Someone on Reddit shipped a weight loss GPT that does something radical.
It refuses to calculate anything until it actually knows you. No numbers. No TDEE. No macros. Not a single calculation until the intake is done.
u/xteaj built it because every diet GPT runs the same tired loop. You hand it your stats, it spits out numbers, you feel judged, you quit. Technically accurate. Practically useless.
Here's why that one design choice is the whole trick, and what it teaches you about building any coaching product.
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The Loop That Kills Diet Apps
Most tools get built for compliance, not sustainability. And compliance without buy-in has a famous shelf life. About two weeks into January.
The numbers aren't wrong. The approach is.
People don't overeat because they lack information. They overeat because they had a rough afternoon, a fight with someone, or got bored at 10pm. Standard calorie trackers ignore all of that. They treat the body like a spreadsheet and the person like a data entry clerk.
So the output is mathematically correct and emotionally useless.
HALT Catches The Spiral Before It Starts
The GPT uses the HALT framework from Kaiser Permanente. Hungry, Angry, Lonely, Tired. When you report overeating, it doesn't log a deficit. It asks what triggered it. Then it zooms out to your full week.
This matters way more than it sounds.
You end up understanding your patterns instead of cataloguing your failures. "I binged last night because I got in a fight with my wife" is a completely different conversation than "you're 800 over." The first leads somewhere. The second leads back to the fridge.
Steal this for anything you're building. Finance apps, habit trackers, coaching tools. Check the state of the person before checking their score.
Add, Reduce, Or Replace (Never "Stop")
Food changes in this GPT get framed three ways. Add. Reduce. Replace. Never "stop eating X."
Instead of "cut out the chips," it might say "add a protein source to your afternoon snack." Same outcome. Completely different headspace.
One path feels like punishment. The other feels like momentum.
That framing shift alone is worth stealing for any behavior-change product you build. People will ditch a plan that makes them feel deprived. They'll stick with one that feels like progress.
The Hard Rule: Zero Math Until All Five Stats
Here's where the prompt gets clever. There's a hard rule baked in. No calculations until it has all five stats. Sex, age, height, weight, activity level. All five.
GPT-4o apparently loves to skip straight to calculating. The builder explicitly blocked that.
This is a product design decision disguised as a prompt constraint. The model is trained to be helpful, and "helpful" by default means giving you an answer fast. But fast answers built on incomplete data are worse than no answers in a coaching context.
So the builder put a gate in front of the model. Earn the right to give advice by completing the intake first.
The builder said it best. "You consumed 800 calories over your target" and "One day doesn't define your week" contain the same information. One makes people quit. The other keeps them going.
That's the whole design philosophy in two sentences. It's not about what you say. It's about what the person on the other end is actually able to hear.
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How It Actually Works
Here's the flow if you want to try it:
Describe your situation first. Don't ask for numbers right away
Let it gather stats conversationally. It asks, you answer
Get your TDEE and one actionable first step. Not a full overhaul
When you slip, tell it honestly. HALT does its thing
Recalculate TDEE every 2-3 kg lost. It'll remind you
That last step is the one most people skip. Your TDEE shifts as you drop weight because a lighter body burns fewer calories at rest. Skip the recalc and you'll hit a plateau, think you're broken, and quit.
The reminder is built into the workflow so you don't have to remember to think about it.
Why The Knowledge Base Closes The Gaps
The prompt is free and open source on Reddit. The builder also loaded custom knowledge docs: the r/loseit FAQ, the Kaiser HALT PDF, and the USDA Dietary Guidelines.
Here's the part worth paying attention to. The knowledge base isn't there to make the GPT smarter in a general sense. It's there to handle the questions that fall outside standard advice.
What about intermittent fasting? What if someone has a thyroid condition? What if they're training for a marathon during a cut?
Without the docs, the model guesses. With them, it has a reference point grounded in actual guidelines. That's the difference between a toy and a tool people keep using.
This is worth studying for any coaching GPT you build. Finance, fitness, habits. The math is never the hard part. Keeping people in the game after their first bad day is.
The intake gate, the reframe, the HALT check. None of this is technically complex. It's just thoughtful. And thoughtful scales.
Try It
The live GPT and full prompt are in the original Reddit post. Grab it, break it, improve it. That's how open source actually works.
And if you're building a coaching tool of your own, the template is sitting right there. Wait for the full picture before giving advice. Talk about the why before the what. And treat the score as the thing you check last, not first.
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