Versioning
Date-based versions on top of the /v1 path.
The current version is 2026-10-01. Pin it with the FormFlow-Version request header; every response echoes the version served. Unsupported values return 400 with the supported list.
Compatibility window
A version stays supported for at least 12 months after its successor ships. Additive changes (new fields, endpoints, event types, enum values) are not breaking: ignore what you do not know.
Deprecations
Deprecated operations respond with Deprecation: true and Sunset: <HTTP date>. The full policy is in docs/api-versioning.md.