How do you decide when a data storage format is final?

Design for the requirements you have now rather than hypothetical features. If a field is only a possibility, leave it out until you actually need it. For binary formats, make records extensible where it is useful—for example, include field identifiers and lengths so older readers can skip fields they do not understand. That gives you flexibility without turning every structure into an overly generic dictionary.

If you are unsure whether binary storage is necessary, prototype the same data as JSON and compress it. The size difference may be smaller than expected, while the format will be much easier to inspect and extend. If binary is required, consider an established extensible format or a small database such as SQLite instead of inventing every part yourself.

For a rigid format, reserve a small amount of space for metadata such as a version number and leave room for future header fields. Variable-length strings or sections can also prevent minor additions from breaking the entire layout. The key is to support plausible evolution, not every imaginable scenario—migration code is usually easier than trying to design the perfect format upfront.

Put a format version in the header from the beginning. When the structure changes, increment the version and keep loaders for older versions when practical. New required fields are the main complication, but you can usually handle those with sensible defaults, an explicit “unknown” value, or a migration step. You do not need to predict every future change—just make it possible to recognize and handle the changes you actually make.