Unknown developer fields in a .fit file — Connect IQ data
Some of the most interesting fields in a modern .fit file were not defined by Garmin at all — an app on the watch added them, and the file explains them itself.
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The file carries its own field dictionary
Beyond the standard profile, FIT lets a Connect IQ app or a third-party sensor write developer data fields: custom values the base format never anticipated. What makes them readable is that the file includes their descriptions inline — each developer field arrives with a name, a unit and a type declared in the same file — so a correct reader can name and label them without knowing the app in advance. This is different from an unknown message type, which is a gap in the reader’s profile; developer fields are self-describing by design.
Why generic tools drop them
Many viewers only decode the standard profile and skip anything developer-defined, so a running-power field from a third-party app, or a custom metric from a Connect IQ data field, simply vanishes from the export. The data is present and self-labelled; the tool chose not to read the part of the file that describes it.
See your developer fields named
This page reads the file on your device, including its developer-field descriptions, and lists those custom fields by the names and units the file gives them — so the third-party data shows up rather than disappearing. It runs locally and uploads nothing.
If this was not your problem, the four things that actually break a .fit file covers the rest, and the file reader opens any file whatever is wrong with it.