Create attachment preflight thread.
Create an empty assistant thread and hidden draft user message for direct attachment uploads before final send.
Authorizations
Bearer JWT token or API Key (sk-*).
Body
Optional assistant runtime selector. v2 routes the draft thread through agent-gateway. v1 is expired and rejected.
v1, v2 Optional execution model identifier enabled for this instance.
Optional assistant reasoning effort for the draft thread: low, medium, high, or xhigh. Defaults to low.
low, medium, high, xhigh Alias for reasoningEffort used by assistant preferences. Prefer reasoningEffort for new clients.
low, medium, high, xhigh Execution mode to persist on the draft thread.
interactive, execute, background, automation Planning policy to persist on the draft thread.
standard, review_required Per-turn approval policy for the eventual first turn. default requires approval for side-effecting tool calls, auto_review uses conditional safe auto-review, and full skips normal approval gates.
default, auto_review, full Optional allowlist of operational tool names for the eventual first turn. Omit to allow the gateway default surface; pass an empty array to admit no tools for direct-answer turns.
Optional registry plugin ids attached to the draft thread and inherited by the first send when omitted there.
Response
Draft thread and hidden draft message created.

