so the issue is that Pub/Sub is super strict about schema compatibility, more than plain protobufs. Adding a new enum value breaks forward compatibility because old subscribers might choke on the new integer (like 5) since they don't know what it means.

In protobuf alone, it's fine (just info loss), but Pub/Sub demands both backward and forward compat, so it rejects the change as incompatible.

Yeah, it's a pain if your enums change often, it does make schemas tricky for evolving messages.

Best fix: Switch those enums to strings for now. That way, you can add new "values" without breaking anything, since strings are flexible.

Just define the possible strings in your app code or docs for validation.

If enums are a must, add a new field with the updated enum and deprecate the old one gradually.

Or create a whole new schema (like v2) and migrate topics/subscribers over time.

Avro schemas might be better for this if you can switch, as they handle enum additions with defaults.

Reserved numbers in enums can help plan ahead, but won't fix existing issues.

Check Pub/Sub docs for updates on this, it's a known limitation.