[malkia]

Learned that at Google, for each service method, introduce individual Request and Response messages, even if you can reuse. This way you can extend without breaking.

Also never reuse old/deleted field (number), and be very careful if you change the type (better not).

[fatnoah]

These were basic principles I put in place at a previous company that had a number of API-only customers. Request/Response messages. Point releases could only contain additive changes. Any changes to existing behaviors, removal of fields, or type changes required incrementing the API version, with support for current and previous major versions.