Protobuf Field Number Tracker

Field numbers in Protocol Buffers are used for binary encoding — they identify fields in the serialized wire format, not names. Changing a field number is a breaking change: old clients and new clients will misread each other's data. Numbers 1-15 use one byte encoding, 16-2047 use two bytes — so frequently used fields should get low numbers.

Field numbers 19000 through 19999 are reserved by the Protocol Buffers implementation for internal use. Using them will produce a compile-time error in protoc. Numbers must also be positive integers between 1 and 536870911 (2^29 - 1).

The 'reserved' keyword prevents reuse of field numbers or names that were previously used by removed fields. This avoids accidental reuse which would cause data corruption when old serialized data is parsed by new code. Example: reserved 2, 15, 9 to 11; or reserved "foo", "bar";

Each field in a message must have a unique field number. If you rename a field, the number stays the same (that's fine). If you remove a field, mark its number as reserved. Never reuse field numbers in the same message — the binary wire format uses the number as the field identifier.

In proto2, fields have explicit rules: required, optional, or repeated. In proto3, all singular fields are implicitly optional and the 'optional' keyword was only re-added in proto3 optional syntax. This tool handles both syntaxes and parses fields correctly regardless of label style.

Fields you expect to be present in most messages — typically IDs, status flags, timestamps, and primary data payloads. Fields 1-15 encode in 1 byte vs 2 bytes for 16-2047, which can matter at scale. Map fields, repeated nested messages, and rarely-set optional fields can safely use higher numbers.