Hi there,
I’ve built a small browser tool for activity files and thought this crowd might find it useful: https://thesplit.fit
It does the structural stuff rather than the data stuff — crop, split, join two files into one (for when the watch dies mid-ride), move a multisport transition boundary after a mistimed lap press, and merge heart rate from a separate strap into a watch file. That last one works out the clock offset between the two devices by correlation, so you don’t have to know that your Polar was 4.7 seconds behind your Garmin.
Deliberately not doing anything Intervals already does well: for fixing bad data points or redrawing a spiky HR trace, the editor here is far better and I use it constantly. This is for the cases where the file’s shape is wrong rather than its values.
Free, no account, runs entirely in the browser — nothing gets stored server side long term. Built it after missing the lap button in my own T1 and discovering the only fix was to post the file on a forum and hope someone took pity.
Feedback very welcome — bug report and idea forms are in the footer.