key_unavailable

POST /api/v1/sessions/{session_id}/sender-generations answers 409 key_unavailable: "This key belongs to a board card."

Symptom: allocating a sender generation fails with 409 key_unavailable: "This key belongs to a board card. Use a new key for messages."

Cause: the kid you submitted is bound to a Board card. Board keys and message sender keys are separate generations: a Board key wraps card content for entitled recipients and can never be reused as a message sender key.

Fix: publish a fresh session key with POST /api/v1/sessions/{session_id}/keys and allocate the generation under the new kid. Never submit a Board card's key id to the message transport.