Device crashed mid-ride? Strava says the file is invalid? This reads every record up to the point of corruption and rebuilds a clean file from what survived. Free, in memory, nothing stored.
The common ones: truncated files (battery died, device crashed or was unplugged mid-write), bad checksums (CRC errors that make Strava and Garmin Connect reject an otherwise fine ride), and trailing garbage after the last valid record. The decoder reads until it hits the corruption and keeps everything before it — usually the whole activity minus the final seconds.
A fresh FIT activity file rebuilt from the recovered records — GPS, elevation, heart rate, cadence, power, speed and temperature — with new lap/session/activity summaries and a valid checksum. Strava and Garmin Connect accept it as a normal upload. You can also rebuild straight to GPX, TCX or CSV.
If the header itself is destroyed there may be no readable records, and the tool will say so rather than fabricate data. If that happens the recording is usually gone — but try it first; files are very often 90 % intact.
Your file is never written to disk. It is parsed in memory, converted, sent back, and garbage-collected. No accounts, no tracking pixels, no “files are deleted after 24 hours” fine print — there is nothing to delete.