API versioning with the Flow-Version header

The API is versioned by date, and the Flow-Version header chooses which version answers a request.

curl https://api.flow.engineer/v1/conversations \
  -H "Authorization: Bearer $FLOW_MESSAGING_KEY" \
  -H "Flow-Version: 2026-11-01"

Rules

The current version is 2026-11-01.
Without the header, the version pinned to your app when it was created is used (app.api_version in GET /v1/app).
Every answer echoes the version that served it in Flow-Version, and webhook deliveries carry it too.
The SDKs send the version they were built for, so their types always match the answers.
Closed lists: error types, event types and content types change only with a new dated version. Within a version you can switch on them exhaustively.
Beta: the API is labelled beta while its shape settles. Breaking changes ship as a new dated version, never inside an existing one.