For recordings that stopped mid-save, show as zero bytes of data, or are refused on upload. Choose the file and you will see what was wrong, what was recovered, and a repaired copy to download.

What it fixes

Problem	What the tool does
The recording was never finalised: the battery died, the device froze or restarted.	Reads every complete record up to the cut, then rebuilds the lap, session and activity summary a platform expects from those records.
The header gives the wrong data size, often zero.	Measures the real data and writes the correct size.
The checksum at the end is missing or wrong.	Recalculates it.
The end of the file is blank padding or leftover data from the device's storage.	Stops at the last real record and leaves the rest out.
Stray bytes sit in front of the file header.	Finds the real start of the file.

What it cannot fix

The tool keeps everything up to the first unreadable record and nothing after it. If the damage is in the middle of the file, the part after the damage is lost even if it looks intact, because a FIT file can only be read in order: each record depends on definitions that came before it. And it cannot bring back time the device never wrote to storage.

The rebuilt summary covers distance, elapsed and moving time, average and maximum heart rate, average cadence and power, and speed. Calories and total ascent are left blank instead of being guessed; most platforms work those out again from the records.

Questions

It says my file is not damaged, but the upload still fails.

Then the cause is something else. The most common is that the platform already has that activity and treats the upload as a duplicate. Another is a file that is valid but is not an activity, such as a course or a settings file.

Why does it ask for the activity type?

Many devices store the sport only in the summary written when you press save. If that part was lost, the tool has no way to tell a run from a ride, so it asks. If the sport survived, it is used without asking.

Is the original changed?

No. The page can only read the file you pick. The repaired copy is a new download with -repaired in its name.

For background on how these files break, read why FIT files get corrupted.