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Every path starts with /v0: this is API V0, the shape before the stable release. /v1 is kept for that release, version 1.0, after which the shape is frozen. Within V0 the API is versioned by date, and the Flow-Version header chooses which version answers a request.

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Rules

  • The current version is 2026-11-01, also with the move to /v0.
  • Without the header, the version pinned to your app when it was created is used (app.api_version in GET /v0/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 message kinds (MessageKind) 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.

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Moving from /v1

The earlier shape, POST /v1/conversations/{id}/messages with content, fallback and channel_options, is replaced by one endpoint per message kind under /v0 (Message kinds). Every other route moves to /v0 unchanged. The flow-messaging-migrate skill moves a project with your coding agent: claude plugin marketplace add flow-engineer/sdk, then claude plugin install flow-messaging@flow-engineer (source). Flow Framework users only upgrade the framework.