonight" sets a reminder at your local time. "If I'm still under my protein by 7 pm, remind me" sets a trigger that fires only if the condition is true. "After every lunch remind me to walk" is another. Each fires at most once a day, and you cancel them by asking.
How to start
Save the number, text "hi", answer a few questions. Text STOP at any time and it stops; START brings it back. Bay is free right now, and the button on the home page opens Messages with the first text already written.
Questions, answered.
Tap a question.
Does an AI personal trainer over text work on Android?
Yes. Bay replies over RCS or SMS on Android and over iMessage on iPhone, so it works on any phone that can text. On SMS, confirmations are "yes" instead of a tapback.
How do I log a workout by text?
In shorthand: "bench 185x5x3" is three sets of five at 185; "pullups 8,8,6" is three sets with the reps listed; "rdl 100kg 3x8 rpe 8" works too. Bay canonicalizes the exercise name, appends to today's workout and reports whether you hit the target.
Will it text me too much?
A typical day is the morning plan, a recap, and a nudge only if nothing was logged. You can move the times, pause everything for a trip, or text STOP to end it entirely.
Is there an app?
No. The thread is the product. Your history and plan are also on a private web page you reach with a sign-in link Bay texts you.
Related reading
Bay is a software assistant, not a doctor, dietitian or certified trainer. Nothing here is medical, nutrition or professional advice; check with a professional before changing your diet or training.