Someone on Reddit just shipped a weight loss GPT that does something radical: it refuses to calculate anything until it actually knows you. u/xteaj built it because every diet GPT runs the same loop. You give it your stats, it spits out numbers, you feel judged, you quit. Technically accurate. Practically useless. The numbers aren’t wrong. The approach is. Most tools are built for compliance, not sustainability. And compliance without buy-in has a well-known shelf life: about two weeks into January.
What’s new
The GPT uses the HALT framework from Kaiser Permanente (Hungry, Angry, Lonely, Tired) to catch emotional eating before the spiral starts. When you report overeating, it doesn’t log a deficit. It asks what triggered it, then zooms out to your full week. This matters more than it sounds. Most people don’t overeat because they lack information. They overeat because they had a rough afternoon, a fight with someone, or just got bored at 10pm. Standard calorie trackers ignore all of that context. HALT forces the conversation to go there first, before any numbers enter the picture. You end up understanding your patterns instead of just cataloguing your failures. Food changes are framed as “add, reduce, or replace.” Never “stop eating X.” Same outcome, completely different headspace. Instead of “cut out the chips,” it might say “add a protein source to your afternoon snack.” You end up in the same place, but one path feels like punishment and the other feels like progress. That framing shift alone is worth stealing for any behavior-change product you build.
The twist
There’s a hard rule baked into the prompt: zero math until it has all five stats. Sex, age, height, weight, activity level. All five. GPT-4o apparently loves to skip straight to calculating, so the builder explicitly blocked it. This is actually a product design decision disguised as a prompt constraint. The model is trained to be helpful, and “helpful” in its default mode means giving you an answer fast. But fast answers built on incomplete data are worse than no answers at all in a coaching context. The builder understood that and built a gate. The model has to 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.
How it works
🔧
- 🎯 Describe your situation first , don’t ask for numbers right away
- Let it gather your stats conversationally (it asks, you answer)
- Get your TDEE and one actionable first step (not a full overhaul)
- When you slip up, tell it honestly , HALT does its thing
- 📊 Recalculate TDEE every 2-3 kg lost; it’ll remind you
That last step is one most people skip. Your TDEE shifts as you lose weight because a lighter body burns fewer calories at rest. If you don’t recalculate, you’ll hit a plateau and think you’re broken when you’re actually just running on stale numbers. The reminder is built into the workflow so you don’t have to remember to think about it.
Pro tip
The full prompt is free and open source in the original Reddit post. The builder also added custom knowledge docs (r/loseit FAQ, Kaiser HALT PDF, USDA Dietary Guidelines) as a knowledge base. The prompt works solo, but the docs close the gaps on edge cases. 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 the first bad day is. The intake gate, the reframing, 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 useful open source actually works. 🙌
Frequently Asked Questions
Q: If GPT can’t count calories accurately, how does this work?
This isn’t trying to be a precision calculator – it’s a behavioral coach. The science (protein targets, BMR formulas, safe calorie minimums) stays as guardrails in the background, but the real magic is in the coaching: the HALT framework catches emotional eating, weekly reframing removes guilt, and the “add/reduce/replace” language makes changes feel sustainable. Users report that behavior changes stick better than chasing perfect calorie counts ever did.
Q: Can I track food with pictures instead of logging apps like MyFitnessPal?
Yes. Users have tested this and found it works – you can send plate photos or describe meals, and the GPT evaluates them without the logging tedium. It won’t be perfectly precise, but if traditional app logging feels like friction, this removes that barrier while keeping you in a coaching conversation. Most people stick with what feels less annoying, even if it’s slightly less accurate.
Q: Why all the emphasis on warmth and support? Why not just give me the facts?
Because behavior change is mostly psychology, not math. A cold calculator tells you to “eat less”; a coach asks “what triggered this?” and helps you find the real problem. Decades of behavioral science show that empathetic guidance beats guilt every time – you’re more likely to stick with advice from someone who understands you.
Q: What makes this different from other diet GPTs or MyFitnessPal?
Most diet tools are calculators with a chat layer; this one flips it by being a coach that uses math as a tool. It won’t calculate your calories until it understands your full situation, and it uses the HALT framework plus “add/reduce/replace” framing – approaches backed by behavioral science. The prompt is open-source, so you can customize it.
I built a Weight Loss GPT that coaches you instead of just counting calories — full prompt included
by u/xteaj in ChatGPTPromptGenius