I’ve built an AI running coach (yes, yet another one out there). It’s not for everyone though. I’ve built exactly the coach that I wanted myself.
I started running more seriously about 4 years ago and have since then become kind of a nerd. I read a lot of science books about running and constantly try to optimize my training. For reference, I run around 50-60 km per week and am currently targeting a sub-17 5K.
Anyway, I started using LLMs as a way to get feedback and inspiration in my planning, and thought it worked pretty well. We all know that LLMs aren’t perfect, but after keeping my chat session going and tuning it, it got better and better. However, that was up until a certain point when it started forgetting and guessing important information. Also, I had to manually give it a lot of information and (of course) it was just reactive.
So, I wanted an AI coach that:
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Knew my schedule and preferences
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Automatically had all the relevant data
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Remembered what really mattered
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Proactively reached out
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Focused on ongoing dialog (not static plans or data questions)
So that’s what I built. I also wanted as good quality as possible, without completely losing sight of the cost. Therefore the more complex parts are using Claude Opus (currently 4.8), while the more simple tasks are using Claude Sonnet (currently 5).
I have just launched on both App Store and Google Play, and of course, it connects directly to your intervals.icu account.
If this sounds interesting, please check it out on www.fartlek.io. All feedback is welcome and appreciated! To make sure my costs aren’t running away I have limited the number of concurrent free trials. If you don’t get a spot, you could just try again later.
I just tested it, as it sounded interesting, but it unfortunately does not work. The connection and syncing to intervals.icu does not work at all. 
Thanks for testing it out, and sorry it didn’t work.
I can see in the logs that the mapping of activities have been skipped for some user. That usually happens because the activities come from Strava, and therefore are unavailable through the api. Could that be the issue? If not, I’ll look deeper into it tomorrow!
Quick update: we just released a new version with a fully redesigned UI. If you want to take another look, it’s at www.fartlek.io.
I’m also looking for around 5 people who want to help test and give feedback. You’ll get a free Pro subscription for the rest of the year in exchange for sharing your thoughts along the way as you use it.
If you’re up for it, reply here or send a message and I’ll get you set up.
Just released an update which includes a new self coached mode. If you don’t want an AI to create the plan for you, but still want to use it to get input and feedback, this might be of interest.
You lay out the plan, Coach Fartlek reviews it weekly and on demand and give suggestions that you can either approve or discard. Also, it still does the other stuff, like analyzing workouts, recovery etc.
More info here: Self-coached running, reviewed by an AI coach — Coach Fartlek .
I’m using your app, but I’ve some issue with straight training they don’t sync from app to interval.icu with the running session is all good. I already enabled all the permission, do you experience something like that?
Thanks for the feedback, I saw your in-app report as well. The integration currently only sync the running activities. But your request is very reasonable and I have added it to the backlog for next release, expected later this week 
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