gRPC Reserved Fields Builder

In Protocol Buffers, what identifies a field on the wire is the number, not the name. Removing a field and later reusing that number makes the new field receive the old one's bytes — no error, no warning, the wrong value in the right place. The reserved keyword exists to make that impossible: the compiler refuses any attempt to reuse a reserved number.

Enter the removed numbers or ranges and the page assembles the declaration. Ranges use a hyphen, and several items are comma-separated. It is worth reserving the field name too, in a separate declaration, in case someone recreates a field with the same name under a different number — which confuses whoever reads the schema without breaking the encoding.

The rule of thumb is reserving whenever you remove, even in an internal schema. The cost is one line; the cost of not doing it is a data bug that only appears once two schema versions coexist in production, and it is among the hardest to track down, because the value arrives intact and in the wrong field. Note too that numbers 19000 to 19999 are already reserved by the protocol itself.