
I've been (vibe) coding a lot lately. I experimented with a few infrastructure projects (logging system, LLM gateway, tool calling permission management, etc.) but one that I found real passion around is an AI workout manager and coach. I built this one in parallel with the other, more "professional", projects out of frustration with other products that I've tried.

In the end I resorted to using Apple Notes and ChatGPT in order to plan out my training sessions. It was ok for a bit but quickly became too much to effectively manage and ChatGPT started losing context. Ok, I'm an engineer, we're in the vibe coding era, I can "fix" this.

![Calendar view](/images/2026-02-03/calendar.png)

# Background

I've been active my whole life, be it sports or even just workouts to stay healthy. As a high-schooler I trained rowing a lot, to the point where it got me to the brink of "do I go professional or not?". I had good success locally as well as at the European U23 level. (Un)fortunately I got a knee injury that required rehab and taking it easy for a while. During that time I got even more into computer science and tech as I had all this extra time all of a sudden (high school + 1-2 training sessions a day keeps you **busy**). Good thing this happened, really, because it was what built the foundation of my career. And I have a really good life because of it.

I stopped rowing at that point pretty much completely, but I remained active through gym and other workouts. I kept doing indoor rowing workouts and have always had a Concept2 rowing machine in my posession no matter where I lived. Later, in the past 5 years, I started two new sports: cycling and skiing. I transitioned to gravel cycling specifically over the past 3 years and added backcountry skiing. To keep my motivation up I signed up for the occasional gravel race and last year I did my first "ultra" (520km, 10k m vertical). And I loved it! For this year I plan that same ultra gravel race, [Seven Serpents: Quick Bite!](https://www.seven-serpents.com/quick-bite/) in May and the [World Rowing Master Regatta](https://wrmr2026.com) in September.

# Why

As you might be able to tell from the above background, I'm not your typical gym rat or weight-loss warrior, my focus is two-fold: develop strength and stamina for better life quality and do well in the above mentioned sports. I would put myself into the "amateur athlete" category. There's apps targeting this type of customer too but they often lack at least one major feature. And since we now live in the AI era, I looked at that as well and have not seen a good implementation of an AI coach targeting athletes just yet. I don't need fancy videos showing me how to perform an exercise. I need a very solid, knowledgeable and to-the-point kind of coach. I need a good and simple UX around seeing the training plan, performing workouts and syncing those to my Wahoo bike computer. The coach should remember a lot about the workouts and me. Oh and I made sure it has more of a to-the-point kind of vibe. No fluff.

# How

It's pretty simple actually. The frontend is a Next.js web app and an iOS native companion app. There's a Postgres database. That's it. The Next.js app handles everything from the frontend to the mobile API. Would I do it like this if I was not vibecoding it? Unlikely. I'd probably split things up a bit more, use multiple services, etc. But honestly, that would be over-engineering. Oh and also, I should say that while I do use the term "vibe coding" it's not really vibe coded. I go to great lengths to review the code but most importantly make sure the architecture and structure is sound.

I started by doing the work with Cursor. I like the integrated review capability, the editor and the fact that you can switch between multiple models. I used mainly Opus 4.5, some GPT-5.2 and the Cursor model, Composer 1, as well. Their model works surprisingly well for refactors and other simple(r) tasks, especially due to its speed. The iteration is super fast. However, what ended up happening as the project grew larger is that I did not really review all the agent changes in real-time. I would end up creating PRs, reviewing those and even then I'd help myself with agent PR reviews. The Cursor interface quickly became a hindrance (especially since it kept changing so fast). Then I briefly tried the Cursor CLI with the idea to keep the model-switching capability but I did not have a good time with that. It's so far behind the other good coding agent CLIs like OpenCode, Codex and Claude Code. I switched to using Claude Code as the main CLI with the occasional use of Codex and OpenCode as well. I like Claude Code's interactivity but for some specific tasks I prefer how Codex and OpenCode need a lot less hand-holding.

# What

This is how Claude describes the project:

> Fitter is an AI-powered workout coaching platform that combines a native iOS app with a Next.js web dashboard to deliver personalized fitness guidance. At its core is an
  intelligent AI coach that conducts onboarding interviews to understand your training background, goals, and physical profile—then provides
  contextual advice and can directly create or modify workouts on your behalf. The platform supports both cardio workouts (cycling, running, rowing, swimming) with structured
  intervals and training zones (Z1-Z5), and strength workouts with sets, reps, and exercise circuits. Users can plan their training via a visual calendar, sync workouts with Wahoo
  devices through OAuth integration, and receive weekly check-in notifications on iOS prompting reflection on recovery, fatigue, and motivation. The AI coach automatically takes
  notes during conversations—tracking feedback about workout difficulty, pain, or performance—building a memory that makes future advice more personalized. Built with Swift 6 and
  SwiftUI on mobile and TypeScript/React on web, Fitter bridges the gap between generic workout apps and having an actual coach who knows your history and adapts your training
  accordingly.

(Fitter is the working name I use for the project.)

![Coach chat](/images/2026-02-03/coach-chat.png)

# I like it because it's mine

I have a huge bias, obviously, but I think it has a unique angle on the AI fitness coach theme. It's not just another gym coach or workout manager, it's not an app to make you a better runner and it's also not an app to log and publish your workouts (keep using Strava for that, I do). But if your goal is to become stronger, fitter and do well in your sport(s) of choice, then it might be interesting to you as well.

Note: This is very much work in progress. I have been using it for months though and if you'd like to try it out let me know! Hit me up on [LinkedIn](https://www.linkedin.com/in/jernejstrasner/) or [X](https://x.com/JernejStrasner).


Jernej Štrasner

Building My AI Fitness Coach