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POST
Start a session (the first turn)

Authorizations

Authorization
string
header
required

fn_live_… or fn_test_… — Authorization: Bearer

Headers

fn-version
string

Dated API version; defaults to the account pin. Honesty note: with one version in existence the gateway echoes any value back as effective WITHOUT validating it — validation against the known-versions set is planned, so send only real dated versions from https://docs.fiveninelabs.com/changelog

Example:

"2026-08-26"

Idempotency-Key
string

24h replay window; a retry never bills twice

Maximum string length: 255

Body

application/json
query
string
required

the natural-language question

max_spend_usd
number
required

this run's reservation and ceiling (per-run, not lifetime); key caps apply

query_budget_ms
integer

whole-query deadline; bounded by the engine's 90-minute backstop

Required range: x <= 5400000
concurrency
integer
Required range: 1 <= x <= 8
webhook_url
string<uri>
metadata
object

Response

SSE stream (when Accept is text/event-stream)